Production of Biogas through Anaerobic Digestion of ...

Intl' Conf. on Chemical, Integrated Waste Management & Environmental Engineering (ICCIWEE'2014) April 15-16, 2014 Johannesburg

Production of Biogas through Anaerobic Digestion of various Waste: Review Rebecca Sebola, Habtom Tesfagiorgis, and Edison Muzenda 

financial outlays associated with establishment, operation and end-of-life management of the landfill site [2]. Hence, the conversion of biodegradable waste into energy has great potential of reducing landfills issues while delivering energy, economic benefits and social stability to the country. Although the government agencies are making considerable effort in tackling waste related problems, there are still major gaps to be filled especially in the solid waste management. Anaerobic digestion (AD) is one of the promising technologies for recovering energy from municipal solid waste. It is already a common alternative method for sewage and manure treatments. Since food waste has the advantage of high organic content compared with sewage or manure, AD is now increasingly considered as a viable alternative for recovering energy from the organic fraction of municipal solid waste, which usually has food waste as a main component. Anaerobic digestion is a biological process performed by many classes of bacteria and generally consists of four steps: hydrolysis, acidogenesis, acetogenesis, and methanogenesis, [3]. The main product this process, methane, can be used as a vehicle fuel or co-generation of electricity and heat, and thus, can lead to reductions in greenhouse gas emissions. Additionally, the transport sector, as one of the major contributors towards energy deficiencies and greenhouse gas emissions, is identified as an area that requires urgent intervention. More efforts are required to address the envisaged fuel shortage and mitigate the environmental challenges. This can be achieved through research and systematic programmes aimed at greening the economy through a low carbon and resource-productive economy [4]. The transport sector is particularly of great interest due to the high social cost of transport in South Africa. As a renewable and sustainable source of energy, several countries have used biogas as a preferred option [5]. However, the process of converting bio-waste to vehicular fuel in the form of compressed biogas (CBG) is a new technology in this country. In addition, there is not much information concerning how the efficiency of the energy recovery from the solid waste can be improved. The primary objective of this study was to review and workout an efficient co-digestion strategy that would maximize methane yield from the complete digestion of selected industrial sludge.

Abstract—Anaerobic digestion is proposed to produce biogas and enhance the methane production by identifying the best substrate. This paper reviews the biogas production from anaerobic digestion of various wastes. Feedstock composition is one of the major factors that affect the production of biogas. High yields of methane depend mainly on the substrates used as feeding material. However, the difference in total methane yield varies based on the type of interactions between different wastes that interfere with digestibility of wastes in the system. The rate of digestion of organic wastes depends mainly on the relative proportion of the component, the amount of the mixture and other physical variables such as temperature and pressure. There is limited information on the optimum conditions that can enhance methane yields and treatment of residues. It is, therefore, recommended that optimum conditions for anaerobic co-digestion must be investigated as well as treatment of sludge to manage the landfill crisis.

Keywords—Anaerobic-digestion, Feedstock Municipal solid waste, Waste generation

composition,

I. INTRODUCTION REQUENT rises in fuel prices and advanced methods of refining conventional fuels from crude oils pose a threat to the environment and calls for a search to find cost effective and environmentally cautious methods of finding alternative fuels and improving engine‘s efficiencies in fuel combustion [1]. A study has shown that Landfill gas (LFG) receives a great deal of attention due to both negative and positive environmental impacts, global warming and a green energy source, respectively. Due to the exhaustion of landfills, continuous complains from the people living in the vicinity of landfills, and environmental impact of landfills [2], like all methods of waste disposal, landfilling imposes both financial and external cost on society. Financial costs refer to actual

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M. Sebola is with the Department of Chemical Engineering Technology, University of Johannesburg, Doornfontein, Johannesburg 2028 (e-mail: Sebola [email protected]) H. Tesfagiorgis is with the Department of Chemical Engineering Technology, University of Johannesburg, Doornfontein, Johannesburg 2028 (e-mail: [email protected]) E. Muzenda is with the Department of Chemical Engineering Technology, Faculty of Engineering and the Built Environment, University of Johannesburg, Doornfontein, Johannesburg 2028, Tel: +27115596817, Fax: +27115596430, (email: [email protected])

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II. CURRENT ADVANCEMENT OF BIOGAS

In contrast, South Africa is one of the highest emitters of greenhouse gases per capita in the world. Carbon emissions per capita are comparable to developed countries, whilst human development indices remain far lower. Therefore, there is an urgent need to decrease the carbon intensity of the South African economy. Again, organic waste in South Africa usually ends up in landfill sites which results in loss of a potential energy resource whilst causing environmentally negative impacts. To introduce biogas technology as an avenue for renewable energy in South Africa will demonstrate the use of available local organic waste in this technology. Furthermore, South Africa is currently facing a potential liquid fuels shortage [17]. It is expected that the transport demand will increase significantly in South Africa, where more than a doubling is expected within the next 30 years [18]. The increase in the transport demand will strengthen South Africa‘s dependency on oil imports which are apparent due to the lack of own oil resources [17], as long as no alternative energy carriers are being used extensively. Currently, Coal to Liquid (CTL) technology is used to satisfy one third of the transport energy demand, which has about three time‘s higher greenhouse gas emissions than conventional petrol and diesel fuels. Therefore, an increasing demand for fossil based petroleum products goes hand-in-hand with an increase in energy related GHG emissions. On the other side the use of biogas as an alternative fuel in South Africa is not easy to determine due to limited research in this field. Even though some areas in SA use it for household use, the use for biogas for vehicular use is not yet explored which can be a possible solution to the current energy crisis.

In the European Union, both the primary production of biogas and the gross electricity production from biogas have increased by almost 18 % between 2006 and 2007 [6]. The greatest share of this growth was achieved in Germany with biogas companies expanding their business despite rising costs for substrate, especially in 2008. By far, Germany has a leading role in Europe with almost 4000 biogas plants, most of them on farms for cogeneration. Feedstock composition is one of the major factors that affect the production of biogas [7]. Therefore, when designing and operating an anaerobic digester, the quantity and characteristics of the feedstock are important and need to be assessed. Germany, Austria and Denmark produce the largest share of their biogas in agricultural plants using energy crops, agricultural by-products and manure [8]. Wastes generated from various industries differ significantly in both their qualities and quantities and depend on the industrial processes and products [7]. Since it is not economically feasible to treat these industrial wastes in separate digesters at each plant, a centralized treatment facility is recommended [8]. Further studies have shown that the total methane yield is linked with the type of interaction between different wastes that interfere with digestibility of wastes in ad processes [8]. Thus, it is necessary to separate the negatively interacting sludge pairs into different batches, as well as keeping all positively interacting pairs together in AD process to improve the overall methane yield [9]. The concept of AD whereby energy rich organic waste material or biogas crops are added to animal manure was realized in large scale biogas plants about two decades ago, have shown the state of the art of co-digestion on sewage sludge, the organic fraction of municipal solid waste (OFMSW) and energy crops with recent progress in research on anaerobic digestion [10], [11], [12], [13]. However, the most used basic substrate in agriculture is pig or cow manure in co-fermentation with biogas crops [14]. In contrast the United Kingdom, Italy, France and Spain predominantly use landfill gas [6]. While the biogas sector grows impressively every year, it hasn‘t received the same attention as for example liquid biofuels for transportation [15]. The majority of people are not aware that natural gas powered vehicles have been available for a long time and that bio-methane could play an important role in the transportation sector. So far, only Sweden has established a market for bio-methane-driven cars. Due to its relatively low prices for electricity, Sweden has traditionally used biogas for heat production (currently around 50 % of biogas) and focused less on electricity (8 %). About 25 % of the produced biogas is upgraded and used as vehicle fuel while the rest is flared or used for other applications [15]. The use of biogas in China began in 1930s and continued to develop until today due to improving technology and management system. Today biogas has become the biggest biomass energy industry in China [16].

III. ADVANTAGES OF USING BIOGAS Due to the increasing population, access to affordable energy services is becoming a prerequisite [19]. There is a strong correlation among energy availability and education, health, urban migration, empowerment, local employment and income generation, and an overall improvement in the quality of life [20]. Understanding and taking into account the current status of developing nations, biogas technology has implicit potential in improving waste management, producing clean energy, and creating employment. A considerable amount of renewable feedstocks in the form of animal manure, crop residues, food and food processing wastes, and OFMSW available in developing countries can be utilized economically for biogas production and at the same time reducing landfilling. In addition, resources currently being used in the management of such wastes can be diverted for establishing biogas plants and harness clean energy in the form of biogas. Mahanty et al [8] studied the effect of AD on methane yield and observed that the reduction of industrial and municipal wastes through anaerobic digestion followed by an aerobic treatment such as composting could be considered as an environmental friendly methodology [8]. It was further noted that although some wastes are poorly biodegradable due to 197


their low solubility or suboptimal C/N ratio, satisfactorily degradation of these substrates can take in certain combinations [21]. Hence, a proper mixture of waste for codigestion can enhance sludge Solubilization, digestion, and biomethane production by ameliorating the antagonistic and synergistic effects of different sludges. This approach provides some practical solutions to treats from diverse industrial sludges in economic and environmental perspectives [8]. Additionally, AD of animal manure and other biogenic wastes offers several environmental, agricultural and socio-economic benefits through the fertilizer value of the digestate, considerable reduction in odor and in activation of pathogens, and ultimately biogas as a clean renewable fuel for multiple end applications. It further offers many possible ecological, technological and economical benefits [8]. Bioenergy production in biogas plants could be enhanced by 40-80% by using organic wastes and by-products as co-substrates [8]. In addition to being a viable alternative of fuel source, it has the potential to reduce the green house gases thereby creating new possibilities of carbon trading in the global market. Other advantages of AD include: dilution of the toxic substances coming from any of the substrates involved, an improved nutrient balance, synergistic effects on microorganisms, a high digestion rate, and possible detoxification based on the co-metabolism process [22]. Moreover, the addition of suitable organic waste favours a more efficient stabilization, enhancing the biogas production [22]. The dilution of toxic substance can reduce GHG emission thus improving air quality. Additionally, it can be produced locally, saving hard currency that are normally used on imported natural gas and fuel [22].

the target groups from buying the gas for different purposes. In addition, the manner in which the gas is stored can raise a concern of fire risks. Again health concerns like allergy and sinuses may arise but unlike firewood, crop residues and dried cattle dung, biogas provides a clean, smoke-free environment. Furthermore, there can be some perception on the environmental pollution. During the production of the gas, Carbon dioxide is emitted to the atmosphere. However, the same carbon dioxide released to the atmosphere is the same released by humans, in that case there are no threat posed to the environment. This is also supported by the theory that the technology utilizes the carbon which is already in the ecosystem, and not through the generation of new carbon. V. FUTURE PERSPECTIVES With the introduction of biogas as an alternative energy source in SA, SA would have taken a step to develop and implement an integrated energy strategy. This will be a noticeable and different development path that ensures energy for all in an equitable and environmentally friendly manner. In decades to come, SA will be powered by a low carbon economy with a significant share of green jobs, where citizens have accessible, affordable, safe, efficient energy services and the transport system that does not affect the health of people. The use of biogas will also make SA to focus on clean energy technology that will promote a visible shift towards low polluting transport sectors, fuels and vehicles. Similarly, all disadvantaged communities will also be provided with effective energy services depending on their needs. A noticeable reduction of fuel poverty, respiratory illnesses and safety threats will be observed through the use of cleaner and safer household fuels.

IV. CHALLENGES THAT HINDER PRODUCTION OF BIOGAS AND UTILIZATION

VI. BIOGAS PRODUCTION FROM VARIOUS WASTE

Besides the fact that there‘s limited knowledge on the technology, the initial cost of installation may be high. Funding for research is also often limited and investors might not be keen as the biogas technology is very new. Hence the level of the technology is not advanced to convince funders. However, the South African National Energy Development Institute (SANEDI) has taken an encouraging initiative to support the energy projects. Furthermore, the production of biogas involves multiple steps which require multi-disciplinary inputs. For instance, the physical components of the system requires proper designing and efficient ways of evaluating the quality and quantity of the product. The process of anaerobic digestion is mainly performed by diverse microorganisms. Hence, understanding the microbiological part of the system is critical for the success of the project. Unfortunately, this is a rare case in most research groups where teams are set based on common background rather than interest. The cost of the gas may be a limiting factor for broad consumption. For instance, the current price 9kg cylinder ranges between R190 to R210. Such prices can exclude low

Anaerobic co-digestion of different organic wastes together can improve nutrient balance, dilute potentially toxic compounds such as sulphur-containing substances, and subsequently increase the processing capacity and biogas yield [8]. Weiland, 2010 [23] reported that bioenergy production in biogas plants could be enhanced by 40-80% by using organic wastes and by-products as co-substrates. Tewelde et al. [24] investigated the biogas production from co-digestion of brewery waste (BW) and cow dung (CW). Total solids (TS), volatile solids (VS), chemical oxygen demand (COD), methane yield (CH4) and carbon dioxide (CO2) were measured as shown in Table I. the study reported 74% conversion of organic solids. The maximum methane yield of 69% was obtained when the ratio CD/BW was 70:30.

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TABLE I CHEMICAL COMPOSITION OF BW AND CD AND CORRESPONDING METHANE YIELDS AT VARIOUS RATIOS [24]

BW CD

CH4 [%] CO2 [%]

Parameters COD BOD [mg/l] [mg/l] 6000-8100 2800-6100 6100 4290 Ratios of CD:BW 60:40 40:60

Nitrogen [mg/l] 40-60 30-38

TS[%] 8.2 8

VS[%] 94 83

pH 4.8 7.3

90:10

80:20

70:30

67

67.5

69

66

63.2

59.6

30.5

30

29.7

31.5

32.8

33.9

Alvarez, and Liden [25] investigated the semi- continuous co-digestion of slaughterhouse waste, manure and fruit and vegetable waste. Their results showed that the co-digestion of slaughterhouse waste with various co-substrates showed positive methane production of 80%. This was supported by other researchers [26] who did an investigation on biogas production from cow dung, cow pea and cassava pealing and attained 76% methane yield. A similar study was done by Ward et al. [27] on optimization of anaerobic digestion of agricultural resources showed 82% conversion of volatile solids, proving an increase in methane production. In Mahanty et al. [8] investigated the optimization of various industrial sludges for biogas production. The waste sludges were collected from waste treatment facilities of paper, chemicals, automobile, food processing and petrochemicals. A polynomial model was used to optimize the gas production as depicted in Fig. 1.

Phosphorous [mg/l] 30-40 10

20:80

studied simplex-centroid mixture formulation for optimized composting of kitchen waste. Furthermore, they noted that methane yield was found to decrease under five batches based optimized co-digestion process. Thus, the digestibility of various industrial sludges is improved under different batches.

Fig. 2 Methane yield from utilisation of sludges in different codigestion process scenarios consisting of one to five independent codigestion batches [8].

The biogas production from co-digestion of corn stover (CS) and chicken manure (CM) was studied by Yegin et al. [29]. Their tests were carried out in triplicates using 1 litre bottles with working volumes 0.5L at 37˚C. The Co-digestion of CS and CM significantly increased methane yield, with methane yield reaching as much as 218.8 mL/g. Xiao, Xingbao and Zheng [30] presented a pilot scale anaerobic co-digestion of municipal biomass waste. The focus was on methane production and green house gas (GHG) reduction. It was reported that 78% methane was produced with. Grisel et al, in [31] also investigated the biogas production from co-digestion of coffee pulp and cow-dung under solar radiation. It was found that during the first month co-digestion at mesophillic conditions, methane content in the biogas obtained was 50%. The content increased up to 60% and remained constant for at least eight months of further digestion. However, Thong et al [32] on the thermophillic anaerobic co-digestion of oil palm empty fruit bunches with palm mill effluent for efficient biogas production, showed 98% biodegradability of the feedstock and 82% methane yield with the corresponding energy content of 36 MJ per m3. This was

Fig. 1 Optimisation scheme flow diagram for sludge co-digestion using polynomial model [8]

It was reported that the maximum possible methane yield is increased from one batch to three batches (specific combination of sludge) of co-digestion process as shown in Fig. 2. This was due to the positively interacting pairs together in co-digestion process described by Abdullah et al, [28], who 199


found to be feasible in the thermophillic acidogenic hydrolysis of lignocellulosic in the empty fruit bunches up to mixing ratio of 2:3:1. This section of the study review the investigation conducted by Bernd, Ivo, Gabriel and Vincent in [14]. [14] Investigated the mesophillic anaerobic co-digestion of cow dung manure and biogas crops in German biogas plants. In this study the effect of hydraulic retention time and volatile solid (VS) crop proportion in the mixture on methane yield was studied. Methane yield as a function of retention time in storage tank for varying temperature with zero pressure, time in storage tank needed to reach a certain degradation of digestate as a function of temperature and methane yields as a function of retention time in storage tank for varying temperature with pressure of 1 are shown in Fig. 3, 4, 5 respectively. Fig. 3 showed that for higher storage tank temperature, maximum methane production is obtained faster. Long time is required to reach satisfactory degradation of feed sock as shown in Fig 5. The study further revealed that co-digestion of organic wastes depends mainly on the relative proportion of the component, the amount of the mixture and other physical variables such as temperature and pressure.

Fig. 5 Methane yields as a function of retention time in storage tank for varying temperature with pressure of 1[14].

Despite the well known benefits of co-digestion, such as optimum humidity, buffering capacity and C/N ratio or inhibitory substances dilution [9], it is not clear whether some co-substrates have adverse impact when they are co-digested with another waste in particular if there is synergisms or antagonisms among the co-digested substrates and if several co-substrates of similar biochemical composition can be codigested [9]. Therefore, it is critical to obtain an optimal mixture of the available co-substrates as well as the optimum operating conditions, which allow high biogas yields without compromising the stability of the process [9]. Pastor et al. [33] also reviewed the composition effect on biogas production. It was observed that an adequate mixture formulation is needed in order to ensure the correct functioning of the anaerobic digestion process. The following parameters have been taken into account in order to obtain an adequate mixture formulation for co-digestion: biogas production improvement, composition, nutrient balance and risk of inhibition by long chain fatty acids (LCFA) [33].

Fig. 3 Methane yield as a function of retention time in storage tank for varying temperature with zero pressure [14].

VII. CONCLUSION Conversion of waste into energy is a technology that has the potential in producing cleaner energy and greener alternative fuel. Anaerobic digestion technology is considered to be a practical method to reduce waste. It is not feasible and economic to treat these industrial wastes in separate digesters at each plant rather to install a centralized treatment facility for all combined waste together. Studies determining the limitations of co-digestion, parameters influencing the anaerobic process and reactions involved to attain methane however, optimum conditions to enhance satisfactory methane yields and treatment of residues have not been reported in literature. It is therefore, recommended that optimum conditions for anaerobic co-digestion must be investigated as well as treatment of sludge to manage the landfill crisis.

Fig. 4 Time in storage tank needed to reach a certain degradation of digestate as a function of temperature [14].

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