Palm oil mill effluent treatment and CO2 sequestration

Environ Sci Pollut Res DOI 10.1007/s11356-017-9742-6

REVIEW ARTICLE

Palm oil mill effluent treatment and CO2 sequestration by using microalgae—sustainable strategies for environmental protection Harizah Bajunaid Hariz 1 & Mohd Sobri Takriff 2

Received: 7 November 2016 / Accepted: 10 July 2017 # Springer-Verlag GmbH Germany 2017

Abstract In this era of globalization, various products and technologies are being developed by the industries. While resources and energy are utilized from processes, wastes are being excreted through water streams, air, and ground. Without realizing it, environmental pollutions increase as the country develops. Effective technology is desired to create green factories that are able to overcome these issues. Wastewater is classified as the water coming from domestic or industrial sources. Wastewater treatment includes physical, chemical, and biological treatment processes. Aerobic and anaerobic processes are utilized in biological treatment approach. However, the current biological approaches emit greenhouse gases (GHGs), methane, and carbon dioxide that contribute to global warming. Microalgae can be the alternative to treating wastewater as it is able to consume nutrients from wastewater loading and fix CO2 as it undergoes photosynthesis. The utilization of microalgae in the system will directly reduce GHG emissions with low operating cost within a short period of time. The aim of this review is to discuss the uses of native microalgae species in palm oil mill effluent (POME) and flue gas remediation. In addition, the discussion on the optimal microalgae cultivation parameter selection is included as this is significant for effective microalgae-based treatment operations.

Responsible editor: Philippe Garrigues * Harizah Bajunaid Hariz [email protected]

1

Faculty of Chemical and Process Engineering, The National University of Malaysia, 43600 Bangi, Selangor, Malaysia

2

Research Center for Sustainable Process Technology (CESPRO), Universiti Kebangsaan Malaysia, Bangi, Selangor, Malaysia

Keywords Microalgae . Wastewater treatment . Industrial effluent . Phytoremediation . Palm oil mill effluent (POME) . Carbon dioxide (CO2) sequestration

Introduction Since the dawn of the global industrial revolution, the increase in greenhouse gases (GHGs) such as carbon dioxide concentration in the atmosphere has become one of the major problems faced in today’s world. The gas emission commonly comes from the industries through the flue gas being emitted throughout manufacturing activities. However, it is not the only problem that the world is currently facing. Wastewater discharged from factories is also a concern that needs to be taken into account in most developing countries. Since water is the crucial substance for all living things, clean water shortage has significantly affected human’s daily activities as well as the country’s development. Furthermore, extensive country developments contribute to polluted wastewater being discharged by the industries, triggering sanitation issues. Hence, rapid industrialization directly influences the status of environmental quality towards an undesired environmental condition. Therefore, the development of green technologies to remediate wastewater and fix the gas emission is very much desired for human and environmental protection. In addition, a cost-effective remediation by using microalgae has promising future advantages that can overcome the non-economic value of conventional water and flue gas remediation methods (Torres et al. 2014). Furthermore, it is undeniable that microalgae has become a significant group of microorganism in the field of biotechnology advancement. Since microalgae can be found almost everywhere including in freshwater and marine water bodies, the commercialization of microalgae biomass has contributed to

Environ Sci Pollut Res

the production of various valuable products such as pharmaceutical supplements and food products that help in promoting a better, healthy life for humans. Moreover, microalgae biomass has become an alternative resource for energy generation such as biodiesel, biohydrogen, and bioethanol production. Apart from that, the biomass commercialization has the potential to offset the cost allocated for microalgae-based wastewater treatment and CO2 sequestration. The amount of carbon dioxide captured by microalgae will define the biomass amount at the end of the cultivation period which is described by one of the researcher where for 1 kg of dry microalgae biomass, about 1.83 kg of carbon dioxide is consumed by the microalgae (Chisti 2007). There are various importance of using microalgae as the approach to have a sustainable environmental treatment technology. Microalgae bioremediation is considered as safe and feasible environmental remediation activities compared to chemical treatment. Microalgae-based treatment technology has the potential to replace the conventional treatment since microalgae is fast and easy to grow at almost everywhere (Lam et al. 2012). Microalgae has higher photosynthetic metabolism compared to the terrestrial plants which will enhance the treatment activities that will be beneficial in term of time and cost. In addition, microalgae cultivation does not required huge land area and can be carried out outdoor in a large scale. The outdoor microalgae cultivation is cost-effective as low energy required at low capital and operation cost since sunlight can be utilized without any cost. Moreover, native microalgae can be used for the cultivation to avoid biosafety issues since the native microalgae strain is isolated from the local environment. By introducing microalgae to treat wastewater, a positive symbiotic relationship between microalgae and other microorganisms such as bacteria can enhance the contaminant reduction in wastewater. As microalgae assimilates nutrient and CO2 from the wastewater and flue gas, the components will be converted into biomass (Suali et al. 2012) for further production of valuable microalgae-based products. Meanwhile, in a large-scale treatment system, a high density of microalgae cultivation is able to be operated with the high supply of nutrient loading from the wastewater such as nitrogen and phosphorus to meet the microalgae growth requirement. Open pond and closed pond systems are able to be operated simultaneously with the aims of reducing or removing the high concentration of carbon dioxide in the flue gas as well as high nutrient in wastewater that are polluting the environment. Although an open pond system such as raceway pond is easy to operate with a low cost of installation, however, the risk of contamination of is higher compare to that of a closed cultivation system. Meanwhile, other challenges faced by an open pond cultivation system are a low-mass transfer rate, high pond exposure to sunlight causing temperature fluctuation, as well as excessive evaporation occur (Chisti 2007). However, a closed cultivation system also has disadvantages

when it comes to a high volume of cultivation where light limitation might be the main issue that will cause microalgae growth limitation. All the shortcomings can be overcome by having an optimal design and parameters of both open and closed systems.

Algae species and characteristics Algae is an organism that can be found in freshwater or marine environment conditions. It lives in a symbiosis environment together with fungus, lichen, and other microorganisms. Algae can be divided into unicellular or multicellular form of microorganisms. Microalgae has the ability to consume nutrients for growth and absorb toxin in suspension. Apart from that, microalgae can be used in CO2 fixation as it undergoes photosynthesis process where CO2 will be consumed for growth and O2 will be released as the respiration product. The presence of microalgae completes the ecosystem as it provides shelter and act as the food source especially for aquatic lives. As microalgae undergoes photosynthesis process, sunlight, carbon dioxide, and nutrients are the requirements for a proper growth of microalgae where nutrients that are rich in ammonia, phosphorus, and nitrate are desired. The classification of algae can be made according to the properties of life cycle, chemical stored, cell structure, and pigmentation. Algae can be classified into a few classes: diatoms, green algae, blue-green algae (cyanobacteria), golden algae, brown algae, and red algae (Becker 2007; Khan et al. 2009). There are differences between microalgae and cyanobacteria in their photosynthesis pathway (Kamarudin et al. 2015). Carbohydrates, proteins, nucleic acids, and lipids are major compositions of microalgae (Williams and Laurens 2010). As stated in Table 1, there are differences in microalgae cell chemical composition relative to the species. The productivity of microalgae in term of lipid production per unit volume is highly dependent on microalgae growth and lipid content in the biomass (Chisti 2007). In comparison to the terrestrial plants, microalgae growth is 100 times faster and it has the ability to double up the biomass within short period of time which is usually less than 24 h (Tredici 2010). The simple cell structure of microalgae with a large surface area per unit volume of microalgae cell explained the rapid growth and cell division of microalgae. This special characteristic of microalgae allows them to consume nutrients at a high rate of consumption (S. A. Khan et al. 2009). Microalgae cultivation under optimum growth conditions allows microalgae optimum cell division for every 3– 4 h. However, most species will take 1–2 days for the population to increase (Williams and Laurens 2010). Microalgae can be cultivated by autotrophic, heterotrophic, or both which is known as mixotrophic way of cultivation. Commonly, photoautotrophic or mixotrophic microalgae

Environ Sci Pollut Res Table 1

Chemical composition of algae expresses in dry weight % (Becker 1994; Lam and Lee 2011)

Algae strain

Lipids

Carbohydrate

Protein

Nucleic acid

Anabaena cylindrica Chlamydomonas rheinhardii Chlorella vulgaris Chlorella pyrenoidosa Dunaliella bioculata Dunaliella salina Euglena gracilis Porphyridium cruentum Prymnesium parvum Scenedesmus obliquus Scenedesmus quadricauda Scenedesmus dimorphus Spirogyra sp. Spirulina platensis

4–7 21 14–22 2 8 6 14–20 9–14 22–38 12–14 1.9 16–40 11–21 4–9

25–30 17 12–17 26 4 32 14–18 40–57 25–33 10–17 21–52 33–64 8–14

43–56 48 51–58 57 49 57 39–61 28–39 28–45 50–56 47 8–18 6–20

– – 4–5 – – – – – 1–2 3–6 – – –

Spirulina maxima Synechoccus sp. Tetraselmis maculata

6–7 11 3

13–16 15 15

46–63 60–71 63 52

2–5 3–4.5 5 -

cultivation method is used (Wang et al. 2010). Autotrophic microalgae cultivation required CO2 supply to meet the requirement of carbon source for microalgae consumption whereas in heterotrophic microalgae cultivation, carbon source can be obtained from the organic compound such sugars and organic acids. Both autotrophic and heterotrophic required CO2 and nutrients for growth (Liang et al. 2009). Economically, microalgae contributes to the production of high-value products as microalgae contains high beta-carotene, glycerol, and alginate that can benefit nutraceutical or pharmaceutical industries. Furthermore, high content of lipid in microalgae can be beneficial in the production of biofuel. Biogas such as biohydrogen and biomethane can be generated from microalgae cultivation system. The amount of protein contained in microalgae is able to meet the requirement of human protein supplement and it can also be used to feed Table 2 Algae growth and habitats

animals. Biofertilizer is another outcome of microalgae biomass where its production can overcome the problem of using synthetic fertilizer that can be harmful to the environment. Other than that, microalgae is known as the natural pollution control agent that can treat waste in forms of water or gas through its growth and development. Furthermore, microalgae-based cultivation system is a cost-effective treatment system as it does not require a huge space for cultivation process compared to other crops. Local algae species Algae growth can easily be found in a damp area or under water as aquatic plant species for algae favours moist and tropical environment compared to a dry environment because of their limitation of vascular tissues and the influence of the

Type of algae

Habitat

Fresh water/marine algae

Grow on animals: turtles, snails, rotifers, worms, crustacean, alligators, three-toed sloths, aquatic ferns, freshwater sponges, and some other animals Aquatic plants: algae grows on or inside water plants (including another algae) Artificial substrates: wooden posts and fences, cans, and bottles Rock (internal and surface) Terrestrial plants—tree trunks, branches, shady sides of trees, damp walls, surface of and inside leaves Sewage (as a growth medium) Billabongs and lagoons: rich microalgae habitats, particularly for desmids


adaptation factor leading to a moist condition being desired for its growth. Algae can be grown at almost everywhere and as shown in Table 2, these are possible habitats for algae growth.

Typical algae strains used for effluent treatment and CO2 sequestration Algae species has also proven to have the capabilities of treating wastewater and mitigating CO2 from industrial flue gas. Several researches are done to study the ability and performance of microalgae in treating sewage, agriculture wastewater treatment, and industrial effluent. It is found that an excess amount of nutrient in the waste stream has the potential to be removed by microalgae as it will consume the nutrient available in the waste stream for its growth requirement. Apart from that, it is possible for heavy metals and toxic substances to be reduced or removed by installing the proposed microalgae-based wastewater treatment system (AbdelRaouf et al. 2012b). In order to demonstrate the performance of microalgae in treating wastewater, some criteria can be assessed such as the microalgae growth rate, percentage of nutrient reduction, and lipid content in the microalgae biomass (Dalrymple et al. 2013). Particularly, through bioremediation of microalgae, nitrate, sulphate, and phosphate can be reduced up to 99, 84, and 73% of reduction percentage (Nandeshwar and Satpute 2014). In addition, different microalgae species possess difference in their nutrient removal performance. For example Scenedesmus obliquus removes up to 100% of ammonium and 97% phosphorus in wastewater (Martınez et al. 2000). In addition, microalgae is also potentially capable of treating palm oil mill effluent (POME) that is also a nutrientrich industrial effluent where up to 93% of removal efficiency is achieved through microalgae-based POME treatment process (Zainal et al. 2012). The performance of Scenedesmus dimorphus shows 99.5, 91.5, 98.8, 97.2, 86, and 86.5% of ammoniacal nitrogen, ammonium, phosphorus, phosphate ion, COD, and BOD in POME (Kamarudin et al. 2013). Meanwhile, Chlorella vulgaris removes 61, 53.8, 84, 66.2, 50.5, and 61.6% of ammoniacal nitrogen, ammonium, phosphorus, phosphate ions, COD, and BOD in POME. The performance of C. vulgaris is slightly lower than Scenedesmus sp. in terms of nutrient removal (Kamarudin et al. 2013) and this can be explained from the presence of chlorophyll a that is higher in Chlorella, causing light limitation and reducing the efficiency of microalgae (Aslan and Kapdan 2006). Meanwhile, Spirulina plantensis is another microalgae species that has a high efficiency in removing nutrient where it is reported that 90% of COD, 87% of ammoniacal nitrogen,

and 80% of total phosphorus being removed in anaerobic POME (Zainal et al. 2012). In the case of CO 2 capture and sequestration, ideal microalgae strains are required for CO2 fixing strategy to achieve most CO2 capture and sequestration aims. A highCO2-tolerance microalgae strain is one of the main criteria during the selection of the strain to be used for the microalgae-based treatment system. A typical industrial flue gas emits about 10–20% CO2 concentration which is considered high and might disrupt microalgae growth. However, there are several microalgae species that are able to withstand this range of CO2 concentration channel to the system. Chlorella sp. Chlorella is one of the species that is able to switch its metabolic pathway between heterotrophic and autotrophic means. While nutrients are being utilized, microalgae undergoes photosynthesis process where oxygen is released as the respiration product. The oxygen released by microalgae will be consumed by bacteria to support the decomposition of organic matters in the wastewater. The sufficient supply of oxygen for the bacteria influences the reduction in BOD and COD of the wastewater. Rapid phosphorus consumption by microalgae will trigger pH increment that causes the formation of calcium phosphate precipitation. This is one of the factors that enhances the removal of calcium in the substrate (Wang et al. 2010). Chlorella sp. is a flexible microalgae species that has a great adaption ability in most of wastewater conditions (Wang et al. 2010). Most of Chlorella sp. have been reported to have a good performance in nutrient removal from wastewater. Other findings have proven that C. vulgaris is able to remove up to 99.7, 89.5, 92, and 75.5% of ammonium nitrogen, total nitrogen, total phosphorus, and COD within 5 days of cultivation period (Wang et al. 2010). These findings are supported by other researches where Chlorella is capable of removing more than 50% of COD (Lim et al. 2010). Chlorella protothecoides is also being used in treating wastewater where the reported performances of nutrient removal were 81.5, 59.7, 88.9, and 96.18% of phosphorus, total nitrogen, COD, and organic carbon (Zhou et al. 2012a). Effective performance of nutrient removal is not the only capability of Chlorella sp., it is also good at producing biomass which is beneficial for high-value products as well as renewable energy source production. Moreover, high content of lipid in Chlorella sp. is obtained by one of the studies where 179 mg/L per day was achieved with the cultivation condition of 2% CO2 and 0.26vvm aeration rate (Chiu et al. 2009). In addition, Chlorella sp. is one of the microalgae species that is able to tolerate high concentration of CO2. Besides that, high tolerance to heavy metal elements in the system is also required and crucial for microalgae especially for treating


wastewater. Wastewater contains heavy metal elements and other miscellaneous matters that might stunt microalgae growth. A microalgae strain with high tolerance to such matters is desired to ensure the effectiveness of microalgae-based wastewater treatment. Chlamydomonas sp. Chlamydomonas sp. is a unicellular green microalgae with the difference in morphological characteristic of flagella observed. This microalgae strain dimensions is about 15–20mm length and 8–12-mm width which is in an oval shape (Ding et al. 2016). Other than their differences in morphological aspect, Chlamydomonas sp. is another microalgae species that is able to switch its metabolism mode between autotrophic and heterotrophic (Fernandez and Galvan 2007; Bell et al. 2013). As reported by previous studies, hydrogen is another gas that is being produced by Chlamydomonas reinhardtii (green algae) throughout its respiration process. This proved that this species of microalgae has the ability to switch from oxygen to hydrogen production throughout respiration. The maximum biomass of C. reinhardtii was reported at 2.0 g/L day in municipal wastewater within 10 days of cultivation period. In addition, the nutrient removal rate achieved from the cultivation was 55.8 mg/L day and 17.4 mg/L day for nitrogen and phosphorus contained in wastewater (Kong et al. 2010). Apart from that, C. reinhardtii uses acetate as its organic carbon source for growth (Boyle and Morgan 2009). The presence of acetate as the carbon source in effluent is from the biological reaction of volatile fatty acids (VFA) through acetogenesis. The acetate will be further converted into carbon dioxide and methane gas through methanogenesis process (K.Wang Lawrence et al. 2006). In reduction of COD value, Chlamydomonas incerta which showed 250 mg/L of COD in POME was removed within 28 days (Kamyab et al. 2015). Isolated native strain of Chlamydomonas sp. UKM6 from POME was reported to have high efficiency in removing ammonium nitrogen in POME (Ding et al. 2016). Scenedesmus sp. S. dimorphus is another microalgae species studied with high nutrient removal efficiency. It was reported that 86% COD, 86.5% BOD, 98.8% phosphorus (P), 99.5% ammoniacal nitrogen (NH3-N), and 91.5% ammonium (NH+4) are able to achieved in anaerobic POME treatment (Kamarudin et al. 2013). The other research found out that S. obliquus able to remove up to 100% of ammonium and 97% phosphorus in wastewater (Martınez et al. 2000). These findings proved that Scenedesmus species can be used for wastewater treatment with the expected outcome of high nutrient removal efficiency to be achieved.

Microalgae growth and performance Medium design and composition influence the growth and performance of microalgae. Insufficient nutrients provided during microalgae cultivation will slow down the growth rate and productivity of microalgae. There are growth and productivity requirements that need to be fulfilled in microalgae cultivation such as optimal supply of macronutrients (carbon, nitrogen, phosphorus), micronutrients (Ca, Mn, Fe, Zn, Cu), growth factors (vitamins, amino acids), functional nutrient for growth and product formation (non-growth associated and growth associated), physical factors (temperature, light, water activity, gas exchange), as well as the addition of additives (protectant, chelating and neutralizing agent). Macronutrient is the major fraction of nutrients required for microalgae growth whereas the micronutrient and other nutrients need to be supplied in small amount. Macronutrient requirement Carbon source is one of the main nutrients required by microalgae. Carbon source in wastewater consists of organic and inorganic form of carbon element. Organic carbons in wastewater present themselves as carbohydrates, proteins, fats, amino acids, and volatile acids whereas the inorganic forms of carbon includes calcium, magnesium, sulphur, potassium, phosphate, bicarbonate, ammonium chlorine, sodium, and heavy metals (Lim et al. 2010). In some cases, microalgae have the ability to switch their metabolism mode based on their current culture condition. As an example, heterotrophic microalgae can switch to autotrophic mode of metabolism when the source of carbon is obtained from carbon dioxide (Su et al. 2012). Sufficient supply of carbon source for microalgae will enhance the growth rate and productivity. However, additional amount of carbon added to the cultivation system is not cost-effective especially in the case of wastewater treatment. Thus, nutrient loading parameter needs to be optimized in order to gain optimal microalgae growth and performance. Furthermore, ammoniacal nitrogen (NH3-N), nitrate nitrogen (NO3-N), nitrite nitrogen (NO2-N), and ammonium (NH4) are nitrogen sources. Nitrogen sources are divided into organic and inorganic nitrogen. The concentration of nitrogen in wastewater or industrial effluent shows a huge defence depending on the activity of that specific industry. For example, the concentration of inorganic nitrogen in POME is approximately five times higher compared to bold basal growth media. The concentration of nitrogen in the medium will influence the growth and biomass yield by microalgae. As reported by previous research, 1 g of ammoniacal nitrogen or nitrate nitrogen available for microalgae consumption results in 15.8 g of biomass produced and carbon source consumed in the process is about 18–24 g in the form of carbon dioxide


(Dalrymple et al. 2013). Moderate concentration of nitrogen (30 mg/L) suggested from the finding of Woertz et al. for effective growth rate and lipid production of microalgae (Woertz et al. 2009). In addition, phosphorus requirement is another form of macronutrient required by microalgae growth. Inhibition of growth will occur if the amount of phosphorus does not meet the required level (Zhu et al. 2013). In the case of CO2 fixation by microalgae, phosphorus limitation leads to low CO2 removal rate. In microalgae-based cultivation wastewater treatment, it is important to evaluate the initial N/P ratio. Different wastewater source indicates differences in their ratio of nitrogen to phosphorus. Higher ratio of N/P in the concentrated effluent was observed in comparison to effluent centrate. N/ P ratio of the effluent was 53.2 which shows that a ratio which is higher than the optimal suggested ratio indicates phosphorus limitation while N/P ratio of centrate was 0.36 which illustrates that ratio below optimal level and nitrogen limited is in centrate (Wang et al. 2010). Theoretically, five times the concentration of nitrogen and 2.5 times phosphorus in POME compared to bold basal growth media (BBM) were used to culture microalgae. Limiting factors A strong relationship is shown between microalgae growth and the availability of nutrients for microalgae consumption. The chemical composition of microalgae biomass, C 106 H 263 O 110 N 16 , represents the fraction of elements contained in the biomass cell and it proves that the elements in the biomass are the keys of microalgae requirement and elements involved. Meanwhile, the concentration of nutrient elements such as nitrogen and phosphorus needs to be evaluated because it will affect the growth of microalgae (Wang et al. 2010). Commonly, nitrogen element will be the limiting factor compared to phosphorus due to the high level of phosphorus available especially in wastewater (Dalrymple et al. 2013). Moreover, the amount of nitrogen source, NH3-N below optimal value will trigger low growth rate of microalgae (Ding et al. 2016). The same effect will occur if there is a phosphorus limitation in the cultivation system as phosphorus is required for growth and energy metabolism (Cai et al. 2013). The nutrient concentration level can be monitored by adjusting the nutrient load into the cultivation system. Nutrients can be diluted to a certain concentration to avoid excessive or highly concentrated nutrient level. A research has proven that highly concentrated POME results in low growth rate and biomass yield of Chlamydomonas sp. UKM 6 which is grown in 100 and 50% factor of POME dilution (Ding et al. 2016). The findings have shown differences when it comes to effluent centrate being used as the cultivation medium. Centrate with high nutrient promotes microalgae

growth (Wang et al. 2010). Thus, microalgae which is grown in high strength of centrate does not face nutrient limitation phenomenon and this can be beneficial for outdoor cultivation system as less area is required for microalgae cultivation (Dalrymple et al. 2013). Apart from the growth rate, the effectiveness of nutrient removal is also related to the level of nutrient content in the effluent. Removal rate of microalgae is highly related to the tolerance of microalgae tissues and the metabolism rate (Wang et al. 2010). Nutrients in substrate are not the only form of nutrients available for microalgae consumption. When the organic substrate is not available, microalgae will switch to autotrophic metabolism mode and use CO2 as their source of carbon (Wang et al. 2010). When there are no nutrient limitation, the rate of photosynthesis can be improved by having optimal irradiance. Increasing the irradiance causes an increment in photosynthetic rate of microalgae until it reaches maximum growth rate as explained in Michaelis-Menten kinetics (Richmond 2004). Meanwhile, light penetration is another factor that is important and causing low growth and productivity when light is limited. In POME microalgae cultivation, light penetration is poor due to the high turbidity of POME which is about 460 NTU and it appears in extremely dark brown in colour which is limiting the light entering the cultivation system. Light penetration factor can be improved by diluting the effluent nutrient for the purpose of reducing the turbidity of the effluent used. This has been done by previous studies where the researcher diluted POME with the factors of 100 and 50% that resulted in turbidity of 460 NTU and 230 NTU (Ding et al. 2016). The limitation of light causes uneven light distribution to microalgae as different amount of light is being exposed to different points in the system. In addition, light intensity at the surface (approximately 20, 000 lx) is higher compared to the area inside of the system (Ding et al. 2016). However, when it comes to low strength or moderate nutrients used for cultivation, the productivity cannot be improved by only considering the light source as nutrient is the crucial need for microalgae growth compared to light (Dalrymple et al. 2013). Cultivation medium for microalgae In order to support microalgae growth, a certain amount of nutrient elements are required. The amount of nutrient required by microalgae has been modified which is known as a synthetic media. There are various types of synthetic media used consisting of different nutrient concentrations for different cultivation purposes. Basal medium, bold basal medium (BBM), BG 11, bold basal with ammonium carbonate (BBMa), and modified Chu 13 medium are among the examples of synthetic medium that have been practically used for microalgae cultivation (Dayananda et al. 2007; Phukan et al. 2011). Different medium composition serves different


purposes for microalgae growth such as in the case of nitrogen source in those synthetic media where BG 11 contains high concentration of sodium nitrate compared to BBM. In modified Chu 13, nitrogen source is present in the form of potassium nitrate while in BBMa ammonium carbonate serves as the nitrogen source for microalgae nutrient requirement. Table 3 shows the composition of the synthetic media available for microalgae cultivation. Growth inhibition caused by nutrient limitation can be avoided when microalgae is being cultivated in the nutrient-rich media such as a synthetic modified media. Study has made a comparison between synthetic media BBM with media recommended by OECD and EPA standard method. As prescribed, BBM contains a higher nitrogen content where most of the nitrogen exists in the form of nitrate and trace amount of ammonium. In comparison to OECD medium, the nitrogen source is

Table 3

totally in the form of ammonium while in EPA medium, the nitrogen source is in the form of nitrate. In terms of nitrogen to phosphorus ratio, BBM shows the best ratio required by microalgae growth compared to EPA and OECD. Moreover, wastewater serves as the nutrient source for microalgae growth that is widely being practised in most wastewater treatment activities. The level of nutrient contained in the wastewater is proven with the capability to support microalgae growth as nitrogen and phosphorus are the main components in the wastewater. Microalgae cannot grow without these two main elements and this can be beneficial for treating wastewater economically while at the same time, an environmental friendly treatment is being practised (Kong et al. 2010). Microalgae’s ability to consume nutrients and absorb metal elements in the wastewater makes them suitable

Compositions of commercial synthetic media

Composition (g/L)

BG 11

BBM

Modified Chu 13

BBMa

Basal

KNO3 NaNO3 K2HPO4 KH2PO4 CaCl2·2H2O MgSO4·7H2O Na2CO3 NaCl FeSO4 EDTA Citric acid Ammonium ferric citrate Ferric citrate Ca(NO3)2·4H2O b-Na2glyserophosphate EDTA-Na2

– 1500 40 – 36 75 20 – – – 6 6 – – – 1

– 250 74 17.5 24 73 – 25 5 45 – – – – – –

200 – 40 – 80 100 – – – – 100 – 10 – – –

– – 74 17.5 24 73 – 25 5 45 – – – – – –

100 – – – – 40 – – – – – – – 150 50 2.71

Vitamin B12 Biotin Thiamine-HCL H3BO3 MnCl2·4H2O ZnSO4·7H2O Na2MoO4·2H2O CuSO4·5H2O Co(NO3)2·6H2O FeCl3·6H2O CoCl2·6H2O Trisaminomethane Ammonium carbonate

– – – 2.86 1.81 0.22 0.39 0.08 0.05 – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

– – – – – – – – – – – – –

0.0001 0.0001 0.01 – 0.108 0.066 0.0075 – – 5.888 0.012 500


for a cost-effective and sustainable treatment method (de-Bashan and Bashan 2010; Mallick 2002). However, there are circumstances where wastewater is used as the microalgae nutrient source. The extremely high concentration of nutrient such as nitrogen and phosphorus can inhibit microalgae growth. The agricultural wastewater is an example of waste stream with high nitrogen and phosphorus concentration (Wilkie and Mulbry 2002) which sometimes might not be suitable for microalgae growth. Moreover, toxic chemicals and heavy metals contained in the waste stream can also cause negative effects for microalgae growth. In some studies, the researchers found out that a high-nutrient concentration in agricultural wastewater gave positive impacts to microalgae growth compared to municipal and industrial wastewater. The low amount of nutrient such as nitrogen and phosphorus in industrial wastewater is unfavourable for microalgae growth. In addition, a typical industrial wastewater contains high amount of toxin which will lower the growth performance of microalgae (Pittman et al. 2011). Meanwhile, the consumption of nutrient in the wastewater is dependent on the microalgae strains. As an example, ammonia is a preferred form of nitrogen source by C. vulgaris and nitrate will be consumed once the ammonia is depleted.

Native phototropic organisms High nutrient level contained in wastewater makes it optimal to support organisms’ growth. Using wastewater as nutrient media for organisms gives added advantages to the wastewater treatment activity where as the organisms consume the nutrients in the stream, the reduction of pollutants can be achieved. Moreover, nutrients in the wastewater support organisms’ growth through the conversion of organic and inorganic forms of nutrient and become a source of energy. Hence, wastewater is an ideal medium for a wide range of microorganisms especially bacteria, viruses, and protozoa (AbdelRaouf et al. 2012a). Meanwhile, the isolation of native microorganisms from the wastewater itself gives benefits to the wastewater treatment process as it increases the adaptability factor of microorganisms to the environment. Thus, high adaptability of microorganism to the culture environment will shorten the adaptation period and increase the effectiveness of system operation. However, in the case of treating POME, native microalgae strains are used to treat the POME through nutrient consumption. This strategy is highly recommended as it is able to reduce pollutants in the waste stream by using natural way of remediation such as native microalgae-based effluent treatment. Native microalgae strains isolated from POME is used for the purpose of avoiding the impact of high concentration of nutrient content and other particulate matters towards microalgae growth. The native microalgae strains will

have high tolerance towards the environment as it originates from the same environmental condition. Activity of heterotrophic bacterial community The decomposition of biodegradable organic matters in wastewater can be related to the activity of heterotrophic bacterial. Bacteria require oxygen to support their respiration and nutrients in the form of organic matters for growth. The efficiency of bacteria in treating wastewater might be lower in comparison to microalgae consumption of nutrient in the wastewater. This is proven by one of the researches where the performance of anaerobic bacteria to reduce nutrient is found to be lower than photosynthetic microorganism such as microalgae due to the lack of bacterial autotrophic metabolism for inorganic nitrogen utilization (Noike et al. 2004). Hence, to overcome the shortcomings of bacterial efficiency in treating wastewater, the combination of microalgaebased treatment system with bacteria is suggested as it gives feasible operation system through their symbiotic relationship. Microalgae undergoes photosynthesis process that releases oxygen as the gas product for the consumption of bacterial to support their respiration requirement. Bacterial consumes oxygen while decomposing organic matters in the stream and carbon dioxide is released as the gas product of it. The carbon dioxide released by bacterial will be consumed by microalgae during photosynthesis process (Su et al. 2012). In addition, chemical oxygen demand (COD) and biochemical oxygen demand (BOD) are reduced due to the oxygen being released by microalgae in the treatment system. Furthermore, anaerobic and aerobic bacterial utilization of oxygen while decomposing the organic matters influences the reduction of COD and BOD of the stream (Kamarudin et al. 2015). The depletion of oxygen in the treatment system will cause the increment of COD, causing limitation to bacterial organic matters decomposition (Jiang et al. 2013).

Microalgae technology Microalgae cultivation treatment becomes a well-known method in treating effluent due to the extraordinary microalgae characteristics that able to reduce pollutant in wastewater as it consumes the nutrient for growth. In addition to that, microalgae is a living thing that undergoes photosynthesis process where carbon dioxide is consumed for the respiration process. This multiple advantages made microalgae cultivation system as the best method to treat pollutants in industrial effluent and at the same time fixing the CO2 from emitted flue gas. Moreover, microalgae cultivation treatment system is considered as a feasible and effective method due to microalgae reducing pollutants contained in effluent and flue gas emitted


while also being beneficial in producing biofuel as an alternative source of conventional fuel. The high growing rate with the ability to synthesize neutral lipids and oils from microalgae biomass proved the sustainability of the system (S. a. Khan et al. 2009). As stated by Chisti (2007), microalgae contributes to several different types of biofuels such as biodiesel through microalgae lipid extraction, bioethanol from fermentation of its biomass, biomethane from anaerobic digestion of its biomass, and biohydrogen being produced photobiologically. Nutrients removal Nutrients in wastewater consist of organic and inorganic compound of carbon, nitrogen, phosphorus, and other forms of elements present in waste effluent. Inorganic nitrogens such as ammonium nitrogen (NH4-N), nitrite nitrogen (NO2-N), and nitrate nitrogen (NO3-N) are the form of nitrogen sources available for microalgae consumption. In industrial effluent treatment, such anaerobic POME contains high organic matters and ammonium nitrogen present which has not been decomposed by microorganisms in the primary and secondary stages of treatment. Therefore, the remaining organic matters and ammonium nitrogen in the effluent will be treated by microalgae-based treatment stage as the tertiary treatment. Approximately, 69.4% of ammonium nitrogen, NH4-N, is present compared to other nitrogen sources. Meanwhile, ammonium nitrogen is the most preferred form of nitrogen by microalgae due to the less energy required for its assimilation (Cai et al. 2013). It was reported that Chlamydomonas sp. UKM6 is capable of reducing ammonium nitrogen at high efficiency (Ding et al. 2016). Moreover, the increment in nutrient removal of the effluent will cause a reduction in COD and BOD. COD removal efficiency is more significant in highly concentrated wastewater such as COD concentration in POME. COD reduction can also be influenced by the stripping factor of volatile organic compound (VOC) from the effluent through gas bubbles (Ji et al. 2015; L. Zhang and Jahng 2010).

Bioreactor design for microalgae cultivation In designing bioreactor system, the dimension is the most important factor that can provide optimal environment, minimum operation cost, and maximum production of the final product. The design concept involves the considerations of optimum cell concentration, rate of product produced, gas exchange involved in the operation, and rate of heat evaluation. The consideration of gas exchange and heat removal was able to improve the productivity and performance of the bioreactor system. For example in fed batch operation, growth inhibition by high concentration of nutrients can be avoided.

In addition, sufficient respiration gases such as oxygen and carbon dioxide are present for utilization by the microorganisms as the bioreactor is being fed from time to time. In the case of microalgae cultivation operation, sufficient light supply, CO2 addition, and nutrient concentration are required for optimal microalgae growth (Brennan and Owende 2010). An effective microalgae-based cultivation system is desired and this can be achieved by having an effective sizing and design of the system which satisfies the requirements for microalgae growth. Microalgae-based treatment system design needs to be able to achieve the target for the treatment process by considering the influent conditions as well as the targeted final effluent coming out from the treatment system. Microalgae-based treatment system can be installed as secondary treatment process when high inoculums are introduced to the system. Moreover, apart from inoculums concentration, light supply is also an important factor to be considered for microalgae cultivation system design. Light supply and its effects to the photosynthetic cultivation influence the microalgae conversion of light as a required energy source. Solar energy is one of the light sources that can be utilized for microalgae-based cultivation system which has the potential to reach maximum microalgae photosynthetic production through nutrient conversion into microalgae biomass (Levine et al. 2011). Besides that, the optimization of light supply is desired for the system design and it is depending on the microalgae metabolism mode. For example, Chlamydomonas sp. UKM 6 is able to survive in low-light penetration as it adopts mixotrophic metabolism mode which requires low light intensity and less energy for growth (Garcıa et al. 2000; Abreu et al. 2012). In some cases, light intensity beyond the optimum level causes photoinhibition. Photoinhibition phenomena occur when irradiance above saturation point causes damage to microalgae light receptors and directly lowers down the photosynthesis rate and microalgae productivity (Richmond 2004). Furthermore, the dimension of the system influences the penetration and distribution of light, nutrient in the suspension, and gas exchange in the cultivation system. The use of narrower diameter column appears to have better hydrodynamic performance and improves microalgae productivity where the accumulation or unstirred suspension nutrient can be avoided and promotes well distribution of light and gas exchange.

Commercial production of microalgae Microalgae is not only capable of treating effluent but also possesses the added value of producing fuel from


high lipid content. In order to meet the requirement of commercial production, a large-scale and cost-effective microalgae-based system technology is desired (Table 4). Phototrophic microalgae cultivation is the best method for a large-scale production. Through phototrophic cultivation system, sunlight as a light source is utilized for energy by the microalgae. Sunlight is considered as a feasible strategy as natural light energy is used rather than an artificial light which will reduce energy and cost allocated for the power source (M. a. Borowitzka 1999). The phototropic cultivation method is divided into two types of cultivation system which are known as open pond system and closed bioreactor system (Chisti 2007; Greenwell et al. 2010).

build a raceway pond. Another suitable material used for a raceway pond is compacted earth-lined pond with white plastic (Brennan and Owende 2010; Chisti 2007). Apart from having a low-cost open pond construction, there are limitations faced when having this system for microalgae cultivation. The limitations include excessive loss of water due to evaporation, high risk of contamination, and the system is highly dependent on surrounding environmental conditions which can hardly be controlled by the operator (Schenk et al. 2008). In addition, lack of temperature control causes excessive water loss and high exposure towards sunlight especially for hot and dry climate areas.

Open system (raceway pond/lake)

A closed bioreactor system is the alternative system that is able to overcome the issues faced in the previous open pond system. However, this system requires a high cost for installation and operation maintenance. By having a closed bioreactor system, the contamination risks can be reduced and a higher yield of biomass respect to reactor volume can be obtained. In addition to that, single-species of microalgae being cultivated in the system prolongs the duration of microalgae in the system (Chisti 2007; Greenwell et al. 2010). Furthermore, there are several closed bioreactor designs which include tubular, plate, and column in shape (Brennan and Owende 2010). A tubular photobioreactor requires a proper temperature control system which increases the operation cost in order to overcome the problem of temperature tolerance of microalgae. A tubular photobioreactor has a large surface area that exposes the cultivated microalgae to the light source. The accumulation of oxygen that leads to inhibition of microalgae in tubular system is another challenge that might influence the productivity of microalgae (Molina et al. 2001). Flat photobioreactor is another type of a closed system that has a large surface area configuration to optimize the illumination of light and reduce light path for microalgae growth requirements (Degen et al. 2001). In the column photobioreactor, air is supplied through injection for the purpose of aeration and agitation. The optimum design and dimension of column bioreactor is important for the productivity of cultivated microalgae. A small radius with less height is desired to meet the requirement of achieving optimum growth and residence time of gas produced such as the oxygen gas released from photosynthesis process of microalgae that can cause inhibition to microalgae growth (Wang et al. 2010). In addition, a closed photobioreactor system is highly recommended for a high density of microalgae cultivation which applies for large-scale system. This approach can enhance the efficiency of microalgae harvesting operation (Pedroni et al. 2001). Physiological and biological parameters can be monitored by a closed bioreactor system which is important for microalgae cultivation system that requires optimum

An open pond system is commonly used for a large scale of microalgae cultivation due to the low cost required for the installation and this system is easy to operate (Mata et al. 2010). The open pond microalgae cultivation system requires less supervision and control due to the simplicity concept of its system operation. There are many types of open pond system including a circular pond and a raceway pond which is widely known for a large-scale microalgae cultivation system (Kamarudin et al. 2015). Circular pond is a round-shaped configuration that is equipped with a rotating impeller for the purpose of agitation. Circular pond system is widely used in Asian region such as Taiwan and Japan (Lee 2001). However, this system is not feasible for a large-scale operation due to the high cost for construction and the great energy required for mixing. Meanwhile, a raceway pond is one of the open pond system design being developed with the advantage of promoting the application of nutrient homogenizer. It is able to counteract the problem of microalgae biomass sediment formed at the bottom of the cultivation system unit. Besides that, it is also the most preferred cultivation system compared to other designs of open pond system due to the factor of operability. The raceway pond is easy to operate and maintain if a scale-up system is desired for microalgae cultivation. Typically, a raceway pond is in an oval shape equipped with mechanical mixing such as paddle wheel to provide adequate mixing and turbulence for microalgae suspension (Chisti 2007; Greenwell et al. 2010). In the aspect of raceway pond sizing, the depth should be at the optimum depth to avoid internal shading effect that might reduce light exposure which is important for microalgae growth. The optimum depth for a raceway pond is usually in the range of 0.2 to 0.5 m (Brennan and Owende 2010). In having an effective design of raceway pond, the factor of material used for the construction of the system is crucial as it will influence the strength and effectiveness of the system. Concrete is one of the materials used to

Closed system (photobioreactor)


conditions for their growth and productivity. The performance and growth rate of microalgae also depend on the strains cultivated (Mata et al., 2009). Growth parameters such as light intensity, temperature, pH, nutrients loading, and CO2 concentration can be controlled easily by having a closed photobioreactor system. However, due to the advance control provided by this system, an added cost is applied for the maintenance of the closed bioreactor system compared to an open pond system (Carvalho et al. 2006). The cost is ten times higher for a closed system compared to an open pond cultivation system (S. a. Khan et al. 2009).

Valuable resource of microalgae biomass Microalgae growth is driven by the consumption of nutrients available in the stream. The consumption of 1-g ammonia nitrogen or nitrate nitrogen generates approximately 15.8 g of final microalgae biomass with 18–24 g of CO2 being assimilated during the process (Dalrymple et al. 2013). Microalgae-based wastewater treatment system is proven to be a beneficial and sustainable strategy to overcome environmental issues. Microalgae not only treats the wastewater but it also generates biomass which can be benefited by the industries. The productivity of biomass and its compositions is highly dependent on cultivation factors such as abiotic, biotic, and operational conditions. However, light, temperature, pH value, salinity, nutrient concentration, respiration gas available, and toxic compounds are considered as abiotic factors that might influence microalgae productivity and biomass compositions. Whereas, biotic factors can usually be related to the cell density of microalgae and the presence of other organisms in the system that can cause competition for microalgae to survive. The favourable operational conditions include optimal agitation rate, residence time, nutrient loading, and system design, and the addition of chemical Table 4

compounds such as bicarbonate can improve the performance of microalgae cultivation and quality of microalgae biomass (Converti et al. 2009; Mata et al., 2009; Rodolfi et al. 2009; Zhou et al. 2012b). Meanwhile, downstream processes include the production of biodiesel, bioethanol, biomethane, and biohydrogen as the source of fuel and energy to counteract energy shortage problem faced (Miao and Wu 2006; Song et al. 2008; Sathish and Sims 2012). On the other hand, non-fuel productions from microalgae biomass is another form of contributions to the industries as many high-value products are able to be produced to satisfy the needs of pharmaceutical and food industries (Chisti 2007; Wang and Lan 2011). Moreover, microalgae biomass can be used as a source of biofertilizer for crops and animal feeds due to high protein content (Ahmad et al. 2011). In the field of biofuel production, microalgae is not having any conflicts faced by sources of biofuel production since there is no competition between food sources. Moreover, the emission of CO2 gas from fossil fuel combustion can be reduced by biofuel as an alternative fuel source (Raupach et al. 2007). The production of biofuels is feasible and suitable for sustainable energy development in the near future. Apart from lipid function in biofuel production, carbohydrates and protein compositions in microalgae can be used as the carbon source or substrate in fermentation process (Harun et al. 2010). The level of protein and carbohydrate contents in microalgae depends on the species as shown in Table 1. Fuel and energy application Harvested biomass will be further processed to extract lipid in the production of fuel or to produce biogas through anaerobic digestion technique (Brune et al. 2009). The presence of amonoalkyl-ester in the form of extracted long fatty acids is converted into biofuel known as biodiesel (Maity et al. 2014) which is biodegradable, non-toxic, and better lubricity with

Comparison of open system and closed system for microalgae cultivation

Cultivation system

Advantages

Disadvantages

Open system

Low installation and maintenance cost High utilization of sunlight due to large surface area

Closed system

Controlled condition/easy to maintain Reduce contamination

Require large area High risk of contamination Temperature fluctuation Excess evaporation High installation cost Light source problem and need to supply light by using device High energy consumption

Reduce evaporation High productivity Easy harvesting


low SOx and CO emission (Van Gerpen 2005; Jacobson et al. 2008). Besides that, the microalgae role in biodiesel production has been proposed to be the potential feedstock due to its ability to grow at a fast rate along with its high lipid content and less space required for cultivation in comparison to the terrestrial crops (Chisti 2007). Microalgae cultivation is flexible as it is able to be cultivated in most areas. Previous findings show that microalgae can be cultivated even in areas with extreme environment conditions (Mata et al. 2010). For example, research done has proved that microalgae can be cultivated in the wastewater (Zhang et al. 2008). The most important criteria of microalgae species used for biodiesel production are the ones with high lipid content. Lipid content of phototrophic and heterotrophic microalgae was reported to have about 1–75% from the total dry biomass weight (Chisti 2007; Mata et al. 2010; Chen et al., 2011). The formation of lipid in microalgae biomass can be induced by manipulating the nitrogen concentration from the nutrient provided. Increase in lipid fraction when the microalgae starved for nitrogen (Chisti 2007; Illman et al. 2000; Richmond 2004). In producing biodiesel, 20% of lipid is desired (Rawat et al. 2011b) and some researchers suggested that 40% of lipid content in microalgae biomass is required for biodiesel production (Mata et al. 2010). This limits the use of wastewater as microalgae medium for biodiesel production due to the high concentration of nutrients especially the great amount of ammonia-nitrogen in the wastewater. Besides that, the moderate to low concentration of nutrient strength is suitable for biodiesel production as lipid fractions in the biomass are at a satisfactory amount which is 20–50% of lipid in the dry microalgae biomass. A higher medium strength triggers lower lipid production which is less than 10% (Dalrymple et al. 2013). In the case of low lipid content of microalgae biomass, the alternative energy production is called biogas. Biomethane and biohydrogen are biogases that are produced anaerobically. Proper fractions of carbon to nitrogen element influence the production of biomethane. About 20 to 25:1 carbon to nitrogen ratio increases the methane production rate by twice the normal rate with the hydraulic retention time (HRT) of 10 days. Previous studies demonstrate that 1 g of microalgae dry biomass is able to generate up to 62.7 mg of biomethane (Dalrymple et al. 2013). Besides that, biohydrogen production as other biogas produced from microalgae cultivation system has the potential to be converted into an electric source. Fuel cell technology is used to store hydrogen which is later used as a power source. Hydrogen can be produced biologically or through electrochemical processes. The biological production of hydrogen known as biohydrogen is produced through photosynthesis or respiration of microorganisms. The production is divided into two stages where in the first stage, hydrogenrich endogenous substrate is produced from the photosynthesis process. In this stage, CO 2 is assimilated through

microalgae respiration with the exchange of oxygen gas. In the second stage, anaerobic environment triggers the conversion of a hydrogen ion into hydrogen gas by the reaction of hydrogenase enzyme in the nucleus cell. In addition, sulphur deprivation is the manipulation factor that causes an anaerobic condition for hydrogen production (Zhang et al. 2002; Melis 2007; Skjånes et al. 2008). Apart from that, bioethanol is another product being produced by microalgae as the replacement for gasoline. It is proven that bioethanol as a biofuel source is environmental friendly due to the low CO2, CO, lead, sulphur, and particulate matter emitted through bioethanol combustion (Costa and de Morais 2011). Furthermore, bioethanol has the ability to broaden the flammability limits with higher flame speed and giving high vapour heat that makes it an efficient alternative biofuel (Balat 2009). Non-fuel and high-value product application The conversion of microalgae biomass into commercialization purposes is not only for the biofuel and biogas fields, non-fuel based products can also be produced for the benefits of the society. The composition of high-value compounds in microalgae biomass benefits the chemical, pharmaceutical, and food industries (Acién et al. 2014). Microalgae biomass contains astaxanthin, carotenoid, chlorophyll, phycobiliproteins, and antioxidant compounds that can be used to manufacture healthcare products or supplements (Lorenz and Cysewski 2000; Del Campo et al. 2000; Orosa et al. 2000; Ip and Chen 2005; Spolaore et al. 2006). Meanwhile, phycobiliproteins is another type of pigments used as natural dyes which can be extracted from cyanobacteria (blue-green algae) Arthropira and Rhodophyte porphyridium. Furthermore, biofertilizer is another potential product that can be manufactured from microalgae biomass. It is produced from microalgae biomass and is environmental friendly and helps to overcome the issues caused by the usage of synthetic fertilizers. The use of synthetically manufactured fertilizer for agricultural purposes leads to an increment in pollutions especially water pollution which is caused by soil runoffs. There are various advantages of using biofertilizers such as improved quality of soils due to high amount of carbon elements stored in soils, better nutrient transfer from fertilizer to crops, increase in the stabilization of soil aggregates (Hu et al. 2008), improved function of soils to retain water for crops, and enhances the microorganisms in agro-activities. In addition, the need to use fossil fuels to produce synthetic fertilizer can be reduced or eliminated as it contributes to the emission of GHGs. Microalgae biomass as animal feed is another effort to utilize the microalgae waste cake which is rich in nutrients. The high content of protein in microalgae biomass can be benefited by aquaculture animals as their food source.


Palm oil mill effluent (POME) Malaysia is the second largest producer of palm oil after Indonesia with the average crude palm oil production being more than 13 million t per year. About 80% of world’s palm oil production is owned by Malaysia and Indonesia where 49.5% of the production is coming from Malaysia (Alias and Tang 2005). Moreover, the increasing number of palm oil mills is rapid throughout the years. Hence, the production of palm oil in Malaysia has contributed to a huge amount of palm oil-based waste. Further expansion of the industry is predicted to cause negative impacts to the country caused by the activities related to the expansion of palm oil production. Deforestation, GHG emission, pollution of river streams, and destruction of wildlife habitats are among the negative impacts caused by the activities ran by the industry (Laurance et al. 2010; Yule 2010). Thus, a sustainable development is desired to remediate and overcome the problems related to the industry for a better future as mentioned by Malaysian Palm Oil Board (MPOB) and Malaysian Palm Oil Council (MPOC). In addition, there are two types of waste generated from palm oil industry operation which can cause water, air, and ground pollution if a proper management of waste is not practised. Solid and liquid wastes are generated from the activity of extracting and producing crude palm oil (CPO). About 2.5–3.75 t of POME is discharged for every ton of CPO produced (Latif Ahmad et al. 2003). In the production of CPO, 5– 7.5 t of water is required causing 50% of wastewater generation in the form of POME (Schenk et al. 2008). Besides that, the palm oil industry is facing a major problem with the POME discharges due to the high palm oil-based nutrient content in the stream that is considered as pollutants. The discharge limits is set up by the authorities in the form of a standard regulation of effluent discharge which is revised from time to time. Composition of palm oil mill effluent (POME) Wastewater can be categorized into several types of effluent discharges such as domestic, industrial, agricultural, and surface runoff. Agricultural activities are the largest consumer of water with 87% usage percentage followed by the industry and domestic about 7 and 8% of water consumption (Samhan and PWA 2008) where the compositions of wastewater demonstrates the lifestyles and technologies practised by the community. Most pollutants contained in wastewater are coming from the industrial effluents and agricultural activities due to their involvement in chemicals such as manufactured by-products or fertilizers. Agricultural activities contribute to ground water pollution from the abundant amount of organic and inorganic substances leading to

eutrophication issues (Abdel-Raouf et al. 2012b). Palm oil mill operation is an example of an industrial activity that generates high concentration of pollutant effluents known as POME that causes major water pollutant (Wu et al. 2010; Mohammed and Chong 2014). Furthermore, POME is one of the wastes coming out from the palm oil production industry in the form of liquid suspension. It is claimed that POME is the most significant form of waste that contributes to the environmental pollution (Poh and Chong 2009). Due to the high value of BOD and COD, POME is categorized as a highly polluted effluent where an extensive treatment is needed before it is being discharged and certified safe, complying with the standard regulation of discharge effluents. In a raw POME, hemicelluloses and lignocelluloses materials are present in the form of carbohydrate polymers that causes high COD (15,000– 100,000 mg/L) value of POME (Chong et al. 2009). Raw POME consists of colloidal suspension with 95– 96% of moisture, 0.6–0.7% of oils, and 4–5% of total solids. POME is an unpleasant, non-toxic suspension waste generated from wet palm oil milling process where a large amount of water and steam are being utilized for the sterilization while washing steps result in a high amount of wastewater discharged in the form of POME (Table 5). Besides that, POME is specifically a discharge coming from the unit operations which include sterilization process through sterilizer condensate, sludge separation process through sludge separator unit, and hydrocyclone as supernatant waste result from the separation process. As shown in Table 6, there are standard regulations for POME discharged by the palm oil mills that need to be followed in order to preserve the environment. However, most of the palm oil mills are unable to meet the industrial wastewater regulation standards, causing serious water pollutions in most rivers in Malaysia (Ahmad and Chan 2009). Thus, effective method with a feasible characteristics of the proposed treatment system is desired by the palm oil industry to treat the wastes coming out from the palm oil mills.

Table 5 Characteristics of POME from the production of CPO (Wu et al. 2010; Mohammed and Chong 2014) Parameters

Value

COD Biological oxygen demand (BOD) Total nitrogen (TN) Total phosphorus (TP) Oil and grease (OG) Total solid (TS)

45,500–65,000 mg/L 21,500–28,500 mg/L 500–800 mg/L 94–131 mg/L 1077–7582 mg/L 33,790–37,230 mg/L

Environ Sci Pollut Res Table 6 POME characteristics and discharge standards (Malaysian Palm Oil Board 2014) Parametersa

POME (range)

Discharge standard (1984 and thereafter)

Temperature (°C) pH Oil and grease BOD COD Total solid Suspended solid Volatile solids Total nitrogen

80–90 3.4–5.2 130–18,000 10,250–43,750 15,000–100,000 11,500–79,000 5000–54,000 9000–72,000 180–1400

45 5.0–9.0 50 100 – – 400 – –

Ammoniacal nitrogen

4–80

100

a

All parameters are in mg/L except temperature and pH

Anaerobic digestion system of POME treatment Biological treatment in treating wastewater is one of the ways to remove dissolved particulate matters and biodegradable matters by the action of microorganisms such as bacteria (Sankaran et al. 2010). The most common biological process of treating POME in Malaysia is ponding (lagoon) system where 85% of palm oil mills are using this system due to its low installation cost (Tong and Jaafar 2004). Moreover, the composition of POME with high content of organic matters makes it relevant for the bioconversion of POME into useful energy sources such as biomethane and biohydrogen. Meanwhile, bioenergy conversion from POME is one of the promising potential ideas for a renewable energy production. These bioenergies can be produced by microorganisms’ reaction in a biological treatment process. Furthermore, a biological treatment is divided into a few stages with their own specific purposes in treating the POME. Anaerobic digestion is in the first stage of the treatment where a high amount of organic matters is utilized for the production of biogas specifically in producing biomethane. This stage is then followed by the facultative pond treatment and will be further treated in the final stage which is known as aerobic pond treatment (Yacob et al. 2006). It is crucial that both the facultative and aerobic pond systems are placed right after the anaerobic pond in order to reduce the remaining organic matters in the stream before it is released into rivers as a discharge (Poh and Chong 2009). Besides that, the pond treatment, which is also known as the lagoon treatment process requires an efficient sizing and design in order to function in the best manner. Meanwhile, another factor that needs to be considered for a lagoon treatment system is the HRT which is the optimum duration required by the microorganisms to

treat effluent in achieving the most pollutant reduction. As studied by previous researchers, the optimum range of depth and HRT are determined for each systems where anaerobic ponds’ optimum range of depth is 5 to 7 m with HRT of 34–45 days while for the facultative pond is 1–1.5-m depth with HRT of 15–20 days (Tong and Jaafar 2004; Yacob et al. 2006). However, it is different for an aerobic pond where it needs shallower pond for the process to be effective with approximately 0.5–1 m with HRT of 24 days (Yacob et al. 2006). There are shortcomings of using a lagoon system for POME treatment such as the formation of particulate scums at the surface and solid accumulation at the bottom of the pond. These shortcomings will directly reduce the efficiency of the POME treatment. However, this issue can be solved by having frequent desludging operations where the scum and solid built-up will be removed out of the system. It is important for the POME treatment system design to be constructed in stages to ensure that the quality of the final effluent complies with the standard regulation of effluent discharge from the mill. This treatment method is being widely practised in Malaysia with an open digestion system or an open pond system. However, the open digestion system contributes to the emission of biogases such as methane gas to the atmosphere as the system is not fully equipped with biogas capturing facilities. Previously, the low demand of renewable energy and the lack of facilities are the reasons behind the capturing of biogas not being practised by the industry. As a result, biomethane emission to the atmosphere has increased the country’s environmental pollution impacts as biomethane is one of the GHGs that has 21 times severe effects in comparison to carbon dioxide, CO2, emissions. Furthermore, the nonrecovery of biomethane directly causes global warming and climate change issues which makes the biological treatment of POME to be a non-environmental friendly method (Subramaniam et al. 2008). In the ratio 65:35 of methane, CH4, and carbon dioxide, CO2, approximately 28 m3 of gases is emitted from 1 t of POME in an anaerobic POME treatment (Yacob et al. 2006). In comparison to other industrial discharges, POME discharge shows the most contribution to the increase in biomethane emission rate and it is crucial for the palm oil industry to treat and utilize the POME in a sustainable way for the production of renewable energies with high beneficial return for the industry. In addition, renewable energies from biogas recovery such as biomethane and biohydrogen have the capability to generate electricity by using settling generator. Studies reported that a net profit of RM 3.8 million per year is gained from the generation of electricity with the production of CPO at 60 t per hour. The electricity being generated from biogas is captured through POME treatment (Chin et al. 2013). Typically, POME will take about 100 days of treatment by series of


ponding system; anaerobic and aerobic treatments. In an anaerobic system, the depletion of oxygen from bacterial activities contributes to the increment in COD value (Jiang et al. 2013). Meanwhile, aerobic being the following treatment process will increase the dissolved oxygen inside the stream through the large surface pond area being exposed to atmospheric air. In the secondary treatment, phosphorus and inorganic nitrogen are abundant, causing eutrophication whereby in a long-term prospect, this is more severe due to the high strength of organics and heavy metals contained in effluent (Abdel-Raouf et al. 2012b).

POME as microalgae growth media Commonly, a commercial medium is being used by most microalgae industries for the purpose of cultivation. However, due to the requirement of nutrient and fertilizer such as NPK, an additional cost is needed for the microalgae cultivation. The cost for the medium sometimes exceeds the gain from the biomass production (Ferrell and Sarisky-Reed 2010). This has proven that the use of commercialized medium is not feasible for the microalgae industries. Thus, the high cost of commercial medium for microalgae cultivation triggers the idea of using an available nutrient source from wastewater as an alternative way of microalgae cultivation. The high content of organic materials and nutrient in wastewater such as POME provides low-cost microalgae cultivation method which will benefit the industries for a longer-term period (Ahmad et al. 2006). Furthermore, the microalgae cultivation in wastewater is able to manage the nutrient loading by the consumption of nutrient for microalgae growth requirement (Wang et al. 2010). This method of microalgae cultivation is considered as economical for a large-scale feedstock where low cost of nutrient sources can be obtained from wastewaters. Besides that, culturing microalgae using POME offers an alternative way to reduce nutrients in the effluent including such amount of nitrogen and phosphorus. The nature of microalgae as an organism that needs nutrients and CO2 as growth requirements has turned the POME microalgae cultivation method into a win-win strategy in treating the POME. The high nutrient content in POME which is considered as a pollutant that needs to be reduced in wastewater treatment can be utilized by microalgae for their growth. In a mass microalgae cultivation system, a large amount of nitrogen is required for a proper growth of microalgae. Huge amount of nitrogen that must be supplied makes the microalgae cultivation process nonenvironmental friendly. However, the high concentration of ammonia from the anaerobic digestion system might inhibit the microalgae growth (Chi et al. 2011). Besides that, an addition of energy and cost is needed to pre-treat the high

content of ammonia stream before it is introduced into the microalgae treatment system.

Effluent treatment Commonly, wastewater is being treated using a biotreatment involving anaerobic and aerobic processes of bacteria degradation. The designed biological system is to eliminate nutrient such as nitrogen and phosphorus that can lead to eutrophication problem. The eutrophication phenomena endanger aquatic lives due to the harmful effects of toxic compounds such as unionized ammonia. Furthermore, current wastewater treatment system treats the waste streams anaerobically and aerobically by the function of microorganisms such as bacteria degradation of organic matters. However, treated effluent coming from the anaerobic and aerobic process still contains inorganic compounds such as ammonium, nitrate, and phosphorus. The possibility for eutrophication that leads to algae blooms is still high even after the biological treatment processes. Moreover, the biological method of wastewater treatment still relies on the self-purification of bacteria which requires a long treatment period. A faster treatment process is desired with higher rate of wastewater recovery while at the same time possesses a cost-effective criteria’s result of domestic, agricultural, and industrial activities and organic and inorganic wastes generated to the water stream, air, and ground that contribute to the increment of environmental pollutions (Lim et al. 2010). Moreover, the palm oil industry is a major contributor to the water pollution problem and therefore an advance wastewater management is desired to solve pollution issues. It is reported that about half of 5–7.5 t of water is used in producing CPO that is generated in the form of POME (Latif Ahmad et al. 2003). The generated POME is considered a pollutant that is deposited to the environment and a proper treatment is needed to reduce the nutrient content and its impact to the environment. In addition, limited water source has forced us to come out with a feasible and sustainable resource recovery. Furthermore, the ponding treatment approach requires a long HRT to complete each of the treatment stages. Meanwhile, a complete series of anaerobic and aerobic processes might take up to 100 days and yet this treatment is still unable to meet the effluent discharge quality standard regulation (Miao and Wu 2006; Song et al. 2008; Sathish and Sims 2012). POME treatment process primarily will have about one and a half day of oil removal activity before it is being transferred into acidification pond which will take 2 days for the acidification process. Then, in an anaerobic pond treatment, the bacteria will utilize the nutrient which will take 30 days or more before the treatment proceeds to facultative pond and aerobic pond treatments. The total treatment


period will take up to 100 days of HRT (Rupani et al. 2010). It is claimed that this secondary treatment of POME is not environmental friendly due to the emissions of GHGs such as methane (CH4) and carbon dioxide (CO2) that leads to global warming issues (Kaku 2011). Generally, methane is generated due to the decomposition of biomass and organic matters retained in the anaerobic system which is known as methanogens process and at the same time reducing the water quality (Chaiprasert 2011). Throughout the primary and secondary treatments, insufficient removal of phosphorus and nitrogen form of nutrients happens where orthophosphate and nitrate are not reduced therefore requiring a further stage of treatment process for a complete removal (Órpez et al. 2009). Primary and secondary treatments are sufficient to eliminate settable solids and promote oxidation of organic matters in the effluent. However, high content of inorganic nitrogen and phosphorus can cause crucial impact in a long-term period due to organics and metal element presence (Abdel-Raouf et al. 2012a). Table 7 shows the characteristics of POME after the anaerobic treatment. Typically, the value of total nitrogen in POME does not meet the discharge standard requirement which is 200 mg/L. As stated in Table 6, the total nitrogen content in the effluent coming from the secondary anaerobic treatment can be as high as 320–1070 mg/L. Thus, the secondary anaerobic discharge needs to undergo additional treatment to further reduce the concentration of total nitrogen in the stream. As an alternative, microalgae will be used to consume and reduce nitrogen content in the streams as the optimum nitrate concentration required is about 200–400 mg/L for an effective microalgae growth (Li et al. 2008a, 2008b). In addition to that, microalgae also needs other micronutrients such as Fe, Zn, P, Mg, Ca, and K to support its growth and these are present in POME (Habib et al. 1998). This microalgae cultivation treatment offers a low-cost and high-efficiency approach to overcome the

Table 7 pond

Characteristics of POME after anaerobic digestion and lagoon

Parameter

Final dischargea range

Mean

pH Oil and grease BOD COD Total solid Suspended solid

7.0–7.4 10–430 110–1800 340–19,680 6090–18,400 760–14,850

7.3 130 610 4820 10,360 4680

Volatile solid Total nitrogen Ammoniacal nitrogen

2680–14,570 320–1070 60–300

5000 520 180

a

All parameters are in mg/L except temperature and pH

conventional treatment limitations in treating effluent as a tertiary treatment before the effluent is being discharged. Secondary treatment, a ponding system by using microorganisms, is satisfactory in terms of a low-cost treatment with a large scale of operation. However, the cause of eutrophication phenomenon is unsolved and a tertiary treatment by the function of microalgae is introduced for further reduction of nutrient content in the effluent. Moreover, the physical and chemical treatments involved in both primary and secondary stages are insufficient which demands a tertiary stage treatment as a backup (Kamarudin et al. 2015). Furthermore, the low efficiency of anaerobic bacteria compared to photosynthetic microorganisms such as microalgae in utilizing the nutrient content in POME is due to the lack of autotrophic metabolism to utilize inorganic nitrogen (Noike et al. 2004). Therefore, further removal of nutrient such as ammonium, nitrate, and phosphate happens in the tertiary treatment. Meanwhile, a quaternary treatment is optional and it is usually operated to remove metal elements, toxicants, and soluble minerals, which requires an even higher cost. Nowadays, the society is aware of pollution causes and effects to the human health (Markou and Georgakakis 2011; Rawat et al. 2011b). Water pollution can be avoided by controlling the quality of discharge through treatment processes. In Malaysia, the discharges coming from any point of source should comply with the standard regulation set by Department of Environment (DOE) Malaysia.

Algae-based wastewater treatment process The potential nutrient removal and wastewater treatment by using microalgae has been proven as a success. Excessive nutrients such as nitrogenous and phosphorus compounds are able to be reduced by microalgae-based treatment system. The same interest is shown by some well-developed countries such as Australia, USA, Thailand, Taiwan, and Mexico to develop this kind of treatment system by utilizing microalgae. The microalgae consumes nitrogenous and phosphorus in wastewater for their growth and cell development while at the same time reducing the abundant amount of nutrient in the wastewater. Cultivated microalgae not only is predicted to remove phosphorus and nitrogen form of nutrient but also has the capacity to reduce or eliminate heavy metals and toxic compounds present in the wastewater (Abdel-Raouf et al. 2012b). In order to demonstrate the performance of microalgae in treating wastewater, some criteria can be assessed such as the microalgae growth rate, percentage of nutrient reduction, and lipid content in the microalgae biomass (Dalrymple et al. 2013). In addition, nutrient reduction will directly influence the reduction in BOD and COD which define the wastewater quality throughout the treatment process. Some researchers propose the microalgae-based wastewater treatment system as the tertiary treatment since further

Environ Sci Pollut Res Fig. 1 Overview design of POME treatment with microalgae cultivation system (Lam and Lee 2011)

removal or phosphorus and nitrogenous by product from the secondary stage effluent is achievable (Cho et al. 2011; Mutanda et al. 2011). Apart from that, a microalgae-based integrated system is one of the potential technologies that is simultaneously involved in air pollution monitoring and wastewater treatment. As shown in Fig. 1, the flow diagram for the integrated microalgae-based treatment system consists of wastewater and flue gas treatment unit operation (Lam and Lee 2011). Meanwhile, a POME treatment by using microalgae has the potential to treat wastewater and flue gas emission within a short period of treatment, while at the same time, valuable product formation can be obtained from the biomass. The concept of treating wastewater by using microalgae is primarily introduced by Oswald in 1950. This is also one of the strategies to mitigate GHG emission to the atmosphere. Since then, there are many researches done by using different species of microalgae to treat industrial and municipal wastewater (Christenson and Sims 2011; Kamarudin et al. 2013; Larsdotter 2006; Zainal et al. 2012). COD and BOD removal A water pollution rate can be defined as the water quality related to the organic and inorganic compounds as well as microorganism content in a water body where the organic and inorganic compounds can be oxidized chemically or biologically. The chemical oxidation rate of compounds present

in wastewater is determined in term of COD while BOD is for the biological oxidation activities of microorganisms to treat the waste. The microorganisms oxidize the biodegradable organic compounds and release carbon dioxide and water as the respiration products. Throughout a respiration process, oxygen is consumed and it is the oxidation agent for microorganisms to perform biodegradation of organic matters. Hence, the demand of oxygen is proportional to the level of organic content in the wastewater and it demonstrates the value of BOD. An oxygen source available for microorganism can be obtained from the photosynthetic microorganisms such as a microalgae which undergoes a photosynthesis process where oxygen is released as the respiration product. However, insufficient oxygen level in the wastewater will limit the function of microorganisms and increase the BOD value of the stream. A BOD level that is too high will harm the aquatic ecosystem. Furthermore, throughout the wastewater treatment stages, the reduction of BOD is significant as organic matters present in the waste stream are being degraded and oxidized. Microorganisms undergo an oxidation process by oxidizing and converting the nutrient compounds in the wastewater. As an example, the available ammonium in the wastewater will be utilized as a nitrogen source for the microorganisms and undergoes conversion into ammonium nitrogen associated products such as ammonia-nitrogen, nitrite-nitrogen, and nitrate-nitrogen. Meanwhile, a significant oxidation of nitrogenous compounds and phosphorus reduces the COD value of treated wastewater (Wang et al. 2010). A COD removal can


also be achieved from microalgae photosynthetic activities which require abundant carbon for microalgae growth requirement (Muñoz and Guieysse 2006). However, a slight increment of COD might happen even though microalgae growth is progressing. The excretion of photosynthesis by-product of microalgae such as glycolic acid might be the cause behind the COD increment throughout the microalgae growth (Merrett and Lord 1973). Commonly, the concentration of dissolved oxygen in the water bodies increases after sunrise and reaches maximum concentration in the afternoon while reducing at night. The rate of photosynthesis influences the concentration of dissolved oxygen available for microorganism respiration. However, a high concentration of dissolved oxygen in the stream triggers a photo-oxidative condition that can damage the receptors of microalgae and disrupt the photosynthesis activity.

Nutrient removal by algae A nutrient removal assessment can be done by observing and comparing the parameters that will influence the growth rate of microalgae. The result of the microalgae growth rate is dependent on cultivation conditions such as HRT, light source, and temperature (Garcia et al. 2000). Meanwhile, the nutrient elements occurring in the wastewater are in the form of ammonia (NH4), nitrate (NO3), nitrite (NO2), and orthophosphate (PO34). Microalgae cell composition shows the element fractions of carbon, hydrogen, oxygen, and nitrogen contents while microalgae biomass compositions are based on the utilization of organic and inorganic compounds of microalgae throughout the photosynthesis process including the assimilation of carbon dioxide (CO2) (Dalrymple et al. 2013). Apart from that, microalgae has the ability to consume and at the same time, reducing nitrogen, phosphorus, and toxic compounds in the waste stream (Boelee et al. 2012; Mata et al., 2009; Sturm and Lamer 2011; Zhou et al. 2012a). Therefore, by introducing microalgae to the nutrient-rich wastewater, a proper management of nutrient loading can be achieved (Wang et al. 2010). The main target of managing the nutrient load and toxicity of the wastewater is to improve the highly polluted wastewater into an acceptable wastewater discharge (Cai et al. 2013). Furthermore, microalgae has the capability to convert inorganic nitrogen into organic nitrogen through assimilation process. The inorganic nitrogen sources such as ammonium (NH4), ammonia (NH3), nitrate (NO3), nitrite (NO2), nitric acid (HNO3), and nitrogen gas (N2) are converted into organic nitrogens such as peptides, protein, enzymes, chlorophyll, energy in the form of ATP and ADP and genetic material (DNA and RNA) (Barsanti and Gualtieri 2006). Therefore, nitrogen existing as ammonium and ammoniacal nitrogens are made available for microalgae consumption (Park et al. 2010).

Wastewater treatment is intended particularly to remove abundant phosphorus and nitrogen that can cause eutrophication phenomenon (Cho et al. 2013). A eutrophication leads to a low oxygen concentration that causes fish kills and the release of toxics such as neutrotoxins. A researcher reports that the efficiencies of nutrient removal can be reached up to 40– 43% (Houser et al. 2014), while 93% of nutrient removal is reported by another researcher through microalgae cultivation in POME treatment (Zainal et al. 2012). Particularly, through bioremediation of microalgae nitrate, sulphate and phosphate can be reduced up to 99, 84, and 73% of reduction percentage (Nandeshwar and Satpute 2014). In addition, different microalgae species possess difference in their nutrient removal performance. S. obliquus removes up to 100% of ammonium and 97% phosphorus in wastewater (Martınez et al. 2000). The performance of S. dimorphus shows 99.5, 91.5, 98.8, 97.2, 86, and 86.5% of ammoniacal nitrogen, ammonium, phosphorus, phosphate ion, COD, and BOD in POME (Kamarudin et al. 2013). Meanwhile, C. vulgaris removes 61, 53.8, 84, 66.2, 50.5, and 61.6% of ammoniacal nitrogen, ammonium, phosphorus, phosphate ions, COD, and BOD in POME. The performance of C. vulgaris is slightly lower than Scenedesmus sp. in terms of nutrient removal (Kamarudin et al. 2013) and this can be explained from the presence of chlorophyll a that is higher in Chlorella, causing light limitation and reducing the efficiency of microalgae (Aslan and Kapdan 2006). Meanwhile, S. plantensis is another microalgae species that has a high efficiency in removing nutrient where it is reported that 90% of COD, 87% of ammoniacal nitrogen, and 80% of total phosphorus being removed in anaerobic POME (Zainal et al. 2012). Ammonium and nitrate are the examples of nitrogen sources in wastewater (Ebeling et al. 2006). The unionized ammonia is toxic towards aquatic life. However, in an activated sludge system, microorganisms will stabilize the high concentration of ammonia by oxidizing it into nitrate. Microorganisms will also consume phosphorus as their growth requirement. Thus, a significant nutrient removal is achieved while at the same time decreasing the value of BOD and COD (Wang et al. 2010). Microalgae consumes ammoniacal nitrogen at a high rate which contributes to a high reduction of ammoniacal nitrogen in wastewater. However, the concentration of ammoniacal nitrogen will start to increase again due to the decomposition of microalgae biomass retained in the water bodies (Cheunbarn and Peerapornpisal 2010; Chuntapa et al. 2003; Zainal et al. 2012). The fact that biomass decomposition increases the ammoniacal content in wastewater is also supported by finding that ammoniacal nitrogen increases after day 12 of S. plantensis cultivation immediately after it reaches the maximum removal of ammoniacal nitrogen on day 11 (Zainal et al. 2012). For a commercial microalgae cultivation especially in the production of biodiesel, the concentration of nitrogen source provided needs to be


limited as a microalgae nitrogen starvation will increase lipid content in the biomass (Chisti 2007; Frac et al. 2010; Illman et al. 2000). In addition, phosphorus is another nutrient element to be removed in the wastewater. The removal can be achieved either by biological assimilation or chemical precipitation. There are factors that cause chemical precipitation of phosphorus such as the temperature, ionic strength, and the impurity content in the stream (Lopez-Valero et al. 1992). Nucleic acids, lipids, and proteins are examples of elements of phosphorus. These elements provide energy for microalgae from the utilization of both organic and inorganic forms of phosphorus. The microalgae shows rapid removal of phosphorus throughout the cultivation period until it reaches the maximum phosphorus removal point. However, the concentration of phosphorus will start to increase again due to the decomposition of organic matters by microorganisms such as bacteria (Craggs et al. 2004; Grönlund et al. 2004; Zainal et al. 2012). Therefore, an effective nutrient removal treatment system can be obtained by considering all factors that influence the microalgae growth and productivity. The aeration system can enhance the reduction of COD where a further reduction of COD is desired which can be achieved by having heterotrophic metabolism microorganisms to consume the nutrient available for their growth and development (Metcalf and Eddy 2003). Therefore, introducing a microalgae cultivation system as microalgae-bacteria symbiosis treatment can shorten the treatment period. In addition, microalgae has a high tolerance and is capable of absorbing metal elements present in the wastewater (Mehta and Gaur 2005). Both marine and freshwater microalgae are able to absorb and store the metal elements in their cells (Afkar et al. 2010; Chen et al. 2012; Kumar and Gaur 2011).

Metals ion and toxic substance removal Microalgae cultivation to treat wastewater is tested by several researchers including the sewage treatment, agriculture wastewater treatment, and industrial effluent because a microalgaebased treatment system has the potential of the removal of heavy metals and toxic substances such as Al, Ca, Fe, Mg, and Mn (Wang et al. 2010). In comparison to bacterial and fungal, microalgae possesses an efficient function to absorb and store heavy metals due to its large surface area and binding affinity (Zhou, Li, et al., 2012a). In addition, the size, shape, and cell wall composition influence the binding affinity factor of a microalgae (Jiang et al. 2013). Theoretically, metal ions will attach to the cell wall followed by a slow uptake of ions into the cell membrane and ions are then transported to the cytoplasm for storage (Wang et al. 2010).

Crucial operation conditions for effective microalgae cultivation system In treating a wastewater, an effective microalgae cultivation system is desired to achieve the best algae growth and performance. The production rate of wastewater treatment is dependent on the wastewater source, location, and culture conditions (Dalrymple et al. 2013). There are several crucial parameters that need to be taken into consideration. These factors can be divided into three categories named physical, chemical, and biological factors. Stated in Table 8 are the factors that influence microalgae growth and productivity in cultivation system. The stoichiometric of an average algae cell is C16H181O45N16P which consists of elements such as carbon, nitrogen, and phosphorus that should be supplied to a microalgae in optimum fractions for optimal growth and productivity. The nutrient concentration is one of the factors that falls under chemical parameters to be considered in a microalgae cultivation where it is in the effluent that needs to meet the microalgae nutrient requirement for the best treatment performance. During photosynthesis process, microalgae uses solar energy as the main energy source which is converted into a chemical energy which gives out oxygen, O2, gas as a by-product. This chemical energy is used to assimilate carbon dioxide, CO2, for the conversion of sugars. The process can be explained from the formula: 6H2 O þ 6CO2 þ light→C6 H12 O6 þ 6O2 : Meanwhile, the carbon element is the most important growth requirement of a microalgae as most fraction of microalgae consists of a carbon element. An inorganic carbon is utilized by microalgae in the form of CO2 and HCO3 and Table 8 Factors that influence microalgae growth and productivity in cultivation system Categories

Parameters

Physical

Light quantity and quality Temperature Mixing Dilution rate Sizing and design (depth) Harvesting frequency Nutrient concentration Oxygen availability Carbon dioxide availability pH addition of bicarbonate Salinity Toxic chemicals Pathogens (virus, fungi, bacteria) Predation by zooplankton Competition between species

Chemical

Biological


some microalgae species are able to utilize organic carbon in the form of sugars, organic acid, acetate, or glycerol as growth nutrients (Wood et al. 1999). In addition, the microalgae species can be divided into several categories such as heterotrophic, autotrophic, and mixotropic species. A heterotrophic microalgae does not depend on CO2 for growth, and up to 50% of organic carbon is being utilized by a heterotrophic microalgae in a high-rate algal pond wastewater treatment system. This shows that microalgae with heterotrophic metabolism uses organic nutrient elements present in the cultivation system as the source of carbon and energy for growth. The heterotrophic microalgae metabolism can be enhanced by adding assailable organic carbon sources such as acetate acid and saccharide (Liang et al. 2009; Perez-Garcia et al. 2011). On the other hand, the autotrophic microalgae species light source is used as the energy source for microalgae to function. In some cases, the metabolism of microalgae can be shifted between both autotrophic and heterotrophic as proven by previous findings where Chlorella sp. and Scenedesmus sp. are the species with this mixotropic characteristic (Becker 1994). Meanwhile, nitrogen is another element required for microalgae growth. Commonly, microalgae takes up nitrogen from the source of ammonium and nitrate available in the effluent (Oliver and Ganf 2002). Other than carbon and nitrogen, phosphorus is also a nutrient element that is needed for microalgae growth. Microalgae consumes phosphorus in the form of orthophosphate, PO−34. An orthophosphate is an inorganic phosphorus source that can be converted from organic sources by the function of enzyme called phosphatases that is present in microalgae cell. This enzyme will start to function and organic phosphates will be converted into orthophosphate when there are insufficient orthophosphate for microalgae to consume (Larsdotter 2006). Apart from that, microalgae has the ability to store phosphorus in the form of polyphosphate granules and it will be consumed once there is an absence of phosphorus in the cultivation system. Insufficient amount of phosphorus does not give a huge impact to microalgae growth in comparison to other factors such as light and temperature (Oliver and Ganf 2002). In addition, the limitation of nutrients such as nitrogen and carbon elements in the effluent can cause a low growth rate of microalgae. However, an amount of nutrient loading that is too high particularly due to the high nitrogen and phosphorus concentrations loaded into the microalgae cultivation system can cause massive algal blooms. Meanwhile, a high ratio of nitrogen to phosphorus, 30:1, will cause phosphorus limitation while a low ratio 5:1 will cause nitrogen limitation in the cultivation system. Typically, in wastewater, phosphorus availability would not be a problem but low nitrogen sources might be the reason that limits the growth (M. a. Borowitzka 1999). Furthermore, an algal bloom must be avoided as it will trigger low performance of microalgae in taking up nutrients and will directly reduce the percentage of nutrients or

pollutant removal from the treatment system. An optimum nutrient concentration or nutrient loading rate is desired and it can be controlled by adjusting the POME concentration through dilution to achieve the optimum nutrient loading value. Macronutrient is not the only nutrient required by microalgae, micronutrient is also required in a small amount for microalgae to perform well in the cultivation system. Manganese, nickel, copper, iron, molybdenum, zinc, boron, and chloride are the micronutrients that can be added to the microalgae system together with the chelating agent such as EDTA (Oliver and Ganf 2002). Meanwhile, the addition of bicarbonate can help to improve photosynthesis efficiency and nitrogen utilization (White et al. 2013). Moreover, the nutrient concentration or loading is the level of nutrient available for microalgae consumption to meet their growth requirement. Nutrient is one of the most critical factors for microalgae productivity (Stephens et al. 2010). Findings show that the average reduction of phosphorus and nitrogen in the form of nitrate is about 43 and 40% from the different nutrient concentrations used (Houser et al. 2014). In a low to moderate range of nutrient concentration, the productivity of microalgae cannot be achieved even by optimizing the light source because of the nutrient-limited environment. In the case of a media with a high concentration of nutrient, nutrient is not a limiting factor thus the area for cultivation system can be reduced for economical purposes (Dalrymple et al. 2013). The presence of heterotrophic microorganisms is based on the nutrient and organic matter content in the stream (Serejo et al. 2015). When organic nutrient is not available in the system, the autotrophic microalgae tends to assimilate CO2 to fulfil its carbon requirement for growth (Wang et al. 2010). Hence, an increase in nutrient loading promotes CO2 reduction as the microalgae uses all of the available CO2 for growth. In addition, different nutrient loadings will give different light illuminations into the wastewater for microalgae utilization. The cultivation of microalgae using POME is a good strategy to provide nutrient for microalgae growth at the same time reducing nutrient through microalgae utilization. However, POME is loaded with an abundant of nutrient which might not be suitable to a certain microalgae species especially the low resistance towards concentrated stream species. A research is done by testing the different ratios of POME dilution to observe the growth and nutrient removal performance of microalgae. POME without dilution has high ammonium nitrogen that inhibits growth and reduces the performance of microalgae in wastewater treatment (Ding et al. 2016). Furthermore, high concentration of cultivation media reduces lipid content in microalgae biomass (Dalrymple et al. 2013). Meanwhile, nitrogen is considered as a limiting nutrient in POME since phosphorus is abundant (Dalrymple et al. 2013). Excessive nutrient such as phosphorus triggers algae bloom which directly contributes to odour and taste problem. An algae bloom creates particulate scums and water discolour


of the system while at the same time causing toxicity condition aquatic life. This phenomena can also be related to the condition of the cultivation medium that is rich in nutrient and the uneven accumulation of organic matters in the system. However, this issue can be solved by having a mixing or water flowing which will increase the gas transfer rate throughout the medium and avoid accumulation of particulates. Meanwhile, a high ammonium level in the stream is associated with a high nutrient load in the system whereby a high ammonium level inhibits microalgae growth (Li et al. 2008a, 2008b; Yuan et al. 2011) and this can be solved by stripping out the ammonium level through ammonia volatilization which can be achieved by adjusting the pH of the medium (Cai et al. 2013). Another method to reduce ammonium concentration is to dilute the nutrient to a certain optimum ammonium concentration such as diluting it with the nitrified secondary effluent which will stabilize the nutrient condition (Yuan et al. 2011). It is observed that microalgae growth rate differences in lag time period with different nutrient concentrations (Tan et al. 2014). Microalgae might take a longer time to adapt to the environment with a higher nutrient load and low-light penetration condition that limits its growth and performance in nutrient removal (Ding et al. 2016). The nutritional factor and environmental condition of microalgae cultivation system affect the assimilation of nitrogen such as nitrate. Furthermore, a test is made by one of the researchers by providing moderate nitrogen to observe the growth and production of lipid content in microalgae biomass as nitrogen influences growth and productivity of microalgae (Woertz et al. 2009. Different microalgae species show differences in their tolerance towards high nutrient concentration. S. platensis is not able to tolerate ammonium level above 200 ppm (Park et al. 2010). The species with highest tolerance is found to be Euglena followed by Oscillatoria, Chlamydomonas, Scenedesmus, Chlorella, Nitzschia, Navicula, and Stigeoclonium. As microalgae undergoes photosynthesis process, carbon dioxide is needed for respiration to occur. For an autotrophic metabolism microalgae species, CO2 availability in the cultivation system is crucial because CO2 supplied to the system will be converted into a carbon source. Meanwhile, atmospheric CO2 can be supplied by the means of aeration. CO2 can be supplied via gas bubbled sparging system, perforated pipe, hollow fibre membrane, or floating gas exchange system. However, the amount of CO2 in the atmosphere is considered low which is 0.033%. An additional CO2 supply can be provided for the cultivation system since microalgae production has shown improvements with optimum CO2 addition (Li et al. 2011). It is crucial to optimize the concentration of CO2 in the cultivation system as low CO2 causes carbon limitation and reduces microalgae productivity. If CO2 addition is at a high rate, it will trigger acidic environment which is

prevents an effective microalgae growth. Besides that, the addition of CO2 is highly related to the pH value of the system as an increasing amount of CO2 causes an increment in pH value of the suspension system. However, at high pH, the inorganic carbon will be in the form of carbonate, CO32−, which cannot be consumed by microalgae and thus contributing to carbon limitation for microalgae growth. The pH factor is strongly dependent on both the CO2 addition and nutrient concentration loading into the system. In addition, pH can be regulated by letting in more organic materials that will enhance respiration process (M. A. Borowitzka and Borowitzka 1988) but the amount of nutrient loading must be at an optimal value for an extremely high level of organic compound inhibits the nutrient uptake by microalgae (Ogbonna et al. 2000). In oppose to that, the increment of CO2 addition to the system increases the acidity of the microalgae suspension which results in a low pH condition. CO2 will be converted into carbonic acid to be consumed by microalgae. A high CO2 (carbonic acid) consumption increases the pH value due to the reduction of carbonic acid in the system (Chevalier et al. 2000). This shows that CO2 concentration is crucial and must be controlled as it gives a huge impact to the environment of microalgae cultivation system. When pH is at a high value, typically pH (>9), most of the inorganic carbons are in the form of carbonate CO3−2 which cannot be consumed by microalgae. Meanwhile, a rapid photosynthesis is causing carbonate and bicarbonate ion dissociation due to the higher rate of CO2 consumption by microalgae than the rate of bacteria respiration, causing a high pH environment. Furthermore, high pH value can cause floc formation in the cultivation system and will directly reduce the nutrient uptake rate by microalgae. Apart from that, nutrient stripping phenomena such as ammonia volatilization and phosphorus precipitation will also occur (Abdel-Raouf et al. 2012a). However, an increasing pH due to photosynthesis can be good for the system as it has the disinfecting effect in the wastewater system. Ultimately, it is a continuous cycle where the pH value will rediscover at night due to the high availability of CO2 in the system as a result from the low rate of photosynthesis (Chevalier et al. 2000). Also, the assimilation of nitrate ions by microalgae will influence the increment of the suspension system’s pH value but if the ammonia is used as the nitrogen source for microalgae, the suspension pH value may decrease. In constructing a mass microalgae cultivation system, a huge amount of CO2 must be supplied to the high concentration of microalgae in the system. However, there are shortcomings such as high energy consumption and high cost for installation and maintenance for CO2 addition facilities. One of the solutions to counteract this is by having CO2 supplied from the industrial flue gas emitted and channelled to the microalgae-based treatment system. This ensures the sustainability and effective microalgae cultivation system as


industrial emissions of CO2 gas are beneficial for microalgae growth while the microalgae is treating the gas emitted to the atmosphere. As a result, the amount of air pollution can be reduced to a minimal value. In addition, during photosynthesis process, microalgae consumes carbon dioxide and gives out oxygen. Hence, the oxygen level in the cultivation system increases. However, an excessive amount of oxygen in the system can cause photo-oxidative and damages microalgae chlorophyll which results in the disruption of the photosynthesis process and reduces the productivity of microalgae (Molina et al. 2001). This photo-oxidative issue can be overcome with an effective design of the cultivation system. For example, in an open pond cultivation system, a large surface area with a shallow depth of the pond can promote mass transfer of oxygen with the atmospheric air while in a closed bioreactor, having an additional compartment for the gas exchange reduces excessive amount of dissolved oxygen in the cultivation suspension. Another consideration parameter that falls under the chemical factor is the microalgae inhibitory substances that can stunt microalgae growth and reduce its performance in treating effluent. There are various substances that can cause inhibition such as heavy metals, detergents, herbicides, and pesticides. A high pH value of the suspension caused by a high ammonia concentration loading also causes inhibition to microalgae growth. The toxicity level of ammonia strengthens as the suspension temperature increases, commonly during the day, while the fractions of ammonia are at a high level; free ammonia form is able to diffuse into the microalgae cell and causes inhibition (Ogbonna et al. 2000). Moreover, light supply is the most crucial factor for microalgae to undergo photosynthesis process where a light source is converted into chemical energy for microalgae to function. Light can be supplied by natural sources such sunlight or from the power sources such as in a lab scale operation. The factor of light supply must be considered by the means of quality and quantity of the supply. Low light quantity happens when there are internal shading effects in the microalgae cultivation system. Such effect is caused by a high density of microalgae covering the surface of the pond such as in an open cultivation system. A low light quantity is also caused by the particulate matters present in the culture system. Since the amount of light supply will be unevenly distributed among microalgae in the suspension, this issue can be solved by having an optimum turbulence provided through mechanical mixing or an effective design of the system with optimum liquid volume and depth of the pond. Meanwhile, for a closed photobioreactor system, light penetration is better compared to an open pond cultivation system. In order to sustain high productivity and biomass, an optimum design can be achieved by having an optimum diameter to height of the closed system column. Commonly, transparent and narrow pipes are used to overcome the shading effect. A multidirectional light can be

illuminated through the transparent pipe for microalgae utilization. Furthermore, light quality can be described as the intensity of light supply. A low intensity of light results in a low microalgae growth. Higher rate of photosynthesis activity can be achieved with a higher light intensity. However, light intensity above the optimum requirement causes photoinhibition to microalgae in the system and damages the microalgae light receptor (Oliver and Ganf 2002). In an outdoor microalgae cultivation system, the utilization of sunlight is important as sun is the main source for microalgae energy to function. Sunlight’s full exposure is about 10,000 FC and if light is not optimized, a high exposure can cause photoinhibition of microalgae. This happens when the light illumination increases beyond the maximum point of microalgae tolerance, causing damage to the microalgae light receptors. This photoinhibition phenomenon will directly reduce the photosynthesis rate and microalgae productivity (Richmond 2004). However, sun plays an important role in providing light energy for microalgae photosynthesis, increasing the dissolved oxygen, and reducing faecal bacteria by applying a heating effect to the wastewater. An indoor microalgae cultivation is light limited since more than 50% of sunlight illumination is reduced for greenhouse light utilization. Although the outdoor microalgae cultivation is ideal for a short period, it is less desired for a long duration due to the excess exposure of light intensity and heating effects (Dalrymple et al. 2013). Another possible method to improve light supply from sunlight is via a vertical orientation of photobioreactor while at the same time avoiding extremely high temperature from mutual shading effects (Cuaresma et al. 2011). Current technology has come out with the idea of producing a device called solar collector that can absorb and store light energy from. This is a hybrid system where the collected natural light will be transmitted and combined with the artificial light. This system makes lighting possible even when there is no sunlight. CO2 sequestration can be improved by having this system and the device can utilize the infrared heat and visible light exposure (Muhs 2000). Besides that, a high light intensity exposure causes pigment degradation and heating effect that limits microalgae growth. Thus, a subdued light is suggested for an optimal microalgae growth due to the low in light strength that can preserve microalgae pigments and enhance its metabolism, promoting growth. Excessive light is also one of the eutrophication causes due to a rapid increase in microalgae growth in the system. As microalgae growth increases, the dead microalgae will proportionally increase, leading to an oxygen depletion in the system as it is used up by the microorganisms such as bacteria to decompose the dead algae. This phenomenon is also known as anoxia. Other than light, an optimum temperature is also crucial as a physiological factor requirement for high microalgae growth


and productivity. The optimum temperature depends on the species of microalgae as some can tolerate high temperature and otherwise. A temperature below the optimum will not kill the microalgae unless it reaches the freezing condition. However, a temperature higher than the optimum will kill the microalgae. A high-temperature microalgae cultivation environment results in moisture loss through evaporation. However, a high-temperature environment can avoid contamination problem due to pathogens and viruses growth. Meanwhile, thermophile microalgae such as cyanobacteria can be used for high-temperature microalgae cultivation system as it has high tolerance towards high temperature (45 °C) compared to other microalgae species. A moderate temperature of microalgae cultivation is suggested for microalgae cultivation to promote optimal CO2 solubility and gas exchange. In operating a microalgae-based treatment system, there are operational conditions that must be optimized and maintained to achieve the best performance of microalgae in treating the wastewater. Turbulence is one of the operational parameters to be controlled despite the type of the cultivation system, open or closed system. Turbulence can be provided by the means of mechanical mixing or through aeration. An example of a mechanical mixing is by using a propeller or paddles while an air bubble injection method is used for the aeration purpose. Aeration promotes mass transfer which allows the removal of excess oxygen released during photosynthesis process by microalgae that can inhibit microalgae growth (Woertz et al. 2009). Besides that, an aeration through a bubble diffuser provides inorganic forms of carbon for photoautotrophic microalgae consumption (Dalrymple et al. 2013). An optimum turbulence is desired as excessive turbulence can cause shear stress and damages the microalgae cell. However, a too low turbulence in the system will not effectively disperse the accumulation of particulate matters and unstirred a boundary layer of suspension. In addition to that, power energy wastage happens as the operational system did not achieve the target for an effective treatment operation. Moreover, the nutrient uptake rate of microalgae can be influenced by the turbulence factor. There are concentration differences between the inside and outside suspension of the cell wall. This can be explained by the hypothesis of a higher concentration difference between suspension and cell wall results in a low diffusion rate for the nutrient to diffuse into the cell. Microalgae will face difficulties in consuming nutrients to meet its growth requirements. Thus, the nutrient uptake rate can be enhanced by minimizing the boundary layer between microalgae cell and the surrounding nutrient concentration. Turbulence provided to the system can overcome this high difference in concentration between microalgae and suspension nutrient where a short period of turbulence is sufficient to overcome the accumulation of particulate matter for an effective microalgae cultivation system. In addition, turbulence can avoid fluctuation in pH value of the suspension and

it promotes gas exchange between the liquid suspension and the surrounding air. It is proven that turbulence influences the microalgae productivity as an optimum turbulence enhances mass transfer rate and overcomes the thick boundary layer of unstirred suspension (M. a. Borowitzka 1999). Besides that, agitation is one of the factors influencing microalgae productivity in reducing pollutants as nutrient in the wastewater (Chaiklahan et al. 2010). Gross et al. (2013) has come out with the finding of an optimum rotation speed of 4 rpm or 3.86 cm/s that is able to solve the problem of a thick boundary layer of suspension with improved light utilization and nutrient uptake rate by microalgae. Microalgae reproduction is also enhanced in a sufficient turbulence system compared to a still medium which can cause biomass accumulation. All in all, in achieving a cost-effective microalgae-based cultivation system, the aim is to have the highest nutrient removal with maximum influent rate in a short distance of system built. However, a slower flow rate enhances the removal of nutrient in the stream (Houser et al. 2014). Thus, the optimization of wastewater flow rate needs to be considered as it influences the nutrient removal efficiency. In addition, the cultivation system design and sizing can be improved for a costeffective system operation. Furthermore, a microalgae cultivation HRT is defined as the period of the microalgae retained in the cultivation system to meet the purpose of the system such as in wastewater treatment activity. HRT for a microalgae-based wastewater treatment should be decided based on the microalgae performance and it is dependent on the microalgae species. It is important to have and optimum HRT for the system as a too short of HRT causes an insufficient time for the microalgae to adapt and efficiently utilize the nutrient in the effluent. If the HRT is too long, the thick boundary of the suspension liquid will start to form and cause limitation of light to penetrate through for microalgae cultivation system. HRT is related to the frequency of the microalgae harvesting activity. As suggested by Gross et al. (2013), the optimum microalgae harvest is every 7 days. It is proven that with an optimum HRT, the competition between species that might lead to nutrient and CO2 limitations can be overcome. Moreover, an algae bloom problem can be avoided. Meanwhile, biological factor is another category of factors that is related to the living organisms’ activities and impacts. In a microalgae cultivation system, there are other species of microorganisms present which causes competitions between species for nutrient and space. In some cases, substances or by-products of microorganisms being produced can inhibit microalgae and this usually happens in an open pond system where the risk of contamination is high due to the exposure to the surrounding. This problem cannot be avoided but the possibility of contamination occurring can be minimized. There are several ways to minimize the contamination effect such as by acidification of the suspension. Acidification done by


adjusting the pH to pH 2 for at least a short period of time is able to kill most rotifers and protozoa. Apart from that, microalgae cell density is another consideration to be accounted that falls under the biological factor. Microalgae cell density can be explained by scientific terms that is the microalgae cell concentration in a system which is known as oligotrophic (less microalgae), eutrophic (excess microalgae), and mesotrophic (optimal and balance microalgae). Microalgae reproduction is influenced by temperature, light supply, nutrient provided, population competition, and the residence time. There are advantages to have a high-density microalgae cultivation system as smaller cultivation area can be used and wastewater treatment period can be reduced with less residence time of microalgae in the treatment unit operation.

conventional systems using chemicals and substances to treat wastewater can cause depositions of harmful wastes to the environment. With a microalgae cultivation system, the waste sludge can be reduced and the sludge biomass is used to produce valuable products such as biofertilizer. In addition, a microalgae cultivation system is able to overcome high energy consumption and non-economical operation issues of the previous wastewater treatment. Mechanical means for aeration are used in the system to provide oxygen for aerobic bacteria in digesting the organic nutrient in the wastewater. However, the residence time of the treatment process needs to be optimized as the longer wastewater is in contact with microalgae, the higher the nutrient removal (Houser et al. 2014) while a microalgae residence time that is too long will cause deposition of dead microalgae and reduced water quality.

Attribute and advantages of algae-based effluent treatment

Challenges associated with algae-based wastewater treatment system

Microalgae-based effluent treatment system serves multiple needs of the environment as well as renewable energy production (Wang et al. 2010). Wastewater treatment using microalgae is the most cost-effective method with high reliability in improving water quality for a continual use. Industrial, agricultural, and domestic wastewater can be used as nutrient sources for microalgae growth requirement which is considered as an economic strategy for a mass microalgae feedstock production. For example, the nutrient-rich characteristic of POME makes the wastewater ideal for a low-cost microalgae cultivation substrate (Ahmad et al. 2006). Microalgae consumes the nutrient while at the same time removing the pollutants as it grows and biomass is generated for further conversions into valuable products (Rawat et al. 2011b). Moreover, microalgae shows the flexibility in its cultivation requirements as it does not need arable land and can be cultivated at almost anywhere. An outdoor mass cultivation is possible for microalgae where sunlight is utilized for energy requirement and power therefore other means of light supplies can be minimized, promoting a cost and energy effective mass cultivation method. Moreover, microalgae possesses continuous production with greater growth rate compared to other conventional crops with the doubling time as fast as 24 h of cultivation period. This is also supported by the fact that microalgae has high tolerance and recovers faster from growth and environmental side effects compared to other crops (Masojidek and Torzillo 2013). Furthermore, it also has the ability to assimilate inorganic forms of carbon, nitrogen, and phosphorus for its growth while at the same time being capable of adsorbing heavy metal elements (Zainal et al. 2012). Furthermore, microalgae cultivation is the best proposed biological treatment system compared to chemical and physical treatments to treat wastewater economically. Previous

The conventional wastewater treatment systems are not costeffective and contribute to the secondary pollutions increment. The secondary treatment operation is not sufficient to remove pollutants and in fact, it increases the GHGs released to the atmosphere. An additional treatment process is required for the secondary treatment process which will increase the cost for each treatment added. The incomplete utilization of natural resources is another shortcoming of the previous wastewater treatment process. However, a microalgae-based wastewater treatment system can overcome the issues faced by the previous wastewater treatment process although there are challenges for a microalgae unit operation. One of the challenges is to scale up the unit operation for commercialization purposes. The scale up of microalgae harvesting, microalgae strain control, and biofuel production are among the issues in developing a potential microalgae cultivation for wastewater treatment and biofuel production (Dalrymple et al. 2013). Other than that, cost is also one of the most important factors for a commercialization purpose. A cost-effective system is highly related to the efficiency of the operation system. The more efficient the operation, the less energy consumed and the lower the operation cost (Huang et al. 2010; Gallagher 2011). Microalgae biomass production efficiency needs to be increased to minimize the cost allocated for the operations. Mixing is one of the factors that will influence both microalgae biomass production and energy consumption for a commercial scale operation (Ahmad et al. 2015).

CO2 capture and sequestration In palm oil industry, CO2 gas is emitted from mills, specifically from the activities of fibres and shell burning treatments. Palm oil fibres and shells undergo a burning process to


generate energy. However, the high carbon content in fibres, 47.2%, and shells, 52.4%, causes a high CO2 concentration being emitted (Mahlia et al. 2001). Approximately 13.7– 161.2 kg of CO2 is emitted into the atmosphere for every production of 1 t of CPO (Subramaniam et al. 2008). Hence, CO2 capture is really crucial to avoid the increment of CO2 concentration in atmosphere where it can reach up to 68.2– 279.6 kg CO2/t of CPO if CO2 is not captured. CO2 availability and fixation by microalgae As a fast growing microorganism, microalgae has the ability to fix the CO2 from the atmosphere and flue gas. In comparison to terrestrial plants, microalgae cultivation has the efficiency 10–50 times greater in CO2 capture and sequestration (S. a. Khan et al. 2009; Li et al. 2008a, 2008b). CO2 is assimilated by autotrophic microalgae when the available organic substrate is used up (Wang et al. 2010). The assimilation of CO2 influences microalgae productivity (Li et al. 2011). The atmospheric CO2 content which is 360 ppm is insufficient for microalgae growth. Therefore, CO2 addition is required for an optimum microalgae respiration process (Brennan and Owende 2010). This makes the microalgae cultivation system economically infeasible due to the extra cost and energy needed for the air pumping system installation and maintenance. In order to overcome this, flue gas can be the alternative source for CO2 supply for microalgae cultivation. As reported by Australian firms, Line Energy & Bioclean Coal in November 2007, up to 90% of CO2 is removed from the coal-fired power plant flue gas emission. A complete CO2 gas removal is possible but an extra cost is needed for the additional treatment. For example, the presence CO2 mitigation operation by some agencies such as EniTechnologie in Italy mitigates 15% of the annual CO2 emission from 700 ha of microalgae cultivation. RWE, Germany is another agency that practises the same method to mitigate CO2 from flue gas and convert it into the valuable microalgae biomass. Microalgae cultivation does not need a large area for the treatment operation as long as a high density of microalgae is cultivated as this strategy is practised by Menova Energy with solar concentrators used to focus the sunlight to photovoltaic solar cells. The concentration of CO2 in the flue gas is up to 20% which can be tolerated by microalgae to this high CO2 concentration condition (Brennan and Owende 2010). In some cases, microalgae cannot withstand a high concentration of CO2 because of the highly acidic environment that can inhibit its growth and productivity. The acidity of the suspension increases as the concentration of CO2 increases, resulting in the high amount of carbonic acid present in the system. Apart

from CO2, flue gas is also composed of N2, O2, NOx, SOx, and soot. A few microalgae species are able to tolerate such amount of NOx and SOx in the flue gases (Brennan and Owende 2010). As reported by Yoo et al. (2010), microalgae species have a high potential in treating flue gases for the purpose of CO2 mitigation. At the same time, they are able to produce high amount of lipid for biodiesel production. In addition to that, Scenedesmus sp. and Botryococcus braunii have better growth rates when flue gases are used as the CO2 source compared to the normal air being channelled into the cultivation system (Yoo et al. 2010). Improving CO2 uptake efficiency CO2 percentage supply to microalgae cultivation system influences the biomass yield of microalgae (Ahmad et al. 2015). In CO2 fixation, Rubisco is the enzyme in the microalgae cell that is responsible with the conversion of CO2 into energy rich molecules such as glucose (Ahmad et al. 2015). Furthermore, a sufficient amount of CO2 must be supplied to the microalgae for growth since the atmospheric available CO2 fraction is insufficient for autotrophic microalgae growth. CO2 captured from the gas phase will be converted into dissolved CO2 for microalgae consumption. In order to improve the CO2 recovery efficiency, the uptake of dissolved CO2 by microalgae must be higher than the mass transfer rate of CO2. In the case of low CO2 recovery, it might be because of the CO2 purged into the system is not dissolved as a liquid form and escapes in the form of gas bubbles to the atmosphere through the head space (Ahmad et al. 2015). Meanwhile, CO2 capture rate can be improved by controlling the nutrient loading. As stated by one of the researchers, limitation in phosphorus available in the system will result in a low CO2 reduction (Zhao et al. 2013). Besides that, CO2 captured efficiency is also influenced by the culture conditions such as the pH value of the liquid medium. An optimal pH influences the solubility of CO2 and mineral elements in the culture media available for microalgae growth requirements and productivity. In the case of flue gas supply as the source of CO2, the composition of SOx or NOx sometimes causes undesired environment for microalgae. Selection the optimal microalgae species and process conditions for CO2 uptake Ideal microalgae strains are required for CO2 fixing strategy to achieve most CO2 capture and sequestration aims. In a largescale microalgae cultivation system, optimum conditions and effective system design are desired while in the aspect of light supply, a huge source of light must be provided to meet the requirement of mass microalgae cultivation. The surface area to volume ratio is also important to design the most efficient large-scale microalgae cultivation system and that the light


limitation problem can be avoided. Besides that, light limitation can also be reduced by improving the suspended solid that is present in the system which will cause a shading effect. Moreover, carbon concentration is another condition factor that must be optimized as it affects the dissolved carbon available for microalgae consumption. Meanwhile, CO2 can be fed to the microalgae cultivation system from the atmosphere or industrial emission gas. Microalgae consumes carbon in form of soluble carbonates directly or the carbonate will be converted into free carbon dioxide by carbonic anhydrase enzyme. CO2 addition serves as the carbon source in microalgae CO2 fixation, and the biomass yield is CO2 consumption-dependent (Ahmad et al. 2015). The consumed CO2 will be converted into sugar to provide energy for microalgae photosynthesis activity (Ahmad et al. 2015). Improvement in the microalgae productivity with CO2 concentration was demonstrated (Li et al. 2011). Mixing is another means that affect the microalgae cultivation condition. In CO2 fixation system, mixing is provided through aeration where optimum aeration rate of inlet gas is importnat because the turbulence affects the liquid suspension. The relation between mixing and biomass production is the main cost subject for commercialization strategy (Ahmad et al. 2015). On the other hand, commercializing the CO2 fixation system not only benefits the production industry but also serves as an environment protection act (Cai et al. 2013).

Microalgae for power plant carbon capture Strategies of using microalgae for carbon capture in power plant Although the installation and operation costs for microalgae cultivation are high, the biomass production of microalgae can be converted for a commercialization purpose which will offset the cost of start-up and maintenance. Meanwhile, hightolerance microalgae species exhibit the ability to consume high fraction of CO2 and are able to withstand the presence of other components in the flue gas which might cause inhibition to a certain species of microalgae. A typical flue gas consists 10–20% v/v of CO2 and other components such as N2, O2, NOx, SOx, and soot. NOx and SOx can be the nutrient source for microalgae; however, if supplied in a high amount, it might cause microalgae growth inhibition. In addition, microalgae is a flexible phototrophic microorganism where a high purity of CO2 is not required while a CO2 supply can be Table 9

directly fed into the cultivation column. Meanwhile, the biomass yield increases as the fraction of CO2 addition increases (Ahmad et al. 2015). In general, microalgae grows under 0.04–100% v/v of CO2 concentration range and 25–100 °C range of thermal cultivation environment. Besides that, the increase of the flow rate of flue gas will increase the agitation rate in a cultivation system resulting in increasing biomass yield. A high gas flow rate provides high CO2 fraction purges into the cultivation system, giving a low recovery of CO2 (Ahmad et al. 2015). This finding can be explained by the factor of mass transfer of gases especially the CO2 gas transition from gas to suspension. In order to improve the mass transfer rate of CO2, a method that is required to enhance dissolved CO2 while at the same time minimizing the forming of bubbles which can cause CO2 to escape out of the suspension is to be studied. Furthermore, the uptake rate of dissolved CO2 by microalgae has to be higher than the mass transfer rate of CO2 from gas to suspension. Effects of the gas components on microalgae Identifying the flue gas composition is important for a microalgae-based CO2 capture and sequestration system design. The selection of microalgae species for the CO2 fixation system is dependent on the composition of the flue gas and the tolerance of microalgae towards the flue gas characteristics. As shown in Table 9, fractions of components of compounds or elements in the flue gas consisting of nitrogen, oxygen, carbon dioxide, sulphur dioxide, nitrogen dioxide, and soot dust. The typical CO2 fraction in the flue gas is within the range of 10–15%, although sometimes, it can increase up to 20% which is influenced by the core activity of the industry. Since the flue gas emitted might contain toxic substances and growth inhibitors that can disrupt the productivity of microalgae, a pre-treatment of the flue gas is suggested. However, an additional process is required, therefore increasing the cost of the operation. However, a proper selection of microalgae species that is able to withstand the flue gas composition can be the solution for this issue. High-tolerance microalgae strains can be used. Therefore, no additional flue gas pre-treatment device is required for the microalgae-based CO2 capture and sequestration unit operation. Chlorella sp. grows with various combinations of trace elements and concentration among other microalgae species. Meanwhile, the concentration of sulphur oxide, SOx, influences the pH value of the cultivation system, and the decrease

Typical flue gas compositions

Flue gas composition

Nitrogen,N2

Carbon dioxide, CO2

Oxygen, O2

Sulphur dioxide, SO2

Nitrogen oxide, NOx

Soot dust

Element Concentrations

82%

12%

5.5%

400 ppm

120 ppm

50 mg/m3


in microalgae productivity is caused by a low pH which is pH