Curr Pollution Rep DOI 10.1007/s40726-017-0068-2
WATER POLLUTION (S SENGUPTA AND L NGHIEM, SECTION EDITORS)
A Recent Overview of Palm Oil Mill Effluent Management via Bioreactor Configurations Supriyanka Rana 1 & Lakhveer Singh 2 & Zularisam Wahid 1 & Hong Liu 2
# Springer International Publishing AG 2017
Abstract Worldwide, crude palm oil industries generate an overwhelming amount of palm oil mill effluent (POME). Since the past few decades, environmental issues associated with POME disposal have challenged the palm oil-producing nations which led them to reevaluate and develop their waste management strategies by using advanced biotreatment technologies. With the help of these technological advances, POME has emerged as a valuable biomass resource with great potential to produce sustainable renewable resources like biogas. This review entails various POME treatment methods in vogue and offers an insight into their improved applicability potential and pollution mitigation strategies by using proposed improved configurations like ponding system, open digesting tanks, anaerobic digestion based-bioreactors, aerobic anaerobic hybrid bioreactors, and membrane bioreactors. This review paper also gives an overview about the recent advancements in POME treatment bioreactor configurations and emphasizing their scope in large-scale applications on an industrial level. This review also critically analyzes their performance level to achieve the standard POME discharge limit by efficiently removing high COD (chemical oxygen demand), BOD (biological oxygen demand), and TSS (total suspended solid).
This article is part of the Topical Collection on Water Pollution * Lakhveer Singh
[email protected];
[email protected]
1
Faculty of Engineering Technology, Universiti Malaysia Pahang, 26300 Kuantan, Malaysia
2
Department of Biological and Ecological Engineering, Oregon State University, Corvallis, OR 97333, USA
Keywords Palm oil mill effluents . POME . Advanced bioreactor configurations . Biofuel
Abbreviations POME Palm oil mill effluent BOD Biological oxygen demand COD Chemical oxygen demand HRT Hydraulic retention time OLR Organic loading rate TSS Total suspended solid OLR Organic load rate GHG Greenhouse gas
Introduction Worldwide population explosion has raised many concerns about our food and energy security [1]. Recent technological advances have turned palm oil mill effluent (POME) from harmful waste to useful sustainable feedstock that can be used to produce valuable by-products like biohydrogen [2] and biomethane [3]. POME is a high organic content containing polluting water released during palm oil milling process, with a brownish color and stingy odor. POME, a major source of inland water pollution which is highly acidic in nature and has a high biological oxygen demand (BOD), 30,000 mg L−1, chemical oxygen demand (COD), 50,000 mg L−1, along with unsafe levels of oil and grease, suspended solids, and total nitrogen content. Consequently, if such wastewater is released into the local water bodies without treatment, it would prove to be hazardous to the aquatic flora and fauna due to its oxygen depletion effects and acidification and eutrophication potential [4]. Several treatment methods are used to treat POME such as conventional treatment systems like the pond system
Curr Pollution Rep
(aerobic or anaerobic or facultative), open digesting tanks, and advanced technology-based bioreactors. However, many drawbacks associated with conventional POME treatment methods like low pH, long HRT (hydraulic retention time), large sludge left outs, large land area requirement, and lack of GHG (CO2 and CH4) entrapment devices [5, 6] paved the way for better advanced alternative methods. Also, these systems have proven to be non-sustainable and ineffective ways of treating palm oil bioresources with minimal financial gain. Despite the existing drawbacks of conventional biological treatment systems, they are the primary methods of choice for POME treatment to date due to the low cost associated with their operation and establishment [6, 7]. Moreover, over the years, biotechnological advancements in POME treatment systems with maximum biogas retrieval capacity have only achieved by combining the various POME treatment methods such as biological methods, membrane filtration, ozonation, biological oxidation, coagulation and flocculation, adsorption, sonication, or integrated methods [8]. The most notable development in terms of design of POME biotreatment configurations is chiefly targeted at acquiring the minimum waste processing time, less establishment area and cost, minimal to zero level BOD- and COD-reducing ability, and highest biogas yield. Presently, several high-rate reactors and advanced hybrid bioreactors configurations are in use to efficiently treat POME such as anaerobic digestion, upflow anaerobic sludge blanket system, expanded granular sludge bed reactor, upflow anaerobic sludge fixed-film reactor [9], continuous stirred tank reactor [10], modified anaerobic baffled reactor [11], integrated anaerobic-aerobic bioreactors [12], membrane bioreactors [13], and ultrasonic membrane anaerobic system [14]. Table 1
The major focus of this review will be to provide the recent overview of POME treatment technologies using various bioreactor configurations in vogue. This review paper’s sole purpose is to provide a viewpoint about the best POME treatment processes in use with the maximum biogas output and to give an understanding about the recent trends and reconsiderations in the bioreactor designs in order to enhance their efficiency and effectiveness. However, the various aspects of POME processing, polishing, and purification of palm oil and the different treatment methods have already been reviewed in the past few years [15, 16].
Current/Future POME Applications Palm oil mills are responsible of generating large quantitities of POME during palm oil production [17]. Malaysia’s national production rate of POME is 0.67 m3 per tonne of FFB processed by mills [18]. POME is produced during the palm milling processes as sterilizer condensate, clarification wastewater, and hydrocyclone wastewater [19]. It is a brownish liquid composed of high organic matter, residual oil and grease, and total solids (with high BOD and COD values), and the collective account of its typical characteristics are given in Table 1. Production of high-strength or low-strength POME depends upon factors like composition of the raw material used and palm oil production process type [19]. Due to the environmental implications associated with POME’s untreated disposal, palm-producing countries like Malaysia and Indonesia have imposed stringent pollution control regulations on its palm oil mills which led them to innovate and
Characteristics of POME [17]
[20]
[21]
[22]
[23]
[7]
[24]
4.2–4.9
3.70–4.70
13,630–15,380
48,300 13,420–37,020 2080–27,040
18,225–23,900 45,818–54,861 46,304–63,192 24,846–30,920
85 4.2 25,000 51,000 40,000 18,000
4.7 25,000 50,000 40,500 18,000
745–935 255–342 -
10,520–31,220 1920–25,800 -
5614–8812 670–780 -
34,000 6000 750 -
34,000 4000 35 750 -
Concentration range (mg L−1) T (°C) pH BOD COD TS TSS
4.18–4.17 30,100 ± 10,390 68,100 ± 7610 28,900 ± 3070 -
80 ± 1 3.4 ± 0.1 37,750 69,500 47,690
TVS VSS O&G NH3-N TN TP TVFA
10,540 ± 920 780 ± 50 608 ± 81 470 ± 240
30,870 692 -
37,000–61,000
All parameters in milligrams per liter except pH, - not reported T temperature, BOD biochemical oxygen demand, COD chemical oxygen demand, TS total solids, TSS total suspended solids, VSS volatile suspended solids, O&G oil and grease, NH3-N ammonia-nitrate, TN total nitrogen, TP total phosphate, TVFA total volatile fatty acid
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upgrade their technologies to comply with these limits [17]. Moreover, Lorestani et al. [25] gave an overall estimate by quoting that a 14.8 million tonne palm oil production creates 53 million cubic meters per year of POME, which is indeed an unmanageable amount to dump. In addition, POME processing utilizes around 5–7.5 t of water to produce one ton of palm oil, more than half of which become POME [26]. Although, non-toxicity and higher nutrient and mineral contents of POME make it a perfect agricultural fertilizer or animal feed, still there is a need of its treatment to bring it to a usable form by removing its higher organic load [27]. Various biochemical and bioconversion technology-based bioreactors have been designed to treat the high organic content of POME, that utilize it to create many useful products like carotenoid used for the production of biogas, biodiesel, bioethanol, biohydrogen, bioplastic, and organic fertilizers [28, 29]. The applicability of POME can be enhanced by further advancements in the design of these bioreactors helping them to enhance its byproducts and reducing their production cost by making them cost effective [30]. Also, carefully designing and constructing these wastewater treatment facilities along with proper safety, operation, and maintenance procedures can solve many processing issues associated with them [31]. Primary POME treatment methods are the combination of mechanical and biological techniques to reduce the total suspended solid (TSS) level and organic load rate (OLR) in treated effluents [32]. Mechanical pretreatment methods are ultrasonication, coalescer filtration, and dissolved air flotation [33]; on the other side, biological means involves microbial consortia aiding biological degradation of POME in a bioreactor or biodigester (either anaerobic or aerobic) [34] and transforming them into methane and other useful inorganic products [35]. The bioreactor, the heart of the bioprocess, is a large vessel designed specifically for waste biotreatment under controlled optimized parameters to achieve maximum yielding results [36]. Whereas, POME treatment ability of the bioreactors has reportedly increased with the use of wellcharacterized microbial consortia (catalyze complex series of biochemical reactions that break the complex organic content into simple compounds such as CO2, CH4, H2S) as compared to the undefined microbial populations. Only few strains with oil-degrading capability have been characterized and successfully used in commercial palm oil treatment [37, 38]. The complexity of the microbial population has limited the efforts to characterize and validate novel microbial strains [39]. Furthermore, various reactor configurations have been explored to efficiently treat POME and their BOD and COD removal rates were compared along with biogas yielding capacity (Table 2). Biogas, a by-product obtained during POME treatment, is a valuable alternative source of energy which is composed of methane (60–70%), carbon dioxide (30–40%), H2S (0 to 0.1%), and H2 (0–10%) and its use as a biofuel is gaining popularity worldwide [59]. In addition, POME-based
power grids, an evolving alternative of renewable electricity supply (especially in palm-producing nations), are the most sustainable way to ensure the energy security needs of an increasing population, without causing any environmental implications on their surroundings [60, 61]. However, use of POME as a potential power source (with reported 10% expected power generation potential) is still in its initial stage whereas palm oil fiber and shell (with 46% expected power generation potential) are reportedly extensively used for a private power generation purpose [62]. POME sludge can be used as an organic fertilizer for agricultural application and is cheap and more effective compared to chemical fertilizers [63, 64]. Moreover, POME treatment systems are still in its infancy; with their reactor design and configurations evolving, there is still tremendous potential in this biomass in terms of its applicability [65]. Developing suitable mathematical models for the bioreactors and integrating the available molecular data in their design and operation will help to create highly automated, advanced, and improved bioreactors with better metabolite efficiency and high cost effectiveness [26]. Also, further research needs to be done in the area of bioprocess engineering and bioprocess microbiology which are still in their initial stages and need to be critically understood and implemented to make processing more efficient and productive.
Various POME Wastewater Treatment Methods Conventional Ponding System and Open or Closed Digesting Tanks Various POME treatment methods are prominently used to efficiently treat palm oil industrial effluent with no or minimum implications on its discharge (Table 3). Ponding system is the most widely used POME treatment method in palm oil mills (85%) of Malaysia followed by open digester tanks [6]. The configuration of POME-treating ponding systems (also called waste stabilization ponds) is a combination of different types of ponds having different functions such as anaerobic, facultative, and aerobic [66]. Additionally, this multistage process also involves few other series of ponds (cooling pond, mixing pond, anaerobic ponds, facultative anaerobic ponds, and algal ponds) with an overall HRT of 100 days [39]. Pond selection in the ponding system depends upon the processing capacity of the palm oil mills or the quantity of effluent produced by them. In a normal ponding system, an oxygendeficient environment inside the pond hinders the aerobic decomposition of wastewater and turning decomposition anaerobic which leads to the release of biogas (containing methane, a harmful greenhouse gas whose content varies with variation in the chemical properties of POME) into the atmosphere. Incorporation of various CH4 emission reduction technologies
Curr Pollution Rep Table 2
POME wastewater treatment ability using various bioreactor configurations
Bioreactor type
pH OLR (g COD L−1 day−1)
Temp HRT (°C) (days)
COD removal (%)
CH4 (%)
CH4 yield
Ref
Open and closed digestion system Closed digesting tank
11.5
7–9
37
N/A
> 50
49
N/A
[40]
8
7
N/A
N/A
> 95
55–61
0.9 m3 m−3 day−1
[41]
Open digesting tank
N/A
N/A
N/A
20
80.7
36
0.51 g day−1
[42]
Anaerobic digestion IAAB IAAB IAAB UASB (2-stage) UASB UASB UASB UASFF
N/A 15.6 19.5 10.5 16.6 12.5 5.8 2.2–9.5 16.5
N/A 7.53 N/A N/A 5.8 7.5 7 7.2–8 7.53–7.58
55 50–55 N/A N/A N/A 35 55 37–57 N/A
7.0 N/A N/A 4.6–6.5 0.9 4 5–10 2.4 2.2
N/A 95 99 99 90 82.4 > 90 76–86 90–94
60 67.50 N/A 64 28 40–48.2 55–66 76 N/A
0.47 0.31 L CH4 g−1 COD removed 0.16–0.24 m3 CH4 (STP) g−1 COD 0.24 L CH4 g−1 COD removed
[43] [44] [45] [46] [47] [48] [49] [50] [51]
UASFF
2.63–23.15
7.2–8.0
38
1.5, 3
89–97
60–82
N/A 0.91 L CH4 g−1 COD 0.60 L CH4 gVS-added 0.03–10.6 L (STP) CH4 day−1 0.309–0.329 L CH4 g−1 COD removed 0.346 L CH4 g−1 COD removed
UASFF
N/A
7.0–8.O
N/A
1.5–2.9 > 90
61–80
> 0.3 L CH4 g−1 COD removed
HUASB MBR
1.83–5.5 N/A
6.5–7.5 5.0–9.0
24 N/A
2.73 N/A
90–98 58.80
N/A N/A
N/A N/A
ABR EGSB
15.08 6.45
6.8 6.8
N/A 35
N/A 9.8
68.78 94.89
EGSB CSTR
5.8 19.8
7 7–8.0
55 55
5–10 3.3
> 90 82
N/A N/A 82.9–88.6 0.29–0.31 m3 biogas (STP) kg−1 COD 51–61 0.55 L CH4 gVS-added 60–72 0.27 L CH4 g−1 COD removed
[52] [53] [54] [55] [56] [57] [49] [58]
N/A data unavailable
such as biogas capture, decanters, co-composting with EFB, and denitrification can be used to minimize methane gas emissions from POME [67]. Modified aerated or aerobic pond is an expensive and energy-dependent method, where mechanical aeration is done using aeration devices to speed up wastewater biodegradation [89]. On the other hand, anaerobic pond is considered as the most effective method to treat highstrength wastewater by anaerobic decomposition [67]. These ponds are highly energy efficient which are less smelly with minimal sludge output and are an efficient way to produce valuable methane (assisted by anaerobic bacteria) in the form of biogas. Emitted biogas can be prevented from getting released into the atmosphere by using sheets to cover these ponds which reduces any chances of environmental issues. POME-derived biogas is a valuable source of energy which is composed of methane (60–70%), carbon dioxide (30–40%), H2S (0 to 0.1%), and H2 (0–10%) [59]. Anaerobic pond under normal operation condition produces 54.4% of CH4, if captured and conserved then it can be used as biofuel or a source of bioelectricity generation [39]. The anaerobic stabilization ponding systems are mainly designed by taking various factors into consideration such as solids retention time, hydraulic retention time, concentration of influent/effluent, and sludge
age. Limited literature availability confines our understanding about the evolving nature of the ponding system. This could be attributed to its drawbacks like large land requirement for pond establishment, inefficiency to produce dischargeable POME, and difficulty in maintaining long-term anaerobic conditions [90, 91]. Moreover, anaerobic pond systems are reportedly said to be more efficient than open digesting tank system for the treatment of POME along with the higher recorded methane emission and higher organic conversion efficiency. Their methane emission pattern is affected by the quality/quantity of discharged POME, which also was the ultimate result of palm mill activities and seasonal cropping of palm oil [68]. On the other hand, open and closed digesting tanks are the second highly used wastewater treatment system after the ponding system with a wide range of volumetric capacity and similar design. These tanks have low capital and operating costs, limited land area requirement, and short HRT (20– 25 days) that make it a popular choice in spite of several environmental implications [42, 69]. Reportedly, open digesting tanks were found to release significant amounts (35%) of harmful biogas (5.5 kg of CH4 per ton) into the atmosphere due to the lack of any gas retrieval device in the
Curr Pollution Rep Table 3
Advantages and disadvantages of various treatment methods
Treatment types
Advantages
Ponding system/open digesting tanks/closed digesting tanks
[41, 42, -Require large treatment area. 66–70] -Extended HRT -Problem associated with the solid accumulation at the bottom -Harmful methane greenhouse gas emission contributing to global warming, which is unacceptable -Inefficient in removing high level nutrients [71, 72] -Long HRT -Yield a valuable biofuel, i.e., biogas by collecting gases released during the process as a by-product -Long start-up required for the granulation process -Sludge generated during the process can be used as -Large plant setup area required fertilizers due to its zero toxicity and high nutritive -Inefficient in utilizing wastes properly; more than half of the waste left untreated that otherwise have generated profile ample amount of valuable biofuel -No external mixing required -Superior quality of products is attained -Comparatively smaller treatment area required -2-stage digestion allows more flexible approach while separating acidogenic and methanogenic phases -High start-up period [43, 49, -Simple to operate -Seeded sludge not required 73, 74] -Minimal process monitoring/control required -Suspended and colloidal components of wastewater -Known for high suspended solid wastewater negatively affect or hinder microbial activities leading treatment to poor bioreactor performance -More robustness and stability while dealing with high OLR, no foaming issue, or biomass washout issue [73, -Larger reactor height/surface area allows it to handle -Preferred for small-scale industries 75–78] -Sludge granulation requires a longer start-up period high OLR -Innovative influent distributor allows proper contact -Rapid mixing will reduce start-up time but affects the methanogenic activity leading to bioreactor perforof biomass with wastewater mance instability -Known for its process stability -Expense associated with the granule purchase and its -Sludge can be used as biofertilizer to enhance soil transportation to plant fertility, a great substitute to chemical fertilizers -Inorganic matter accumulation can pose a problem making sustainable farming possible -Foaming problem at high OLR -Suspended solids may interfere with its performance, so [53, -Ability to deal with high OLR and high HRT 79–82] reducing its OLR capacity -Good hydraulic contact between the sludge and the -Less efficient gas production at high treatment volume wastewater -Less biomass retention -Withstand shock loading -Can treat mid to low organic content wastewater with -High volatile fatty acids can obstruct the granulation process, ultimately affecting the bioreactor the same effectiveness as the highly organically performance loaded wastewater [58, -Difficulty to withstand high OLR - Thorough mixing allowing more wastewater to 83–86] -Continuous stirring sometime lead to biomass washout, biomass contact thereby adversely affecting the bioreactor’s -Improved biogas productivity performance [20, 45, - Cost effective, efficient, and with smaller footprints -High start-up period -Its design and process development are still in its infancy 46] -Best choice with constraints like space utility due to limited research in this area -Stepped organic loading leads to rapid biomass -Despite excellent treatment ability, its industrial-level development studies awaited -Hollow centered packed bed minimizes operational problems like foaming and clogging -Compartmentalization adds to the microbial robustness and enhances their ability to efficiently biodegrade -Less area required for establishing its treatment setup -Highly expensive [13, 87, -Gives best result with pretreated effluents or as -Membrane fouling responsible for shortening the 88] posttreatment method membrane life
Anaerobic digestion
UASB/H-UASB
EGSB
UASB-FF
CSTR
IAAB
MBR/MAS
-Cost effective-low capital cost associated with its establishment -Ability to handle high OLR -Least maintenance required -Reliable, stable and energy efficient
Disadvantages
References
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system [59]. In addition, POME treated using an open digested tank also further needs to be treated and refined using facultative and aerobic ponds to bring down the effluent organic load (BOD and COD) to an acceptable discharge limit thereby making it a more time-consuming process. While on the other hand, closed digesting tanks are armed with advanced controlled system along with safety valves and sophisticated monitoring devices which offer better optimization of the process and better biogas recovery. They are fully covered with covers which prevent the emitted biogas from getting released into the atmosphere. Comparatively, the methane production rate of closed digesting tank is significantly higher (5019 kg d−1) than open digesting tanks (518.9–1043.1 kg d−1) [41]. However, close digesters experience some stability issues like fluctuation in biogas yield in the long run; this is primarily due to various factors such as seasonal palm oil cropping or harvesting, variation in industrial wastewater type, and quality or quantity of wastewater produced. Also, merits and demerits of this process were also discussed in a comprehensive review by Salihu et al. [34]. Moreover, a comparative account was made between the open and closed tank digestibility tests and found that open digestive tanks’ durability rate was lesser (60%) as compared to the closed digesters’ (85%), due to oxygen transfer into the close tank while tank feed induction [59]. Also, there is a large gap between establishment costs of the open and closed digesting tanks over other highly advanced POME treatment systems thereby making it an attractive choice to date [70].
Anaerobic Treatment Methods Various efforts have been made to create a sustainable and financially less demanding biological process to treat high organic load-containing wastes like POME [40]. As a result, anaerobic wastewater treatment technologies were innovated, which have proven their effectiveness over the years in treating high-strength wastewaters with great efficiency and with high ability to retrieve bioenergy gases [92]. These methods have gained popularity among people due to their technological simplicity, economical reliability, and sustainability [93]. In the late 1980s, Jans et al. [94] had presented the most practical design for anaerobic treatment-based bioreactors that could efficiently treat various types of industrial wastewaters. Furthermore, Khalid et al. [95] have classified the anaerobic treatment-based bioreactors based into three categories like batch reactors, one-stage continuously fed system, and two stage or multistage continuously fed system. Also, Poh et al. [6] gave a comprehensive review of various anaerobic treatment practices in use for treating POME wastewater. Recently, Khanal et al. [96] had broadly discussed about the recent developments and advancements in the area of anaerobic bioreactors.
Anaerobic Digestion Anaerobic digestion is an anoxic way to degrade complex organic wastes in an anaerobic digester by using a consortium of anaerobic microorganisms (chiefly methanogens) which produce biogas as an end product [43]. In this process, the microbial sludge acts as the chief reaction center where four different types of microbes (hydrolysis, acidogenic, acetogenic, and methanogenic complex) collectively convert complex organic compounds into a simpler one [47]. A difference in the physiology and the nutritional requirements of these four groups of bacteria increases their acclimatization time in the reactor [97]. In addition, methanogens perform best at neutral pH while other groups require less pH to thrive; such conflict of interest increases biogas production time [98]. So, two-stage (acidogenesis stage and methanogenesis stage) anaerobic digestions can solve this kind of pH issue and also can improve the biogas retrieval time at the same time [99]. Furthermore, anaerobic digestion of palm oil mill effluent can result in 85% of COD removal by using recycle sludge in a thermophilic continuous digester [71]. Also, the organic binding properties of microbes bring down the higher level of POME’S COD to acceptable dischargeable COD values [72]. A better understanding of design and innovations of anaerobic biodigester helps in the effective applicability of this reactor. In addition, Lee et al. [100] have given an elaborate account of the best anaerobic biodigester designs used in the wastewater treatment industry and also discussed about its stage of development to make this digester more efficient and productive in terms of metabolite yield (i.e., biogas) and minimal remaining residues. In addition, decoupling of the solid retention time to HRT was suggested to prevent the washout of slow-growing methanogens from the reactor (by using granulation and flock formation, membrane-assisted biomass retention, microbial biomass recycling, biomass immobilization in attached growth systems) to improve its wastewater processing ability. Anaerobic Granular Sludge Treatment Processes Anaerobic treatment processes are evolving ever since they came into existence in the late 1960s. Due to their cost effectiveness, low energy demand, ability to handle high OLR, better dewaterability, and great process stability, these processes were opted worldwide to treat diverse varieties of wastewater. A few years back, Lim et al. [101] wrote a comprehensive review about anaerobic granular sludge treatment processes, i.e., upflow anaerobic sludge blanket (UASB), expanded granule sludge blanket (EGSB), and static granular bed reactor (SGBR), and also gave their comparative account while discussing about the characteristics, application trends, and limitations associated with them. The former two techniques are dominantly in use to treat high-strength wastewater,
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while the last one is in its initial stage of development and is used to treat mid- or low-strength wastewater [102]. In addition, applicability of these processed to treat wastewater along with their characteristics, involved anaerobic granulation type, industrial scope, and future prospects was reviewed in detail by Liu et al. [103]. Upflow Anaerobic Sludge Blanket System Upflow anaerobic sludge blanket (UASB) is a promising way to treat highstrength wastewater like POME. This reactor is a granulated sludge-based high-rate anaerobic treatment technology used to treat various types of wastewater [104]. Granulated sludge helps in retaining a large number of microorganisms inside a reactor and in attaining high waste stabilization, which enhances the performance of the UASB reactor. Many factors like upflow velocity rate, pH, and nutrient level can influence the quality of granulated sludge type [105]. Furthermore, the chief features of the UASB reactor are better sludge-retaining capability, high efficient particle separation, short HRT, high OLR, better energy efficiency, low setup land requirements, and enhanced ability to harness useful bioenergy by-products [106]. Moreover, UASB reactors have reported to treat POME with 90% COD removal at 5 days of HRT, OLR of 5.8 gVS (L-reactor.day) −1 , to produce methane yields of 436 mL-CH4 g−1 VS-added [73]. While in other research, COD removal efficiency of the UASB at mesophilic conditions (57 °C) was observed to be 76–86% at an OLR of 2.2– 9.5 g COD L−1 day−1 along with 13.2 L biogas day−1 (with 76% methane content) [49]. However, many drawbacks are also associated with this technology like upflow velocity variations, higher hydraulic pressure-leading granules washout, and longer start-up period calls for the modification of UASB reactor design [48]. A detailed review on upflow anaerobic sludge blanket reactor was written by Bal et al. [50] that gives us in-depth insights into the processing ability and biogas retrieval ability of the UASB reactor. Earlier, this technology has already been used to treat the various types of industrial wastewater like POME, distillery wastewater [107, 108], slaughterhouse wastewater [108], and industrial wastewater [109]. Moreover, Borja et al. [43] have developed a hybrid UASB (H-UASB) with capacity to form granules in a twostage process by combining the UASB and anaerobic filter (AF) concepts together with improved efficiency and better biogas production rate. However, pilot-scale or even industrial-scale implementation of these advanced versions at the industrial level is still awaited. Also, H-UASB run on the mesophilic temperature of about 49 °C reportedly had shown 91% COD removal rate as compared to the room temperature-operated reactor and suggested the possible role of sludge granule morphology in the effective anaerobic treatments [110]. Also, H-UASB offers sludge immobilization and higher loading rate with shorter HRT and generates higher biogas production [74]. In addition, H-UASB had
successfully treated the POME with the COD strength of ± 5000 mg L−1, OLR of 1.83 g COD L−1. Day and HRT of 2.73 days for the start-up operation which finally resulted 98% of COD removal with shortening of start-up period from 60 to 47 days [74]. Other wastewater treated using H-UASB are pharmaceutical wastewater [110], pulp and paper mill wastewater [52], and poultry slaughterhouse wastewater [111]. Also, the applicability of H-UASB technology to treat various types of wastewaters has been reviewed in detail by Rajathi [112]. Advanced Anaerobic Expanded Granular Sludge Bed Reactor Expanded granular sludge bed (EGSB) is an advanced anaerobic system; popularly known as thirdgeneration anaerobic systems, it is a granular sludge-based wastewater treatment technology to treat low- or highstrength POME [75]. Its staged reactor structure is preferred over the vertical slim column structures due to its ability to make processing more efficient [113], and further, several efforts have been made to optimize its structure by using 2D/3D computational fluid dynamics, hydrodynamics, and biokinetic studies [114] to check for the applicability of this reactor industrially; industrial-scale pilot EGSB reactors were used and reported better performance in treating POME [5, 76]. It is a modified version of granular sludge-based traditional UASB technology [both granular sludge-based technologies], which works at a higher superficial rate or upflow velocity (7–10 m h−1) in comparison to the UASB (0.5– 1.5 m h−1), leading to higher recirculation rates, better hydraulic mixing, improved sludge bed expansion further increasing wastewater to sludge contact, and mass transfer; ultimately, biogas production increased [57, 115]. However, the performance of both EGSB and UASB to treat raw and de-oiled POME was compared and the latter reactor was found to be more stable with more methane gas emissions [73]. The impact of HRT at a similar OLR was evaluated in a two-stage EGSB and found that dealing with higher COD at a slower rate yields better results than lower COD at a faster rate [77]. A few years back, Wang et al. [76] using a pilot-scale EGSB reactor to treat high-strength POME operated at HRT of 9.8 days achieved a COD removal efficiency rate of 94.89% and 27.65 m3 biogas per cubic meter of discharge (65–70%) was also obtained. In addition, a bench-scale EGSB reactor was applied to treat POME and reported COD removal of 91% at HRT of 2 days and OLR of 17.5 kg COD/(m3 × day) at 35 °C [78]. However, the reliability of the laboratory-scale EGSB trial results is still a matter of further investigation. Recently, this issue has been addressed by Connelly et al. [116] using various advanced methods to monitor the effect of scalability on bioreactor performance, microbial ecology, and microbial community physiology and found acetotrophic and hydrogenotrophic methanogenesis in laboratory-scale experiments with better overall performance level in comparison
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to the full-scale plant with predominant hydrogenotrophic methanogenesis. Also, the microbial communities associated with the EGSB were analyzed to understand their bioderivative role in the POME treatment process [117, 118]. This technique has also already been used to treat dairy wastewater [119] and slaughterhouse wastewater [120].
Upflow Anaerobic Sludge Fixed-Film Reactor Upflow anaerobic sludge fixed-film (UASFF), a modified UASB-fixed film (UASB-FF) bioreactor, is an anaerobic hybrid technology which is a combination of UASB and upflow fixed-film (UFF) in a single reactor. The performance of this hybrid anaerobic reactor is chiefly dependent upon the quality and biological behavior of granular sludge under different sets of conditions [54]. Unlike UASB reactors, the treatment of POME using UASFF bioreactors usually shows a short start-up time with an accelerated anaerobic sludge granulation which helps it achieve 97% of COD removal at 3 days of HRT [79]. This hybrid reactor overcomes the drawbacks associated with these two reactors like instability, lower efficiency, and less loading rate to attain better performance in terms of higher biofuel production [51] to help in improving biofuel yield. Moreover, biohydrogen production using UASB-FF reactors has reportedly outperformed the UASB reactors by overcoming the drawbacks like long start-up period, upflow velocity fluctuations, and granule washout. Mohammadi et al. [80] reportedly used UASB-FF to achieve hydrogen production yield of 0.31 L H2 g−1 COD from POME using response surface methodology (RSM) at feed flow rate and upflow velocity of 1.7 L day−1 and to 0.5 m h−1. Likewise, similar combinations of technology and operating variables were analyzed to achieve better POME treatment efficiency, and the best results were obtained with the feed flow rate and upflow velocity of 2.45 L day−1 and 0.75 m h−1, respectively, with 82.4–90% COD removal [81]. Also, Zinatizadeh et al. [82] further documented the maximum methanogenic activity (0.99 g CH4-COD/g VSS day) by using influent biomass concentration of 2000 mg VSS L−1 along with influent COD value and initial alkalinity of 10,000 mg COD L−1 and 2000 mg CaCO3 L−1, respectively. In another study, they reported high COD removal rate (28 g COD L day−1) at an OLR of 34.73 g COD L day−1, i.e., equivalent influent COD of 34,725 mg COD L−1 and 1 day HRT [53]. Overall, these results suggests that the UASFF reactor is one of the most efficient technologies to successfully treat POME with maximum COD removal rate at short HRT due to its capacity to retain high biomass even under high OLR. The UASB-FF bioreactor has been already used to treat various biological wastewater types like cheese whey wastewater [121], suspended mixed wastewater [122], and antibiotic plant wastewater [83].
Continuous Stirred Tank Reactor Continuous stirred tank reactor (CSTR) is another reactor type which is in wide use for the treatment of high-strength raw POME. It is a methanogenic anaerobic digestion process with continuous mechanical or hydraulic agitation. Continuous stirring in CSTR helps in maximizing biogas production by increasing contact between POME and the microorganisms by providing uniform pH and temperature [83]. Plant-scale POME treatment using this reactor has reported 83% of COD removal efficiency along with 62.5% of biogas production [84]. However, this mixing also poses problems like active biomass washout at short HRT leading to the decline in the CSTR performance which is one the major problems that need to be improved upon by designing a better version of this reactor [85]. Also, shear stress generated through excessive mechanical agitation in CSTR (high OLR) can lead to decrease in biogas production [10]. Similarly, CSTR’s instability at high OLR used to be another drawback that was limiting it only to be an indirect means of POME treatment (used only to polish treated POME), but recent technological advancements in its design made it an important direct method of POME treatment [10]. So, efforts to achieve stable CSTR operation at high OLR (an OLR of 26.5 kg m−3 day−1) were reportedly made possible by periodic addition of chitosan which helped in improving POME treatability by flocculating the anaerobic sludge (which increases the number and the retention time of methanogens inside the reactor), leading to a higher methane production rate (9.39 m3 m−3 day−1) [86]. Also, modified CSTR had been used to treat raw POME at 55 °C, OLR of 19.0 g COD L day−1, and reported biogas production rate of 6.23 L L day−1 and 82% COD removal efficiency. And, Clostridium sp. was found to be the major dominant bacteria in the reactor responsible for the hydrolytic biodegradation of POME to maximize methane production [10]. On the other hand, POME treatment using CSTR is reported to produce more biohydrogen (2.16 L day−1) with 44.7% COD removal rate whose continuous mixing helped to achieve maximum substrate to biomass contact and its performance was found to be stable in long-term operations [58]. The CSTR system also has been applied for the treatment of different types of wastewater like zinc-containing wastewater [123], food wastes [124] and hydrocarbon-rich industrial wastewater effluents [125], and jam wastewater [126]. Modified Anaerobic Baffled Reactor Anaerobic baffled reactor (ABR) is a promising technology that can effectively deal with high organic character of POME [127]. Its ability to effectively acquire the separation between the HRT and SRT allows microbes to remain undisturbed despite continuous flow of wastewater. Other ABR advantages are its simple cost-effective design with shock loading-
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enduring capacity and its ability to handle high-volumetric loads [128]. But, few drawbacks in the conventional design of ABR had led to the construction of the modified version to fix the issues pertaining to its efficiency. In addition, the modified form of ABR (MABR) is the modified version of the old ABR that offers many significant features lacking in the previous one with great improvement in its ability to treat complex wastewater released from POME industries. Recent advances in the ABR are reviewed in detail by many researchers explaining its applicability in various fields and suggested few modifications to enhance its treatment efficiency to make this reactor a potentially useful technology in the long run [129]. The effect of the recycling ratio on maintaining the alkalinity in the reactor and a suggested recycle of more than 15 times would be of great help in stabilizing the internal pH to 6.8 without the need of any external alkalinity supplementation [11]. In addition, temperature conditions could be of great help to improve reactor performance. Thermophilic MABR was found to offer the maximum COD removal rate (90–95%) with reported increase in biogas production as compared to the mesophilic one [130]. Moreover, a significant modification in the general anaerobic baffled reactors allows it to treat the various degrees of complex wastewater with minimal usage of power thereby reducing the capital costs associated with POME treatment. The mechanism associated with the MABR and its kinetic and hydrodynamic studies was discussed in detail by Hassan et al. [56]. This reactor type has also been successfully used to treat different types of wastewater which have also been treated using ABR, that include recycled paper mill wastewater [131], yeast manufacturing wastewater [132], tapioca wastewater [133], and synthetic wastewater [134]. Integrated Anaerobic-Aerobic Bioreactors Integrated anaerobic-aerobic bioreactor (IAAB) is a high-rate anaerobic bioreactor with stacked configurations which are known for its processing efficiency, stability, and shorter start-up period [12]. Although, design of IAAB and advancements in its operation and processing ability are still in its developmental stage, many novel researches with great waste treatment ability assure its significant contribution in the area of waste treatment in the coming future. IAAB is an integration of three compartments, i.e., anaerobic, aerobic, and settling chambers in a single bioreactor with improved waste degradation capacity and methane production [46]. Its anaerobic compartment (a hollow cantered packed bed) is armed with UASFF technology along with a gas-liquid-solid separator (GLSS), while the aerobic compartments are loaded with activated sludge. Using this reactor, Chan YJ et al. [45] (laboratory scale) was able to achieve 99% BOD and COD removal in POME wastewater at an OLR up to19.5 g COD L day−1 and better biogas yield (with 48–64% of
methane, 0.16–0.24 L CH4 (STP) g−1 COD removed) [20]. However, a previous study also reported that high OLR leads to decline in COD removal rate of the anaerobic chamber, while increase in the aerobic one by using precise performance estimating models like Stover-Kincannon model for anaerobic and Monod model for aerobic chambers. Similar results were obtained using IAAB for POME treatment with an overall COD, BOD, and TSS removal around 99% at 10.5 g COD L day−1 OLR and 45 days HRT to produce biogas (with 64% methane) and methane retrieval around 0.24 L CH4 g−1 COD was achieved [45]. Thermophilic IAAB reactors (50–55 °C) were reportedly found to perform better while treating POME as compared to the mesophilic systems and its overall BOD, COD, and TSS removal rate was optimized to be 99.6% using RSM along with the methane production rate of 0.31 L CH4 g−1 COD removed [45]. Such ability to handle higher OLR with excellent POME-treating efficiencies makes IAAB as one of the highly efficient reactors till now, but its scale up needs to be further optimized and thoroughly researched before taking this technology to the industrialscale level. The IAAB reactor also has been used to treat other wastewater types like slaughterhouse wastewater [44], food processing wastewater [135], and synthetic domestic wastewater [136]. Membrane Bioreactors and Ultrasonic Membrane Anaerobic System Membrane technology is used as the potentially useful treatment method in terms of bioresource recovery like biogas, where consortia of microorganisms actively treat POME using membrane filtration methods like microfiltration or ultrafiltration and efficient removal of suspended solids. It is a combination of the activated sludge process based on the membrane separation process that is used as a substitute clarifier to bring down the COD and BOD values to within the acceptable discharge standard limits, and increasing its biogas productivity as well. Membrane bioreactor (MBR) technology provides several advantages over others like high-quality effluent, short HRT, higher OLR, and less sludge production, but few disadvantages like membrane fouling, higher energy consumption rate, higher membrane costs, and short membrane life often trouble MBR users [137]. Membrane fouling mitigation solutions have been reported like using biofunctionalized super paramagnetic nanoparticles with an ability to prevent membrane fouling [138]. Also, as a solution to this problem, a modified version of MBR, i.e., ultrasonic membrane anaerobic system (UMAS), is used to overcome the MBR fouling problem [14]. In addition, application of bacterial quorum sensing has been found to be a successful novel weapon against membrane fouling in MBR [139]. POME has been treated using MBR pilot plant
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with a reported decline in their BOD and COD levels of about 61.2 and 58.9%, respectively, and an ultrafiltration membrane system can be further used to achieve 99.9% overall waste degradation. Such excellent treatment efficiency of MBR is responsible for its growing popularity worldwide [13]. In addition, effective treatment of POME using MBR in combination with baffled air flotation to achieve 97% COD removal with 93.9% TSS removing rate achieved 99.8% of turbidity [87]. A comprehensive review was written about the current trends associated with MBR bioreactor development and role in treating various types of wastewater [55, 140]. MBR also has been used to treat various types of wastewater like industrial wastewater [138] and cosmetic industry wastewater [141]. Recently, model-based methodology has been assisting the plant operators in designing proper strategies to maximize the functionality of MBR wastewater treatment plants with reported 40% enhanced nitrogen removal capacity [142]. Also, Guglielmi et al. [143] proposed a steady-state CODbased approach for MBR with maximum nitrogen removal along with the possible criteria for membrane bioreactor selection and appropriation of its design and they also widely discussed about the role of constituent materials, membrane processes involved, and factors on the overall performance of MBR. Similarly, insight into the design, operational aspects, and maintenance of MBR are widely discussed by Park et al. [144]. A scope of a new methodology, reverse membrane bioreactor, still needs to be explored to treat high-strength effluents like POME, and recently, its future prospects and applicability potential have been discussed in detail by Mahboubi et al. [88] (Table 4). Such systematic approaches would help to explore the potential of operational improvements of a plant by carefully calculating and identifying the optimum input variable values (using algorithms) to achieve optimum operational strategies with minimal cost function.
Conclusion Undoubtedly, biotechnological advances have paved the way to sustainable reuse like high-strength effluents like POME. Once considered as an unmanageable agriwaste, it is now considered as a valuable biomass resource with their potential applicability in various areas. In an era of sustainable and cleaner production, its industrial or domestic applications like biofuel generation, bioelectricity, and biofertilizer production have grabbed worldwide attention. The importance of the various bioreactor configuration types (either conventional or advanced bioreactor types) to turn this biomass from waste to an asset cannot be denied. Many technological advances have already been made in the form of various types of bioreactor presently in use and many more innovative breakthroughs are still waiting in this field that could offer an efficient and sustainable way to manage hazardous wastes. Innovating the highly advanced and cost-effective bioreactor configurations needs an in-depth understanding of the pros and cons of their design to achieve their unfailing ability to treat/handle smallscale to large-scale load-bearing capability, only then will the palm oil industry be fully unburdened from the weight of POME. However, the ponding system is still the most prevalent POME treatment method in major palm oil-producing nations like Malaysia, Indonesia, the USA, and Brazil, but sustainable advanced bioreactor configuration’s popularity is gaining momentum day by day and would soon catch up with that of the conventional POME treatment methods.
Compliance with Ethical Standards Conflict of Interest interest.
The authors declare that they no conflict of
References 1. Table 4
New emerging POME treatment methods
POME treatment method type
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