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Analysis of the biogas productivity from dry anaerobic digestion of organic fraction of municipal solid waste
Renewable and Sustainable Energy Reviews 81 (2018) 2328–2334
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Analysis of the biogas productivity from dry anaerobic digestion of organic fraction of municipal solid waste
MARK
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Anthony Njuguna Matheri, Vuiswa Lucia Sethunya, Mohamed Belaid , Edison Muzenda Department of Chemical Engineering, University of Johannesburg, Doornfontein, Johannesburg 2028, South Africa
A R T I C L E I N F O
A BS T RAC T
Keywords: BMP Biodegradable Co-digestion Mesophilic temperature OFMSW Renewable energy
In this study, it was observed that in experimental work under laboratory scale using conventional biomethane potential (BMP) analyser under the mesophilic optimum temperature of 37 °C and pH of 7. Organic fraction municipality solid waste (OFMSW) inoculated with cow manure had higher biodegradability rate leading to high methane production under shorter hydraulic retention rate. The co-digestion of OFMSW and cow manure stabilises conditions in digestion process such as carbon to nitrogen (C: N) ratio in the substrate mixtures as well as macro and micronutrients, pH, inhibitors or toxic compounds, dry matter and thus increasing methane production. It was concluded that the organic waste generated in the municipality co-digested with manures to produce methane can be used as a source of sustainable renewable energy.
1. Introduction
1.1. Biogas
Many African nations have been motivated to look for sustainable renewable energy sources such as; hydropower, wind energy, solar power, biomass energy, geothermal power, tidal power as well as wave power, to solve the problem of the extinction of fossil fuels and the need for green and clean energy [1]. Biomass energy is one of the renewable energy sources that has gained momentum because of its environmentally friendly aspect [2]. In addition to the carbon dioxide pollution from fossil fuels, the world is also faced with a waste pollution in the form of leftover food, which is proven also to be one of the contributing factors to global warming [3]. As research develops the solutions to handle all waste management issues have been addressed as some of them include pyrolysis, gasification and incineration of solid waste [4]. The difference between pyrolysis and gasification is the degree of air/oxygen present for combustion. While incineration can be done in the presences of oxygen. The heated waste material will create gas, liquid and solid deposits [4]. Though these technologies offer a practical approach to managing waste, they have been found to require a great deal of energy to operate, and some consume more energy to operate than the energy that can be produced from them [4]. The use of biogas has proven to be an effective way to use renewable energy sources and reduce these greenhouse gases [5,6]. The main objective of this study was the analysis of the production of methane from dry fermentation of organic fraction of municipal solid waste (OFMSW) on the bio – methane potential (BMP). To analysis biogas production of this substrate, the focus was given at optimum temperature and pH level.
Biogas is the by-product from the anaerobic digestion (AD) process of biomass and is used as a clean fuel. Biogas products play a major role in the biogeochemical carbon cycle. Biogas is a mixture of approximately 60% methane, 39% carbon dioxide and a small fraction of 1% as the water vapour, hydrogen sulphide and some other gases by volume. When it is purified to over 99% methane it becomes identical to a natural gas known to be bio-methane [7]. Bio-methane like biogas is used to generate heat using boilers, for lighting households, cooking and as fuel for vehicles. The compositions of biogas are outlined in Table 1 [7]. Biomethane potential tests (BMP) are done to determine the amount of biogas or biomethane per gram of volatile solids (VS) contained in the substrate used in the AD process. The tests are used for many other properties such as the process operational conditions that have to be monitored to avoid process malfunctions, environmental considerations, the time it will take for a substrate to degrade and the average bio-methanation for each substrate examined and integrated into the biogas production process [7].
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1.2. Anaerobic digestion AD is one of the alternative renewable energy technologies which has proven to be the acceptable option among most of these waste management's technologies. AD is a biochemical process where organic matter is decomposed in the absence of oxygen by various types of
Corresponding author. E-mail addresses: [email protected] (A.N. Matheri), [email protected] (V.L. Sethunya), [email protected] (M. Belaid), [email protected] (E. Muzenda).
http://dx.doi.org/10.1016/j.rser.2017.06.041 Received 9 March 2016; Received in revised form 18 May 2017; Accepted 16 June 2017 Available online 29 June 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
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Table 1 Percentage of gases by volume present in biogas. Biogas Composition
Percentage (%)
Methane (CH4) Carbon dioxide (CO2) Hydrogen sulphide (H2S) Hydrogen (H2) Ammonia (NH3) Carbon monoxide (CO) Nitrogen (N2) Oxygen (O2)
50–70 30–45 1–2 1–2 1–2 trace trace trace
Fig. 2. Trend of low and high solid anaerobic digestion plants in Europe [12].
1.2.1. Wet anaerobic digestion (WAD) Wet digestion requires water greater or equal to the biomass being processed while in the dry digestion process, the biomass or feedstock is digested as received [10]. Wet anaerobic digestion (WDA) process is an effective process yet it has a water wasting problem which should be avoided since water is one of the scarce resources that can run out. Also, the percentage of water in the digested feedstock will need to be dried. This requires a lot of energy and nutrients are also lost in the process [11]. Contrarily, dry anaerobic digestion has proven to be better for wet digestion due to its versatility, robustness and better water management strategies as shown in Fig. 2 [12]. In this current study, the focus is on dry anaerobic digestion.
1.2.2. Dry anaerobic digestion (DAD) The dry anaerobic digestion process is an energy and water saving process. It does not require an addition of a lot of water to the substrate, meaning that it does not require dewatering and energy used to dry the digestate [13]. The dry anaerobic digestion process takes place within bioreactors, which are batch processes operating independently, hence the malfunctioning of a reactor do not affect the functionality of the others. Unlike wet digestion process, the substrate in dry fermentation does not need stirring or being pumped through pipes which sometimes experiences blockage [13]. DAD process is been given much attention in the energy sector and research based environments for laboratory scale studies because of its low operation cost and potential by- products. However, despite all its advantages, the process may show inhabitation problems which are due to the requirement of large inoculums, long retention time, accumulation of VFA and the type of water material used [14]. Therefore, for a development of a suitable DAD process various aspects of the process, operational parameters, environmental impacts of the process, economic analysis, mass balance and energy flow needs to be monitored carefully [15]. Fig. 3 shows the mains steps that are undertaken in a DAD process for the production of biogas. Biogas production undergoes four distinct chemical and biological processes. These processes do not differ in either wet or a dry digestion process and they include hydrolysis, acidogenesis, acetogenesis and methanogenesis. The major functional groups of bacteria according to their metabolic (activity) reactions are [17,18]: Fermentative, hydrogen‐producing acetogenic, hydrogen‐consuming acetogenic, carbon dioxide reducing methanogens and aceticlastic methanogens bacteria
Fig. 1. Type of feedstock for anaerobic digestion [7,9].
anaerobic micro-organisms [7]. The rate at which this process take place in the production of biogas depends on a number of parameters that include, pH, temperature, nature of the substrate used, nutrients, digester construction and size [7,8]. AD uses a wide range of biomass as feedstock/substrates for the production of biogas. The type of feedstock or substrate that is mostly used can be animal manure, agriculture waste, garden waste, market vegetable waste, slaughter houses or abattoir waste, sewage sludge, a mixed organic fraction of municipal solid waste (OFMSW) and other commercial and industrial organic waste. Fig. 1 shows the classification of feedstock used for the AD from different sources. Feedstock for AD varies according to its composition, homogeneity, fluid dynamics, dry matter content, methane yield and biodegradability [7]. The pathways for anaerobic digestion are either wet or dry digestion depending on the need for the fluidity of the substrate. The definition of both wet and dry anaerobic digestion are defined as followed: 2329
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Fig. 3. Main steps of dry anaerobic digestion process [3,11,16].
1.3.1. Temperature Methane is formed over a wide range of temperatures from low temperature to high temperature though not over 65 °C. The three different temperature ranges for methane formation can be defined by the microbial activity as given below [27]:
are the four major functional groups of bacteria in metabolic reaction activity in anaerobic digestion [11,16]. Hydrolysis is theoretically the first step of anaerobic digestion, during which the complex organic matter (polymers) are decomposed into smaller units (mono- and oligomers). During hydrolysis, the longchain molecules, such as carbohydrate, protein and fat polymers, are broken down to monomers (small molecules). Different specialised microbial produce a number of specific enzymes which catalyse the decomposition, and the process is extracellular. During hydrolysis, polymers like carbohydrates, proteins, lipids and nucleic acids are converted into glucose, glycerol, purines and pyridines [19]. In the acidogenesis process, the acidogenic bacteria transform the products of the hydrolysis into short chain volatile acids, alcohol, ketones, carbon dioxide and hydrogen. Some of the major acidogenesis stage products are acetic acid, propionic acid, formic acid, butyric acid, lactic acid, ethanol and methanol. From these products, the carbon dioxide, hydrogen and acetic acid skip the third stage, acetogenesis, and be utilized directly by the methanogenic bacteria in the final stage to produce biogas; methane and carbon dioxide [20–22].
• • •
Psychrophilic temperature from 10 °C to 20 °C Mesophilic temperature from 20 °C to 40 °C, or transition temperature of 35 °C and 37 °C Thermophilic temperature from 50 °C to 65 °C, usually 55 °C.
Psychrophilic digesters were mostly used in the 1980s when biogas was used for heating purposes. At that time, at 23 °C the average heating production was higher than that of mesophilic digesters [28]. In history, no anaerobic psychrophilic bacteria has been found at temperatures below 20 °C because under these conditions the psychrophilic anaerobic digestion was not feasible, had low microbial activity and biogas production [29]. In recent years mesophilic digesters are the most popular. The temperature of digesters depends mostly on the feedstock composition and the type of reactor, but it has been observed from literature that for maximum gas production rate, the temperature should be maintained at an approximately constant level [30]. A number of mesophilic and thermophilic anaerobic bacteria are described in the temperature ranges between 28 °C and 42 °C and between 55 °C and 72 °C respectively. It has also been found that the thermophilic digesters have lower retention time that is due to the high catalytic activity of thermophiles [30].
1.3. Conditions for the anaerobic digestion The rate of biogas production depends on a number of conditions (parameters) that include; hydraulic retention time (HRT), temperature, trace metals, C/N ratio, organic loading rate, partial pressure, pH level, nature of the substrate, microbes balance, and oxygen exposure to anaerobic [23–26]. 2330
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highest and will not occur at low pH [40]. 1.3.4. Retention time Retention time (RT) is the time required for the complete degradation of the organic material to occur. It is defined as:
RT =
Liquid Volume DailyFlow
(1)
The RT completion of the anaerobic digestion reaction differs with process parameters such as waste composition and temperature. The RT for a biomass digestion in mesophilic conditions varies from 10 to 40 days. However, RT in thermophilic conditions is lower than that of mesophilic. A high solids reactor operating under thermophilic ranges has a retention time of about 14 days. RT is directly proportional to the degradation rate, the lower the degradation rate, the shorter the RT [35]. Schaefer and Sung [41] studied a thin corn silage (94 gTCOD/L and 61 gTS/L) in a digester that was completely mixed at different hydraulic retention time (HRT) i.e., 30, 20, 15, and 12 days also at different volumetric organic loading rates (OLRs) of 3.2, 6.1, 6.4 and 7.6 g TCOD/L-Day, respectively. At steady state, the HRT results were found to be 30, 20 and 15 days, but the reactors were then found to be inactive after 12 days giving total volatile fatty acids (TVFAs) of 7 g/L. Average RT for mesophilic digestion was then found to be 12–18 days [11].
Fig. 4. Graphical representation of temperature range in anaerobic digestion [32].
Thermophiles are said to provide additional benefits in terms of low contamination [30]. The experimental work done by the University of Alaska Fairbanks shows that a 1000 L digester using psychrophilic temperatures produces 200–300 L of methane per day from the digesters in warmer climates [31]. Thermophilic digestion systems are considered to be less stable while the energy input is much higher, so more biogas is removed from organic matter in an equal amount of time. The higher the temperature, the faster the reaction and the faster the gas yields in anaerobic digestion [32]. Fig. 4 shows the temperature range in the process of anaerobic digestion.
1.3.5. Organic loading rate Organic loading rate (OLR) is the capacity of AD system for the biological conversion or the feed amount of organic material, expressed as carbon oxygen demand (COD) or volatile solids (VS) to the system daily per m3 of the digester volume. OLR can be expressed as:
1.3.2. Carbon to nitrogen ratio (C/N ratio) C/N ratio represents the relationship between the amount of carbon and nitrogen in the organic materials. Fricke reported that the optimum C/N ratio for anaerobic digestion is said to be in a range of 20–30 [32,33]. In situations where C/N ratio happens to be higher than 25, the methanogens consume nitrogen rapidly, which results in lower gas yields, and a lower C/N ratio will cause ammonia accumulation and pH being greater than 8.5, which results in a toxic methanogenic bacteria [33]. Optimal C/N ratio is a function of the type of feedstock and may vary with the type of feedstock used. C/N ratio can be maintained at the required or acceptable range by mixing ratios of high and low C/N ratios. C/N ratio ranging from 22 to 25 is most suitable for anaerobic digestion of fruits and vegetable waste [34]. Romano and Zhang in a research paper written by Norberg [35] suggested that the optimal C/N ratio of onion juice and digested sludge should be maintained at 15 [33,35]. Li et al., [36] studied DAD of organic wastes and found C/N ratio between 15 and 18 when corn stover was inoculated with digested sewage sludge. Digestion rate decreased when C/N ratio increased to 21 or higher due to low pH in the first 7 days at 37 °C [33]. Thus pH has an effect on the digester rate and the C/N ratio.
OLR =
DailyflowxVSconcentration Liquid Volume
(2)
Gas production in an AD system is dependent on the OLR. When the feeding capacity in the system exceed the OLR, the gas production decreases [42]. This is due to accumulation of fatty acids in the digester slurry. Thus, OLR is one of the most important controlling parameters in the continuous system because if not carefully monitored, the system faces overloading and the system fails to function properly [43]. With regard to content, AD is classified into three categories which are: low solids (LS) AD system that is less than 15% TS, medium solids (MS) processes which are about 15–20% TS, high solids (HS) processes at 20–24% TS range [44]. Fernandez et al. [29] optimised OLR for mesophilic systems and found that values for OLD to be 2.5–3.5 kgVS/ m3- day for cattle manure, 5–7 kgVS/m3-day for cattle manure with cosubstances and 3–3.5 kgVS/m3-day for pig manure [11]. 2. Methodology 2.1. Substrate collection
1.3.3. pH level The pH value is a measurement of acid or basic concentration in aqueous substances, i.e., the concentration of hydrogen ions in solution. Anaerobic bacteria, e.g. the methanogens are sensitive to acid concentrations and their growth can be lowered if the digester system is acidic [33]. Lee et al. [37] show that they have optimised the pH value in AD systems and it was found that the pH value for methanogenesis was around 7, but the pH value differed in all the AD stages. Lee et al. [38] found that the optimal pH range for methanogens in AD to be 6.5 – 8.2, while Kim et al. [39] reported a pH value at 5.5 6.5 for hydrolysis and acidogenesis. The pH of a digestate varies with retention time, but the initial step, acetogenesis process in a batch reactor occurs at a rapid pace and in the acidogenesis process acid is produced since it is lower in the digestion tank. It is important to constantly measure the pH to ensure the well-being of methanogens so that methane can be produced. Methane has been observed to occur when the pH is at its
The main objective of this study was to evaluate the bio-methane potential from the dry fermentation of organic fraction of municipal solid waste (OFMSW) using cow manure for inoculation under controlled experimental conditions. The substrate was quantified and characterised before the onset of production. Fig. 5 shows a summary flow diagram of the feedstock or substrate quantification. The food waste collected was sorted in weekly basis according to the following categories: starch, carbohydrates, protein, vegetables and fruits. The characterised food waste was weighed on a mass scale and the masses of the various food waste samples were then recorded. Waste characterization was done to ascertain the composition. These included physical and chemical composition with regards to C/N ratio, volatile solids, total solids and elemental analysis for carbon, nitrogen, sulphur and hydrogen in accordance with the standard method (APHA 1995) [45]. 2331
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Table 2A Substrate characterization for the anaerobic digestion. Substrate
C (%)
H (%)
Vegetables and Fats Fruits and Proteins Starch and Fibres Cow manure
24.18 22.77 32.93 14.87
1.04 1.96 1.12 1.65
N (%)
S (%) 3.28 2.53 2.99 0.84
0.00 0.00 0.00 0.00
Table 2B Substrate characterization for the anaerobic digestion.
Fig. 5. Flow diagram of the feedstock quantification.
The experiment was run at mesophilic temperature 37 °C and pH of 7 under controlled 500 ml digester. The amount of biogas produced was measured using water displacement method. Biogas was analysed using gas chromatograph containing flame-ionization detector. The operating conditions of oven temperature were 70 °C, detector 150 °C, injection port 80 °C and helium was used as carrier gas at flow rates of 20 ml/min. Fig. 6 shows the biomethane potential set up.
TS (%)
VS (%)
C/N Ratio
Vegetables and Fats Fruits and Proteins Starch and Fibres Cow manure
63.72 70.40 60.01 60.01
30.27 29.02 37.00 91.55
76.16 80.67 71.24 78.72
7.37 9.00 11.02 17.70
where; Mdried = Amount dried sample (mg), Mwet = Amount of wet sample (mg) and Mburned = Amount of burned sample (mg).
(F *CF ) + (S*Sf ) C = N (F *Nf ) + (F *Nf )
The purpose of this experiment was to determine methane production in the dry anaerobic digestion of organic fraction of municipal solid waste (OFMSW) inoculated with cow manure at the mesophilic optimum temperature (37 °C) and initial pH of 7 using water displacement method and Bio-Methane Potential (BMP) analyser. Tables 2A, 2B shows the substrate characterization. TS is the sum of dissolved solids and suspended solids. TS and pH are the important parameters to assess anaerobic digestion process efficiency [14,21]. VS is the organic portion of TS that biodegrade in the anaerobic process. C/N ratio is an important factor in bacteria stability in the anaerobic process [46,47]. TS, VS and MC are calculated using Eqs. (3)–(5) respectively while C/N ratio is calculated using Eq. (6).
VS (%) =
Mdried − Mburned *100 Mwet
(3)
TS (%) =
Mdried *100 Mwet
(4)
(6)
where; F = first substrate, S = second substrate, Cf = carbon composition for the first substrate, Cs = carbon composition for the second substrate, Nf = nitrogen composition for the first substrate and Ns = nitrogen composition for the second substrate. From the characterised study of the food waste. It was found that the fruits and proteins (FP) had low solids content (29.02% TS) due to the fact that greater portion of the sample contained fruits with high moisture content whereas the sample containing starch and fibres (SF) contained a high concentration of solids (37% TS). Volatile solids represented an organic matter of the feedstock without considering the inorganic salts and ash. Total solids percentage represented organic and inorganic material in the feedstock. The proportion of volatile solids in total solids was much higher in the fruits and protein sample (80.67%) compared to the samples containing starch and fibre (71.24%) as well as vegetable and fats (76.16%). The average moisture content (MC) of all the three samples was found to be 63.72%, 70.40% and 60.01% respectively. A high moisture content percentage favoured optimum biogas production since it allows bacteria to release methane and metabolic processes to occur. Hence moisture content was significant for optimal digestion as it aids the digestion process to yield high-quality biogas rich in methane (CH4). Zhu et al. [34] reported that substrates with MC of 75% such as food and yard waste are suited for digestion. It was also recommended by Abbasi et al. [48] that the MC for optimum conditions to be 90%. It was also stated that the wetter the material, the more volume and area it takes up relative to the levels of gas production, but we can observe that the values obtained from this current study were within the dry digestion range. The carbon and nitrogen (C/N) ratio for the three samples was found to be 7.37, 9 and 11.01. From these results, it was found that the carbon ratios were too low compared to the study carried by Sreekrishnan et al., [49] that shows, the best anaerobic digestion range should be between 16 to 30 C/N ratios for food waste. Substrates with low C/N ratio was most likely result in ammonia accumulation and volatile fatty acids (VFAs) in the digester, which could hinder the methanogenic activity and system failure [49]. From the results summarized in Table 2B, it was comprehended that maintaining of proper composition of the substrate was necessary. The higher carbon content in the system gave more carbon dioxide formation and lower pH value which was also a factor that needed to be maintained and controlled according to Dioha et al. [50]. When the levels of C/N ratio
3. Results and discussion
Mwet − Mdried *100 Mwet
MC (%)
where; C – carbon, N – nitrogen, H – hydrogen, S – sulphur, MC- moisture content, TS – total solids, VS – volatile solids and C/N – carbon-nitrogen ratio.
2.2. Experimental procedure
MC (%) =
Substrate
(5)
Fig. 6. BMP test experimental setup.
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Fig. 7. Methane yield of OFMSW from BMP analyser in hours.
toxic to non-toxic forms of compounds such as ammonia. Li et al., [54] suggests that ammonia accumulation of the system hinders the production of methane as well as causing system failure [54,55]. The problem of pH causing ammonia accumulation was addressed by adding another substrate to the system (co-digestion). OFMSW are rich in substrate essential for bacteria to grow and produce high-quality methane. Using gas chromatography (GC) to analysis the biogas production before carbon dioxide fixing or absorption, the composition of the biogas was found as follows; fats and vegetables 70% CH4 and 30% CO2, starch and fibre 70% CH4 and 30% CO2 and lastly fruits and proteins had biogas consisting of 60% CH4 and 40% CO2.
was high, this suggested higher depletion of nitrogen used by the methanogenic bacteria that produces methane to satisfy their protein needs, therefore, resulting in lower biogas production rate. To operate an anaerobic digester at optimum C/N ratio, biodegradable material of high C/N ratio was blended with the biodegradable material of low C/N ratio. This enhanced the optimum conditions of C/ N ratio to be within 15–30:1 [46]. It was observed from Fig. 7 that methane production increases with retention time increases. Hence, it can be stated that there was a directly proportional relationship between retention time and methane production. Fig. 7 illustrates methane yield of OFMSW from BMP analyser in hours. There was an absence of lag phase due to presence and balance of active microbial for digestion supplied by the inoculate. The peak methane production of the vegetables and fats sample (249.6 Nml) was achieved up to 130 h (5 days) demonstrating the maximum degree of methane formation out of all the three samples. Samples containing fruits & proteins as well as starch and fibres had methane volumes of about 219.7 Nml and 128.5 Nml respectively and were achieved within 48 h before the samples stopped producing any gases. From the sample of vegetables and fats, it was observed that the maximum methane production was achieved in a shorter time because of the availability of agitation on the system since maximum methane is achieved as agitation rate increases thus indicating the effective influence of the agitation rate on the overall conversion [51]. The substrate time and the microbial balance in the system also places a role in the maximum production of methane at a shorter retention time. High biodegradability matters produce methane at a faster rate and afterwards ease production. The pH changes during anaerobic fermentation were due to the accumulation of volatile fatty acids (VFA's) by acidogenic bacteria, the pH value for sample 1, samples 2 and sample 3 were initially at 7.2, 6.99 and 6.98 respectively in the start-up of the run. Since the digestible compounds of organic matters were hydrolysed and converted into fatty acids quickly, the pH began to decrease gradually to a higher pH value which can be observed to be the reason why the two sample only produced methane in the two days [52]. The acidogenic bacteria population increases more than the appropriate ratio required, these created an excess accumulation of acids medium inside the digester. Thus increasing acidity and eventually decreasing pH, causing deactivation of methanogens and so the digestion process [53]. The pH had an influence on the equilibrium of
4. Conclusion It was concluded that in experimental work under laboratory scale using conventional biochemical methane potential and had optimum methane yield after retention time in hours. Co-digestion of OFMSW and cow manure stabilises conditions in anaerobic digestion process such as C: N ratio in the substrate mixtures as well as macro and micronutrients, pH, inhibitors or toxic compounds and dry matter. Although BMP results only showed methane production in hours because of the small loading rate, these results reinforce the validity of OFMSW as a strong candidate for use in dry anaerobic digestion. It was then concluded that OFMSW was indeed a better feedstock to be used in dry anaerobic digestion and should be co-digested with a secondary substrate to minimise ammonia accumulation and sample acidity.
Acknowledgement The authors wishes to express their appreciation to Process Energy Environmental and Technology Station (PEETS) funded by South Africa National Energy Development Institute (SANEDI), Technology Innovation Agency (TIA) and City of Johannesburg (CoJ/UJ/WTE/ FS003) (CoJ, Green Economy), Chemical Engineering and Applied Chemistry Departments at the University of Johannesburg for allowing us to work in their laboratories. Dr Robert Huberts, Prof Jane Catherine Ngila, Dr Tumisang Seodigeng, Dr Ludger Eltrop and Prof Shivani Mishra for consultancy.
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References [1] Berndes G, Hoogwijk M, van den Broek R. The contribution of biomass in the future global energy supply: a review of 17 studies. Biomass- Bioenergy 2003;25(1):1–28. [2] G.A.F.I.A. Pyrolisis and plasma incineraton in Europe. Case study of gasification; 2006. 〈www.no-burn.org〉. [3] Matheri AN, Belaid M, Seodigeng T, Ngila CJ. The kinetic of biogas rate from cow dung and grass clippings. iieng org; 2015. [4] Zheng YH, Wei JG, Li J, Feng SF, Li ZF, Jiang GM, Ning TY. Anaerobic fermentation technology increases biomass energy use efficiency in crop residue utilization and biogas production. Renew Sustain Energy Rev 2012;16(7):4588–96. [5] Allan H. Grass productivity. Washington DC: Island Press Conservation Classics Series; 1998. p. 56–89. [6] Latinwo GK, Agarry SE. Modelling the kinetics of biogas production from mesophilic anaerobic co-digestion of cow dung with plantain peels.. Int J Renew Energy Dev 2015;4(1):55. [7] Al Seadi T. Biogas handbook. Syddansk Universitet; 2008. [8] Matheri AN, B., M, Seodigeng T, Ngila CJ. Modelling the kinetic of biogas production from co-digestion of pig waste and grass clippings. In: Proceedings of the world congress on engineering, WCE 2016. London, U.K; 2016. [9] Schnurer A, Jarvis A. Microbiological handbook for biogas plantsSwedish Waste Management U 2010; 2010. p. 1–74. [10] Chen X, Romano RT, Zhang R. Anaerobic digestion of food wastes for biogas production. Int J Agric Biol Eng 2010;3(4):61–72. [11] Kothari R, Pandey AK, Kumar S, Tyagi VV, Tyagi SK. Different aspects of dry anaerobic digestion for bio-energy: an overview. Renew Sustain Energy Rev 2014;39:174–95. [12] Annachhatre AP. Dry anaerobic digestion of municipal solid waste and digestate management strategies. Asian Institute of Technology; 2012. [13] Angelidaki I, Ahring BK. Effects of free long-chain fatty acids on thermophilic anaerobic digestion. Appl Microbiol Biotechnol 1992;37(6):808–12. [14] Themelis NJ, Verma S. The better option-anaerobic digestion of organic wastes in MSW. Waste Manag World 2004:41–8. [15] ISAT, GTZ. Biogas digest volume I: Biogas basics. Information and Advisory Service on AppropriateTechnology. [16] Matheri AN, Mbohwa C, Belaid M, Tumisang S, Ngila JC. Multi-criteria analysis of different technologies for the bioenergy recovery from OFMSW. Newswood and International Association of Engineers. Lect Notes Eng Comput Sci 2016(1):2226, [888–893]. [17] Henze M. Biological wastewater treatment: principles, modelling and design. London: IWA Publ; 2008. p. 401–37. [18] Matheri AN, Belaid M, Seodigeng T, Ngila JC. Role of impact of trace elements on anaerobic co-digestion in biogas production. In: Proceedings of the 24th world congress on engineering (WCE 2016) London, UK (Unpublished); 2016. [19] Ostrem K, Themelis NJ. Greening waste: anaerobic digestion for treating the organic fraction of municipal solid wastes. Earth Engineering Center Columbia University; 2004. p. 6–9. [20] Mata-Alvarez J. Biomethanization of the organic fraction of municipal solid wastes. IWA publishing; 2002. [21] Jørgensen PJ. Biogas-green energy: process, design, energy supply, environment. Researcher for a Day; 2009. [22] Matheri AN, Mbohwa C, Belaid M, Tumisang S, Ngila JC. Mesophilic anaerobic codigestion of cow dung, chicken droppings and grass clippings. In: Proceedings of the international conference on systems engineering and engineering management. San Francisco, USA; 2016a. [23] Angelidaki I, Ahring BK. Anaerobic thermophilic digestion of manure at different ammonia loads: effect of temperature. Water Res 1994;28(3):727–31. [24] Ogejo JA, Wen Z, Ignosh J, Bendfeldt ES, Collins E. Biomethane Technol 2009. [25] Laura RD, Idnani MA. Increased production of biogas from cowdung by adding other agricultural waste materials. J Sci Food Agric 1971;22(4):164–7. [26] Tchobanoglous G, Theisen H, Vigil S. Integrated solid waste management: engineering principles and management issues. McGraw-Hill Science/Engineering/ Math; 1993. [27] Henze M, van Loosdrecht MCM, Ekama GA, Brdjanovic D. Biological wastewater treatment. IWA publishing; 2008. [28] N.R.E.L. US data summary of municipal waste management alternatives; 1992. [29] Fernández J, Pérez M, Romero LI. Effect of substrate concentration on dry mesophilic anaerobic digestion of organic fraction of municipal solid waste
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Renewable and Sustainable Energy Reviews journal homepage: www.elsevier.com/locate/rser
Analysis of the biogas productivity from dry anaerobic digestion of organic fraction of municipal solid waste
MARK
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Anthony Njuguna Matheri, Vuiswa Lucia Sethunya, Mohamed Belaid , Edison Muzenda Department of Chemical Engineering, University of Johannesburg, Doornfontein, Johannesburg 2028, South Africa
A R T I C L E I N F O
A BS T RAC T
Keywords: BMP Biodegradable Co-digestion Mesophilic temperature OFMSW Renewable energy
In this study, it was observed that in experimental work under laboratory scale using conventional biomethane potential (BMP) analyser under the mesophilic optimum temperature of 37 °C and pH of 7. Organic fraction municipality solid waste (OFMSW) inoculated with cow manure had higher biodegradability rate leading to high methane production under shorter hydraulic retention rate. The co-digestion of OFMSW and cow manure stabilises conditions in digestion process such as carbon to nitrogen (C: N) ratio in the substrate mixtures as well as macro and micronutrients, pH, inhibitors or toxic compounds, dry matter and thus increasing methane production. It was concluded that the organic waste generated in the municipality co-digested with manures to produce methane can be used as a source of sustainable renewable energy.
1. Introduction
1.1. Biogas
Many African nations have been motivated to look for sustainable renewable energy sources such as; hydropower, wind energy, solar power, biomass energy, geothermal power, tidal power as well as wave power, to solve the problem of the extinction of fossil fuels and the need for green and clean energy [1]. Biomass energy is one of the renewable energy sources that has gained momentum because of its environmentally friendly aspect [2]. In addition to the carbon dioxide pollution from fossil fuels, the world is also faced with a waste pollution in the form of leftover food, which is proven also to be one of the contributing factors to global warming [3]. As research develops the solutions to handle all waste management issues have been addressed as some of them include pyrolysis, gasification and incineration of solid waste [4]. The difference between pyrolysis and gasification is the degree of air/oxygen present for combustion. While incineration can be done in the presences of oxygen. The heated waste material will create gas, liquid and solid deposits [4]. Though these technologies offer a practical approach to managing waste, they have been found to require a great deal of energy to operate, and some consume more energy to operate than the energy that can be produced from them [4]. The use of biogas has proven to be an effective way to use renewable energy sources and reduce these greenhouse gases [5,6]. The main objective of this study was the analysis of the production of methane from dry fermentation of organic fraction of municipal solid waste (OFMSW) on the bio – methane potential (BMP). To analysis biogas production of this substrate, the focus was given at optimum temperature and pH level.
Biogas is the by-product from the anaerobic digestion (AD) process of biomass and is used as a clean fuel. Biogas products play a major role in the biogeochemical carbon cycle. Biogas is a mixture of approximately 60% methane, 39% carbon dioxide and a small fraction of 1% as the water vapour, hydrogen sulphide and some other gases by volume. When it is purified to over 99% methane it becomes identical to a natural gas known to be bio-methane [7]. Bio-methane like biogas is used to generate heat using boilers, for lighting households, cooking and as fuel for vehicles. The compositions of biogas are outlined in Table 1 [7]. Biomethane potential tests (BMP) are done to determine the amount of biogas or biomethane per gram of volatile solids (VS) contained in the substrate used in the AD process. The tests are used for many other properties such as the process operational conditions that have to be monitored to avoid process malfunctions, environmental considerations, the time it will take for a substrate to degrade and the average bio-methanation for each substrate examined and integrated into the biogas production process [7].
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1.2. Anaerobic digestion AD is one of the alternative renewable energy technologies which has proven to be the acceptable option among most of these waste management's technologies. AD is a biochemical process where organic matter is decomposed in the absence of oxygen by various types of
Corresponding author. E-mail addresses: [email protected] (A.N. Matheri), [email protected] (V.L. Sethunya), [email protected] (M. Belaid), [email protected] (E. Muzenda).
http://dx.doi.org/10.1016/j.rser.2017.06.041 Received 9 March 2016; Received in revised form 18 May 2017; Accepted 16 June 2017 Available online 29 June 2017 1364-0321/ © 2017 Elsevier Ltd. All rights reserved.
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Table 1 Percentage of gases by volume present in biogas. Biogas Composition
Percentage (%)
Methane (CH4) Carbon dioxide (CO2) Hydrogen sulphide (H2S) Hydrogen (H2) Ammonia (NH3) Carbon monoxide (CO) Nitrogen (N2) Oxygen (O2)
50–70 30–45 1–2 1–2 1–2 trace trace trace
Fig. 2. Trend of low and high solid anaerobic digestion plants in Europe [12].
1.2.1. Wet anaerobic digestion (WAD) Wet digestion requires water greater or equal to the biomass being processed while in the dry digestion process, the biomass or feedstock is digested as received [10]. Wet anaerobic digestion (WDA) process is an effective process yet it has a water wasting problem which should be avoided since water is one of the scarce resources that can run out. Also, the percentage of water in the digested feedstock will need to be dried. This requires a lot of energy and nutrients are also lost in the process [11]. Contrarily, dry anaerobic digestion has proven to be better for wet digestion due to its versatility, robustness and better water management strategies as shown in Fig. 2 [12]. In this current study, the focus is on dry anaerobic digestion.
1.2.2. Dry anaerobic digestion (DAD) The dry anaerobic digestion process is an energy and water saving process. It does not require an addition of a lot of water to the substrate, meaning that it does not require dewatering and energy used to dry the digestate [13]. The dry anaerobic digestion process takes place within bioreactors, which are batch processes operating independently, hence the malfunctioning of a reactor do not affect the functionality of the others. Unlike wet digestion process, the substrate in dry fermentation does not need stirring or being pumped through pipes which sometimes experiences blockage [13]. DAD process is been given much attention in the energy sector and research based environments for laboratory scale studies because of its low operation cost and potential by- products. However, despite all its advantages, the process may show inhabitation problems which are due to the requirement of large inoculums, long retention time, accumulation of VFA and the type of water material used [14]. Therefore, for a development of a suitable DAD process various aspects of the process, operational parameters, environmental impacts of the process, economic analysis, mass balance and energy flow needs to be monitored carefully [15]. Fig. 3 shows the mains steps that are undertaken in a DAD process for the production of biogas. Biogas production undergoes four distinct chemical and biological processes. These processes do not differ in either wet or a dry digestion process and they include hydrolysis, acidogenesis, acetogenesis and methanogenesis. The major functional groups of bacteria according to their metabolic (activity) reactions are [17,18]: Fermentative, hydrogen‐producing acetogenic, hydrogen‐consuming acetogenic, carbon dioxide reducing methanogens and aceticlastic methanogens bacteria
Fig. 1. Type of feedstock for anaerobic digestion [7,9].
anaerobic micro-organisms [7]. The rate at which this process take place in the production of biogas depends on a number of parameters that include, pH, temperature, nature of the substrate used, nutrients, digester construction and size [7,8]. AD uses a wide range of biomass as feedstock/substrates for the production of biogas. The type of feedstock or substrate that is mostly used can be animal manure, agriculture waste, garden waste, market vegetable waste, slaughter houses or abattoir waste, sewage sludge, a mixed organic fraction of municipal solid waste (OFMSW) and other commercial and industrial organic waste. Fig. 1 shows the classification of feedstock used for the AD from different sources. Feedstock for AD varies according to its composition, homogeneity, fluid dynamics, dry matter content, methane yield and biodegradability [7]. The pathways for anaerobic digestion are either wet or dry digestion depending on the need for the fluidity of the substrate. The definition of both wet and dry anaerobic digestion are defined as followed: 2329
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Fig. 3. Main steps of dry anaerobic digestion process [3,11,16].
1.3.1. Temperature Methane is formed over a wide range of temperatures from low temperature to high temperature though not over 65 °C. The three different temperature ranges for methane formation can be defined by the microbial activity as given below [27]:
are the four major functional groups of bacteria in metabolic reaction activity in anaerobic digestion [11,16]. Hydrolysis is theoretically the first step of anaerobic digestion, during which the complex organic matter (polymers) are decomposed into smaller units (mono- and oligomers). During hydrolysis, the longchain molecules, such as carbohydrate, protein and fat polymers, are broken down to monomers (small molecules). Different specialised microbial produce a number of specific enzymes which catalyse the decomposition, and the process is extracellular. During hydrolysis, polymers like carbohydrates, proteins, lipids and nucleic acids are converted into glucose, glycerol, purines and pyridines [19]. In the acidogenesis process, the acidogenic bacteria transform the products of the hydrolysis into short chain volatile acids, alcohol, ketones, carbon dioxide and hydrogen. Some of the major acidogenesis stage products are acetic acid, propionic acid, formic acid, butyric acid, lactic acid, ethanol and methanol. From these products, the carbon dioxide, hydrogen and acetic acid skip the third stage, acetogenesis, and be utilized directly by the methanogenic bacteria in the final stage to produce biogas; methane and carbon dioxide [20–22].
• • •
Psychrophilic temperature from 10 °C to 20 °C Mesophilic temperature from 20 °C to 40 °C, or transition temperature of 35 °C and 37 °C Thermophilic temperature from 50 °C to 65 °C, usually 55 °C.
Psychrophilic digesters were mostly used in the 1980s when biogas was used for heating purposes. At that time, at 23 °C the average heating production was higher than that of mesophilic digesters [28]. In history, no anaerobic psychrophilic bacteria has been found at temperatures below 20 °C because under these conditions the psychrophilic anaerobic digestion was not feasible, had low microbial activity and biogas production [29]. In recent years mesophilic digesters are the most popular. The temperature of digesters depends mostly on the feedstock composition and the type of reactor, but it has been observed from literature that for maximum gas production rate, the temperature should be maintained at an approximately constant level [30]. A number of mesophilic and thermophilic anaerobic bacteria are described in the temperature ranges between 28 °C and 42 °C and between 55 °C and 72 °C respectively. It has also been found that the thermophilic digesters have lower retention time that is due to the high catalytic activity of thermophiles [30].
1.3. Conditions for the anaerobic digestion The rate of biogas production depends on a number of conditions (parameters) that include; hydraulic retention time (HRT), temperature, trace metals, C/N ratio, organic loading rate, partial pressure, pH level, nature of the substrate, microbes balance, and oxygen exposure to anaerobic [23–26]. 2330
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highest and will not occur at low pH [40]. 1.3.4. Retention time Retention time (RT) is the time required for the complete degradation of the organic material to occur. It is defined as:
RT =
Liquid Volume DailyFlow
(1)
The RT completion of the anaerobic digestion reaction differs with process parameters such as waste composition and temperature. The RT for a biomass digestion in mesophilic conditions varies from 10 to 40 days. However, RT in thermophilic conditions is lower than that of mesophilic. A high solids reactor operating under thermophilic ranges has a retention time of about 14 days. RT is directly proportional to the degradation rate, the lower the degradation rate, the shorter the RT [35]. Schaefer and Sung [41] studied a thin corn silage (94 gTCOD/L and 61 gTS/L) in a digester that was completely mixed at different hydraulic retention time (HRT) i.e., 30, 20, 15, and 12 days also at different volumetric organic loading rates (OLRs) of 3.2, 6.1, 6.4 and 7.6 g TCOD/L-Day, respectively. At steady state, the HRT results were found to be 30, 20 and 15 days, but the reactors were then found to be inactive after 12 days giving total volatile fatty acids (TVFAs) of 7 g/L. Average RT for mesophilic digestion was then found to be 12–18 days [11].
Fig. 4. Graphical representation of temperature range in anaerobic digestion [32].
Thermophiles are said to provide additional benefits in terms of low contamination [30]. The experimental work done by the University of Alaska Fairbanks shows that a 1000 L digester using psychrophilic temperatures produces 200–300 L of methane per day from the digesters in warmer climates [31]. Thermophilic digestion systems are considered to be less stable while the energy input is much higher, so more biogas is removed from organic matter in an equal amount of time. The higher the temperature, the faster the reaction and the faster the gas yields in anaerobic digestion [32]. Fig. 4 shows the temperature range in the process of anaerobic digestion.
1.3.5. Organic loading rate Organic loading rate (OLR) is the capacity of AD system for the biological conversion or the feed amount of organic material, expressed as carbon oxygen demand (COD) or volatile solids (VS) to the system daily per m3 of the digester volume. OLR can be expressed as:
1.3.2. Carbon to nitrogen ratio (C/N ratio) C/N ratio represents the relationship between the amount of carbon and nitrogen in the organic materials. Fricke reported that the optimum C/N ratio for anaerobic digestion is said to be in a range of 20–30 [32,33]. In situations where C/N ratio happens to be higher than 25, the methanogens consume nitrogen rapidly, which results in lower gas yields, and a lower C/N ratio will cause ammonia accumulation and pH being greater than 8.5, which results in a toxic methanogenic bacteria [33]. Optimal C/N ratio is a function of the type of feedstock and may vary with the type of feedstock used. C/N ratio can be maintained at the required or acceptable range by mixing ratios of high and low C/N ratios. C/N ratio ranging from 22 to 25 is most suitable for anaerobic digestion of fruits and vegetable waste [34]. Romano and Zhang in a research paper written by Norberg [35] suggested that the optimal C/N ratio of onion juice and digested sludge should be maintained at 15 [33,35]. Li et al., [36] studied DAD of organic wastes and found C/N ratio between 15 and 18 when corn stover was inoculated with digested sewage sludge. Digestion rate decreased when C/N ratio increased to 21 or higher due to low pH in the first 7 days at 37 °C [33]. Thus pH has an effect on the digester rate and the C/N ratio.
OLR =
DailyflowxVSconcentration Liquid Volume
(2)
Gas production in an AD system is dependent on the OLR. When the feeding capacity in the system exceed the OLR, the gas production decreases [42]. This is due to accumulation of fatty acids in the digester slurry. Thus, OLR is one of the most important controlling parameters in the continuous system because if not carefully monitored, the system faces overloading and the system fails to function properly [43]. With regard to content, AD is classified into three categories which are: low solids (LS) AD system that is less than 15% TS, medium solids (MS) processes which are about 15–20% TS, high solids (HS) processes at 20–24% TS range [44]. Fernandez et al. [29] optimised OLR for mesophilic systems and found that values for OLD to be 2.5–3.5 kgVS/ m3- day for cattle manure, 5–7 kgVS/m3-day for cattle manure with cosubstances and 3–3.5 kgVS/m3-day for pig manure [11]. 2. Methodology 2.1. Substrate collection
1.3.3. pH level The pH value is a measurement of acid or basic concentration in aqueous substances, i.e., the concentration of hydrogen ions in solution. Anaerobic bacteria, e.g. the methanogens are sensitive to acid concentrations and their growth can be lowered if the digester system is acidic [33]. Lee et al. [37] show that they have optimised the pH value in AD systems and it was found that the pH value for methanogenesis was around 7, but the pH value differed in all the AD stages. Lee et al. [38] found that the optimal pH range for methanogens in AD to be 6.5 – 8.2, while Kim et al. [39] reported a pH value at 5.5 6.5 for hydrolysis and acidogenesis. The pH of a digestate varies with retention time, but the initial step, acetogenesis process in a batch reactor occurs at a rapid pace and in the acidogenesis process acid is produced since it is lower in the digestion tank. It is important to constantly measure the pH to ensure the well-being of methanogens so that methane can be produced. Methane has been observed to occur when the pH is at its
The main objective of this study was to evaluate the bio-methane potential from the dry fermentation of organic fraction of municipal solid waste (OFMSW) using cow manure for inoculation under controlled experimental conditions. The substrate was quantified and characterised before the onset of production. Fig. 5 shows a summary flow diagram of the feedstock or substrate quantification. The food waste collected was sorted in weekly basis according to the following categories: starch, carbohydrates, protein, vegetables and fruits. The characterised food waste was weighed on a mass scale and the masses of the various food waste samples were then recorded. Waste characterization was done to ascertain the composition. These included physical and chemical composition with regards to C/N ratio, volatile solids, total solids and elemental analysis for carbon, nitrogen, sulphur and hydrogen in accordance with the standard method (APHA 1995) [45]. 2331
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Table 2A Substrate characterization for the anaerobic digestion. Substrate
C (%)
H (%)
Vegetables and Fats Fruits and Proteins Starch and Fibres Cow manure
24.18 22.77 32.93 14.87
1.04 1.96 1.12 1.65
N (%)
S (%) 3.28 2.53 2.99 0.84
0.00 0.00 0.00 0.00
Table 2B Substrate characterization for the anaerobic digestion.
Fig. 5. Flow diagram of the feedstock quantification.
The experiment was run at mesophilic temperature 37 °C and pH of 7 under controlled 500 ml digester. The amount of biogas produced was measured using water displacement method. Biogas was analysed using gas chromatograph containing flame-ionization detector. The operating conditions of oven temperature were 70 °C, detector 150 °C, injection port 80 °C and helium was used as carrier gas at flow rates of 20 ml/min. Fig. 6 shows the biomethane potential set up.
TS (%)
VS (%)
C/N Ratio
Vegetables and Fats Fruits and Proteins Starch and Fibres Cow manure
63.72 70.40 60.01 60.01
30.27 29.02 37.00 91.55
76.16 80.67 71.24 78.72
7.37 9.00 11.02 17.70
where; Mdried = Amount dried sample (mg), Mwet = Amount of wet sample (mg) and Mburned = Amount of burned sample (mg).
(F *CF ) + (S*Sf ) C = N (F *Nf ) + (F *Nf )
The purpose of this experiment was to determine methane production in the dry anaerobic digestion of organic fraction of municipal solid waste (OFMSW) inoculated with cow manure at the mesophilic optimum temperature (37 °C) and initial pH of 7 using water displacement method and Bio-Methane Potential (BMP) analyser. Tables 2A, 2B shows the substrate characterization. TS is the sum of dissolved solids and suspended solids. TS and pH are the important parameters to assess anaerobic digestion process efficiency [14,21]. VS is the organic portion of TS that biodegrade in the anaerobic process. C/N ratio is an important factor in bacteria stability in the anaerobic process [46,47]. TS, VS and MC are calculated using Eqs. (3)–(5) respectively while C/N ratio is calculated using Eq. (6).
VS (%) =
Mdried − Mburned *100 Mwet
(3)
TS (%) =
Mdried *100 Mwet
(4)
(6)
where; F = first substrate, S = second substrate, Cf = carbon composition for the first substrate, Cs = carbon composition for the second substrate, Nf = nitrogen composition for the first substrate and Ns = nitrogen composition for the second substrate. From the characterised study of the food waste. It was found that the fruits and proteins (FP) had low solids content (29.02% TS) due to the fact that greater portion of the sample contained fruits with high moisture content whereas the sample containing starch and fibres (SF) contained a high concentration of solids (37% TS). Volatile solids represented an organic matter of the feedstock without considering the inorganic salts and ash. Total solids percentage represented organic and inorganic material in the feedstock. The proportion of volatile solids in total solids was much higher in the fruits and protein sample (80.67%) compared to the samples containing starch and fibre (71.24%) as well as vegetable and fats (76.16%). The average moisture content (MC) of all the three samples was found to be 63.72%, 70.40% and 60.01% respectively. A high moisture content percentage favoured optimum biogas production since it allows bacteria to release methane and metabolic processes to occur. Hence moisture content was significant for optimal digestion as it aids the digestion process to yield high-quality biogas rich in methane (CH4). Zhu et al. [34] reported that substrates with MC of 75% such as food and yard waste are suited for digestion. It was also recommended by Abbasi et al. [48] that the MC for optimum conditions to be 90%. It was also stated that the wetter the material, the more volume and area it takes up relative to the levels of gas production, but we can observe that the values obtained from this current study were within the dry digestion range. The carbon and nitrogen (C/N) ratio for the three samples was found to be 7.37, 9 and 11.01. From these results, it was found that the carbon ratios were too low compared to the study carried by Sreekrishnan et al., [49] that shows, the best anaerobic digestion range should be between 16 to 30 C/N ratios for food waste. Substrates with low C/N ratio was most likely result in ammonia accumulation and volatile fatty acids (VFAs) in the digester, which could hinder the methanogenic activity and system failure [49]. From the results summarized in Table 2B, it was comprehended that maintaining of proper composition of the substrate was necessary. The higher carbon content in the system gave more carbon dioxide formation and lower pH value which was also a factor that needed to be maintained and controlled according to Dioha et al. [50]. When the levels of C/N ratio
3. Results and discussion
Mwet − Mdried *100 Mwet
MC (%)
where; C – carbon, N – nitrogen, H – hydrogen, S – sulphur, MC- moisture content, TS – total solids, VS – volatile solids and C/N – carbon-nitrogen ratio.
2.2. Experimental procedure
MC (%) =
Substrate
(5)
Fig. 6. BMP test experimental setup.
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Fig. 7. Methane yield of OFMSW from BMP analyser in hours.
toxic to non-toxic forms of compounds such as ammonia. Li et al., [54] suggests that ammonia accumulation of the system hinders the production of methane as well as causing system failure [54,55]. The problem of pH causing ammonia accumulation was addressed by adding another substrate to the system (co-digestion). OFMSW are rich in substrate essential for bacteria to grow and produce high-quality methane. Using gas chromatography (GC) to analysis the biogas production before carbon dioxide fixing or absorption, the composition of the biogas was found as follows; fats and vegetables 70% CH4 and 30% CO2, starch and fibre 70% CH4 and 30% CO2 and lastly fruits and proteins had biogas consisting of 60% CH4 and 40% CO2.
was high, this suggested higher depletion of nitrogen used by the methanogenic bacteria that produces methane to satisfy their protein needs, therefore, resulting in lower biogas production rate. To operate an anaerobic digester at optimum C/N ratio, biodegradable material of high C/N ratio was blended with the biodegradable material of low C/N ratio. This enhanced the optimum conditions of C/ N ratio to be within 15–30:1 [46]. It was observed from Fig. 7 that methane production increases with retention time increases. Hence, it can be stated that there was a directly proportional relationship between retention time and methane production. Fig. 7 illustrates methane yield of OFMSW from BMP analyser in hours. There was an absence of lag phase due to presence and balance of active microbial for digestion supplied by the inoculate. The peak methane production of the vegetables and fats sample (249.6 Nml) was achieved up to 130 h (5 days) demonstrating the maximum degree of methane formation out of all the three samples. Samples containing fruits & proteins as well as starch and fibres had methane volumes of about 219.7 Nml and 128.5 Nml respectively and were achieved within 48 h before the samples stopped producing any gases. From the sample of vegetables and fats, it was observed that the maximum methane production was achieved in a shorter time because of the availability of agitation on the system since maximum methane is achieved as agitation rate increases thus indicating the effective influence of the agitation rate on the overall conversion [51]. The substrate time and the microbial balance in the system also places a role in the maximum production of methane at a shorter retention time. High biodegradability matters produce methane at a faster rate and afterwards ease production. The pH changes during anaerobic fermentation were due to the accumulation of volatile fatty acids (VFA's) by acidogenic bacteria, the pH value for sample 1, samples 2 and sample 3 were initially at 7.2, 6.99 and 6.98 respectively in the start-up of the run. Since the digestible compounds of organic matters were hydrolysed and converted into fatty acids quickly, the pH began to decrease gradually to a higher pH value which can be observed to be the reason why the two sample only produced methane in the two days [52]. The acidogenic bacteria population increases more than the appropriate ratio required, these created an excess accumulation of acids medium inside the digester. Thus increasing acidity and eventually decreasing pH, causing deactivation of methanogens and so the digestion process [53]. The pH had an influence on the equilibrium of
4. Conclusion It was concluded that in experimental work under laboratory scale using conventional biochemical methane potential and had optimum methane yield after retention time in hours. Co-digestion of OFMSW and cow manure stabilises conditions in anaerobic digestion process such as C: N ratio in the substrate mixtures as well as macro and micronutrients, pH, inhibitors or toxic compounds and dry matter. Although BMP results only showed methane production in hours because of the small loading rate, these results reinforce the validity of OFMSW as a strong candidate for use in dry anaerobic digestion. It was then concluded that OFMSW was indeed a better feedstock to be used in dry anaerobic digestion and should be co-digested with a secondary substrate to minimise ammonia accumulation and sample acidity.
Acknowledgement The authors wishes to express their appreciation to Process Energy Environmental and Technology Station (PEETS) funded by South Africa National Energy Development Institute (SANEDI), Technology Innovation Agency (TIA) and City of Johannesburg (CoJ/UJ/WTE/ FS003) (CoJ, Green Economy), Chemical Engineering and Applied Chemistry Departments at the University of Johannesburg for allowing us to work in their laboratories. Dr Robert Huberts, Prof Jane Catherine Ngila, Dr Tumisang Seodigeng, Dr Ludger Eltrop and Prof Shivani Mishra for consultancy.
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