Investigation of Biogas Generation from the Waste of a Vegetable and Cattle Market of Bangladesh

In Bangladesh, the collection and disposal of solid wastes of markets (bazaars) are done haphazardly. The practice mainly includes accumulation of solid wastes at different locations of a market by sweepers/cleaners irregularly, then loading the wastes in wheelbarrow or rickshaw van, and dumping the wastes on nearby road side/open area or into khal/river causing public annoyance and environmental pollution. This practice is aesthetically displeasing, and possesses a great threat to the environment by emitting greenhouse gases, polluting soil, surface water and groundwater, producing odorous gases and blocking the drains. When disposed on lands and waste dumps become dry, these are burnt to reduce the volume. The burning generates smokes and particulate matters polluting the surrounding air. A better waste collection, treatment and disposal system is necessary to improve the overall environment of a market. Waste reduction, reuse, recover, recycling and ultimate disposal of residual wastes are the key elements of an effective solid waste management plan. To improve the quality of life, the consumption of energy has been increasing rapidly worldwide causing high rate of depletion of fossil fuel. The use of fossil fuel as the primary source of energy has led to global warming and environmental pollution. In this backdrop, alternative sources of energy (water, sun light, biomass, wind etc.) have become the focus of attention worldwide. Energy recovery from solid wastes is gaining importance day by day. There are many technologies for producing energy from solid wastes such as anaerobic digestion, incineration, co-incineration *Corresponding author: Jalil A, Department of Civil Engineering, Bangladesh University of Engineering and Technology (BUET), Bangladesh, Tel: + 880–2–966 5639; Fax: + 880–2–966 5639; E-mail: majalil@ce.buet.ac.bd


Introduction
In Bangladesh, the collection and disposal of solid wastes of markets (bazaars) are done haphazardly. The practice mainly includes accumulation of solid wastes at different locations of a market by sweepers/cleaners irregularly, then loading the wastes in wheelbarrow or rickshaw van, and dumping the wastes on nearby road side/open area or into khal/river causing public annoyance and environmental pollution. This practice is aesthetically displeasing, and possesses a great threat to the environment by emitting greenhouse gases, polluting soil, surface water and groundwater, producing odorous gases and blocking the drains. When disposed on lands and waste dumps become dry, these are burnt to reduce the volume. The burning generates smokes and particulate matters polluting the surrounding air. A better waste collection, treatment and disposal system is necessary to improve the overall environment of a market. Waste reduction, reuse, recover, recycling and ultimate disposal of residual wastes are the key elements of an effective solid waste management plan. To improve the quality of life, the consumption of energy has been increasing rapidly worldwide causing high rate of depletion of fossil fuel. The use of fossil fuel as the primary source of energy has led to global warming and environmental pollution. In this backdrop, alternative sources of energy (water, sun light, biomass, wind etc.) have become the focus of attention worldwide. Energy recovery from solid wastes is gaining importance day by day. There are many technologies for producing energy from solid wastes such as anaerobic digestion, incineration, co-incineration and refuse derived fuel [1]. Among them anaerobic digestion has become a promising technology particularly for recovery of energy from biodegradable organics. Anaerobic digestion of market wastes is a potential environment friendly technique which produces energy in the form of biogas [2][3][4][5][6][7][8] and the residue can also be used as fertilizer. Biogas is gaining popularity as a substitute for other fuels to save energy and to protect the environment. Successful dissemination of domestic biogas technology has occurred worldwide in countries where governments and institutions are involved in the subsidy system, in planning, design, construction, operation and maintenance of biogas plants. Several [11,12]. In Bangladesh, biogas is currently being used mainly for cooking and lighting purposes in rural areas, and there have been some experiments on the use of biogas to generate electricity or drive irrigation water pumps [10]. Huge amount of biodegradable solid wastes (i.e., food waste, fish waste, slaughtered animal waste, animal feces, vegetable waste, fruit waste etc.) are generated in the markets of Bangladesh and can be used for biogas generation. However, in Bangladesh, no study on biogas generation based on market solid wastes has been reported in the literature. The authors of the paper have undertaken a research project on solid waste management of rural markets of Bangladesh. As a part of the project, investigation on the type and quantity of solid wastes produced in a vegetable and cattle market was carried out, and the biogas generation from the wastes was quantified through laboratory scale reactors. This paper presents the results of the investigation.

Anaerobic Digestion Process
A consortium of microorganisms working synergistically accomplishes the anaerobic digestion of organic matter in the absence of oxygen with ultimate products being carbon dioxide and methane. The general process of anaerobic digestion is a series of four metabolic processes like enzymatic hydrolysis, acidogenesis, acetogenesis, and methanogenesis ( Figure 1) and each metabolic stage is assisted by a series of microorganisms. A brief description of the processes is given below.
1. First, large protein macromolecules, fats and carbohydrate polymers (such as cellulose, hemicellulose, starch, pectin, and lignin) are broken down through hydrolysis by extracellular enzymes to amino acids, long-chain fatty acids, and sugars before being taken up by acidogenic bacteria. The rate of hydrolysis is a function of pH, temperature, composition, particle size of the substrate and concentrations of intermediate products [13].
2. After that, soluble organic components including the products of hydrolysis are fermented to form three, four, and five-carbon volatile fatty acids (such as lactic, butyric, propionic, and valeric acid), alcohols, hydrogen, and carbon dioxide by acidogens.
3. In acetogenesis, bacteria consume these fermentation products and generate acetic acid, carbon dioxide, and hydrogen. 4. Finally, methane is produced by methanogenic bacteria from the acetic acid, hydrogen, and some of the carbon dioxide as well as directly from other substrates of which formic acid and methanol are the most important [13]. Three biochemical pathways are used by methanogens to produce methane gas. The pathways along with the stoichiometries of the overall chemical reactions are: a.
Acetotrophic methanogenesis: CH 3  Methanol is shown as the substrate for the methylotrophic pathway, although other methylated substrates can be converted. Sugars and sugar-containing polymers such as starch and cellulose yield one mole of acetate per mole of sugar degraded. Since acetotrophic methanogenesis is the primary pathway used, theoretical yield calculations are often made using this pathway alone. From the stoichiometry above, it can be seen that the biogas produced would theoretically contain 50 percent methane and 50 percent carbon dioxide. However, acetogenesis typically produces some hydrogen, and for every four moles of hydrogen consumed by hydrogenotrophic methanogens, a mole of carbon dioxide is converted to methane. Substrates other than sugar, such as fats and proteins, can yield larger amounts of hydrogen and acetate can be biochemical substrates for a number of other products as well. Therefore, the overall biogas yield and methane content will vary for different substrates, biological consortia and digester conditions. Typically, the methane content of biogas ranges from 40-70 percent (by volume).
For healthy methanogenesis to occur, anaerobic conditions are essential. The digesters used must be well sealed which allows the biogas to be collected for energy conversion and eliminates methane and carbon dioxide emissions during the anaerobic digestion process. In addition to methane and carbon dioxide, small amount of hydrogen sulfide and ammonia are produced. The most important nutrients for bacteria are carbon and nitrogen, and must be provided in the proper ratio. Otherwise, ammonia can build up to levels that can inhibit the microorganisms.
In general, the optimal conditions for anaerobic digestion of organic matter are near-neutral pH, constant temperature (thermophilic or mesophilic), and a relatively consistent feeding rate. Imbalances among the different microorganisms can develop if conditions are not maintained near optimum. Imbalances occur if too much feed is added, if inhibitory compounds accumulate, or if the feed stream lacks natural pH buffers. Solid concentrations higher than about 40 percent total solids can also result in process inhibition, likely due to the reduced contact area available to the microorganisms. Higher temperatures result in faster reaction kinetics. However, the microorganisms themselves are adapted to relatively narrow temperature ranges. Mesophilic and thermophilic microbes are adapted to roughly 30-40°C and 50-60°C respectively. In a well-balanced anaerobic digestion process, all products of a previous metabolic stage are converted into the next one without significant buildup of intermediate products [14]. The overall results are nearly complete conversion of the anaerobically biodegradable organic matter into end products like methane, carbon dioxide, hydrogen sulfide and ammonia.

Selected market and composition of waste
Madbarer Char bazaar (a large size market) of Shibchar upazila of Madaripur district of Bangladesh was selected for the study. It consists of both permanent and temporary shops (about 1500 nos.) including open spaces for cattle and goat market. This market is usually functional twice a week -on haat days (Sunday and Thursday). On Thursday, the haat is usually the largest. Cows and goats are sold only on that day. Large amount of vegetables are sold on both Sunday and Thursday. Twenty restaurants are operated only on the haat days and 7-8 tea stalls serve on other days of the week. On non-haat days, milk, fishes and very limited amount of vegetables are sold. Not much commodities are sold on other five days of the week.
Two labors were appointed to collect, separate, store and measure the wastes on two consecutive haat days. They collected all the wastes produced in a whole day and separated each item and kept them in different sacks. Figure 2 shows collection of wastes by two labors in a plastic bucket. The amount of individual item was measured using an electronic weighing scale after the hat was over. The composition of the wastes is shown in Table 1. From Table 1, it is found that the amount of wastes generated on Thursday was about 25% greater than those on Sunday. On the haat day, the average amount of easily biodegradable waste was found to be 644 kg out of the total waste of 1054 kg. About 61% of the waste generated was biodegradable. Cow dung, fish waste, ginger, cursed lobe, guava, and banana leaf were the major biodegradable wastes. Goat droppings, bitter melon, brinjal, dhundul, and pointed gourd were the minor biodegradable wastes.

Experimental setup
In order to generate basic data on biogas generation, experiments were run in two phases. In the first phase, batch reactors were operated and in the second phase, daily feed reactors were operated. The same composition of the easily biodegradable portion of the market waste was used in both the phases. Daily average composition of the biodegradable wastes was determined on the basis of the average waste generation on the haat days per week. Experiments were conducted using this daily average composition. At the onset of the experiments, the TS (Total solids) and VS (Volatile Solids) of the biodegradable wastes were determined. The biodegradable wastes were cut into small pieces (maximum dimension of 4 mm) before feeding a reactor (anaerobic digester). A glass bottle of 2.5 liter capacity was used as batch reactor for biogas generation. Two batch reactors were set up in the Environmental Engineering Laboratory of BUET in the first phase of the experiments. The effluent of the biogas plant of Jahangirnagar University was used as the liquid of the digester to ensure the abundance of active microbes for biogas generation. The digester was placed in a closed chamber made of Thai Aluminium and two electrical room heaters were placed inside the chamber to maintain a constant temperature. The heaters were operated alternatively. Biogas was collected in a gas holder using water displacement method. The experimental setup is shown in Figure 3. In the second phase of the experiments, two daily feed reactors -one  single chamber reactor and one double chamber reactor (two reactors connected in series) were set up in the Environmental Engineering Laboratory of BUET. A plastic container fitted with an inlet tube at the mid height and an outlet tube near bottom on the opposite side of the container was used as the single chamber reactor. Two plastic containers were connected with a tube near the bottom and an inlet tube was fitted to the first container at the middle while an outlet tube was attached near the bottom of the second container to make the double chamber reactor. Other arrangements were the same as those in the first phase of the experiments. The experimental setups are shown in Figure 4.

Operation of anaerobic digesters
In the first phase, the experiments were started on the 18th August, 2016 and ended on the 2nd October, 2016 (total 45 days of operation).
In this phase, 500 g and 750 g of the waste were added in two 2.5 L digesters separately and the amount of liquid added was 1500 mL for 500 g waste and 1400 mL for 750 g waste to make the effective volume of 2.1 L. The temperature of the chamber and the volume of gas generation were recorded daily during the experiments. The pH of the contents of the digesters was sometimes measured. Gas leakage from the digester was sometimes not realized, and when this problem was understood/ encountered, remedial measures were taken immediately. The experiments of the second phase were started on the 25th October, 2016 and ended on the 18th December, 2016 (total 53 days of operation). In the single chamber reactor, 750 g waste and 1500 mL inoculum were added at the start of the experiment. Each container of the double chamber reactor was loaded with 375 g waste and 1000 mL inoculum at the beginning of the experiment. The data of the 1st phase experiments revealed that the optimum retention time was 40 days. Hence, 18.75 g of waste was mixed with 37.5 mL of water to prepare the daily feed for the single chamber reactor. From the 2nd day of the operation, this mixture was fed at a time every day through the inlet of the reactor after taking out 55 mL slurry from the reactor through the outlet. For the double chamber reactor, the daily feed consisted of a mixture of 18.75g waste and 50 mL water. The first chamber of the reactor was fed daily with this mixture once after taking out 67.5 mL slurry from the second chamber. The temperature within the Thai Aluminum chamber and the volume of gas generation of each reactor were recorded daily during the experiments. On the 3rd December, 2016, the heater was out of order and the experiments were continued at the ambient temperature up to the end.

Methods of measurement
The biogas generated was measured daily by water displacement method using an inverted measuring cylinder filled with water and placed in a water jar. The inverted cylinder was refilled as and when needed. The temperature within the closed Thai Aluminum chamber was measured by a thermometer. The temperature was also displayed on the heater. The TS and VS of the wastes, and pH of the content of the reactor were determined according to the APHA Standard Methods [15].

Results and Discussion
The TS and VS of the biodegradable portion of the market waste were found to be 17.94% and 13.87% respectively. The batch experiments of the 1st phase were run for 46 days with 500 g and 750 g waste in two reactors. The variation in the quantity of daily gas production in the reactors and the daily average temperature within the Thai Aluminum chamber with time are shown in Figure 5. The results of the 1st set of experiments reveal that for each digester, the gas production per day was very high during the initial few days of operation and then fluctuated for the remaining days. The relatively high quantity of gas production in each digester during the initial few days can be attributed to the biodegradable organic matter of the added liquid and the amount of readily biodegradable organic matter (cow dung) added to the digester. Variability in the biodegradation processes of the components of the waste was the main reason for fluctuation in the daily gas production. After the first three days, the variation in daily gas production was small for 750 g waste up to about 39 days of operation and then gas production reduced to low values in general. But the daily gas production reduced to low values for 7 days after the initial high values. Undetected leakage and gas collection problem might be the reasons of this poor performance. Then the daily gas production increased significantly for 9 days. After about 20 days of operation, the daily gas production of 500 g waste was low and the fluctuation was insignificant. The highest gas production per day was 1700 mL for 500g waste, and 3100 mL for 750 g waste. The daily average temperature varied from32-36°C and it is revealed that this small variation in the temperature has no effect on the rate of gas generation. The pH of the reactors was measured at the end of the experiments and was found to be 7.6 for 500 g waste and 7.7 for 750 g waste indicating favorable condition for microorganisms. It can be assumed that the pH was in favorable range judging the rate of the gas production throughout the experiments.
The cumulative gas production with time curves for the digesters are shown in Figure 6. The nature of the curve for 750 g waste appears to be reasonable. Although the curve for 500 g waste appears reasonable for the first 3 days corresponding to generation of high amount of biogas, later the production rate reduced abruptly -the reason appears to be gas leakage from the digester which could not be detected. Additional sealing of the reactor was done and the rate of gas production increased  which is reflected in the nature of the curve. The total gas production for 45 days of operation was about 20.6 L for 500 g waste and 41.6 L for 750 g waste. It reveals that this higher waste concentration was more favorable for biogas generation.
The experiments of the second phase were operated on daily feed basis with 750 g waste as initial substrate and run for 53 days. The variation of the daily gas production in each system (single chamber and double chamber) with time and the daily average temperature with time are depicted in Figure 7. From the results of the 2 nd phase of experiments, it is observed that the gas production per day was very high during the first two to three days owing to availability of high amount of easily biodegradable organic matter (cow dung). Then a sharp drop was noticed in the daily gas production and it fluctuated for 10 to 11 days. After that a more or less stable condition was obtained and continued as long as the average daily temperature was within 32-38°C. When the temperature dropped suddenly to 22-25°C, the daily gas production rapidly decreased to low values. Sudden drop of the closed chamber's temperature came as a shock to the microorganisms resulting in slow down of their metabolic activity greatly and the gas production was negligible. It should be noticed that the daily gas production was generally higher for double chamber in comparison with that of the single chamber. Greater retention time and prevention of short circuiting of daily added feed are the reasons for the higher rate of gas generation. Under favorable conditions, the average daily gas production was 635 mL for single chamber and 780 mL for double chamber. About 23 percent higher gas production was achieved under stable condition. The cumulative gas generation in the single chamber and double chamber with days of operation is presented in Figure 8. In general, the trend of the curves appears to be reasonable. The initial part of the curves is steeper and then the slope decreases up to 13 days of operation. After that both the curves become straight lines revealing stable rate of gas generation up to 40 days of operation. The slopes of the straight lines are different -higher value for double chamber and lower value for single chamber. Then the slope of each curve decreases due to sudden drop of the temperature within the closed chamber. The cumulative gas production for 40 days was 39.1 L for single chamber and 44.6 L for double chamber. Based on the data of the batch reactors presented in this paper, it is found that for 40 days of hydraulic retention time (HRT), the daily average biogas generation per unit volume of the reactors were 0.225 and 0.482 m 3 /m 3 for organic loading rate (OLR) of 0.83 and 1.24 g VS/L/d respectively. The corresponding rates of biogas producton were 0.273 and 0.389 m 3 / kg of VS added. For the daily feed reactors having HRT of 40 days, the stable rate of daily biogas generation per unit reactor volume was 0.289 m 3 /m 3 for both the single chamber and double chamber reactors. The stable rates of biogas production were 0.244 and 0.30 m 3 /kg of VS added for the single chamber reactor (OLR=1.18 g VS/L/d) and the double chamber reactor (OLR=0.96 g VS/L/d) respectively. The double chamber reactor was more productive than the single chamber reactor because short circuiting of fresh feed could not occur in case of the double chamber reactor. For batch studies, Bouallegui et al. [4] reported biogas generation rate of 0.16 m 3 /kg of VS added for OLR of 1.06 having HRT of 47 days and 0.26 m 3 /kg of VS added for OLR of 0.9 g VS/L/d with HRT of 32 days. In case of daily feed reactor, Babaee and Shaygen [6] obtained biogas generation rate in the range of 0.30 -0.47 m 3 /kg of VS added for vegetable wastes with OLR in the range 1.4-2.75 g VS/L/d with HRT of 25 days. They also reported biogas generation rate of 0.26-0.47 m 3 /kg of VS added for OLR in the range of 0.3-1.6 g VS/L/d for fruit and vegetable waste and municipal solid wastes from literature. Sridevi et al. [7] conducted daily feed two phase studies (acidogenic, HRT=2 days and methanogenic, HRT=15-25 days) with OLR varying from 1.5-4.5 g VS/L/d and found biogas production in the range of 0.24-0.72 m 3 / kg of VS added. Patil and Deshmukh [8] reported the biogas yield from a mixture of vegetable wastes in the range of 0.36-0.90 m 3 /kg of VS added from literature. The results obtained from the present study agree well with those reported in the literature.

Conclusion
The composition of a rural market wastes was determined and experimental investigation of biogas production from the biodegradable portion of the market wastes was conducted using batch reactors and daily feed reactors. Based on the results of the study, the following conclusions can be made: