How to Generate Biogas from Organic Wastes?

The following article will guide you about how to generate biogas from organic wastes.

Biogas Yield from Crop Residues:

A.C. Varshney, et al., studied biogas potential of certain crop residues such as castor shells and leaves, pearl millet husk, mustard straw and potato haulm. M.S. Kalra and J.S. Pawar carried out experimental studies to investigate biodegradability of rice-straw which caused biogas yield of 4 ft³/lb (200 litre/kg) under batch digestion from experimental digesters of 50 gallon (190 litres) capacity.

Rice-husk did not undergo complete digestion following its high lignin content and unfavourable non-lignin carbon-to-nitrogen ratio. A mixture of rice-straw and cattle dung in equal proportion on dry weight basis yielded 9.1 per cent more gas than rice-straw alone. A. Rahman, et al., reported yield in batch loaded laboratory digesters from rice-straw, sawdust and water-hyacinth inoculated with cowdung.

Under identical conditions, rice-straw produced 310 litre biogas with 77.8 per cent methane content sawdust 144.8 litre and 62.28 per cent and water-hyacinth 368 litre with 77.13 per cent (methane) respectively. The pH of the digester contents was initially low and found to be decreasing as the digestion picked up.

Total microbial populations of bacteria, fungi, actinomycetes, cellulolytic organisms and coliform bacteria were found to decrease with time. D.M. Badger, et al., carried out laboratory-scale studies for anaerobic fermentation of crops and organic wastes at loading concentrations of 3-10 per cent total solids.

In general the concentration of an element as a percentage of the dry weight was higher in the residue than in the undigested material. This occurs on account of conservation of nutrient elements during digestion and gasification of some of the organic material which makes it rich fertiliser. Biogas yield decreased with increasing loading concentrations.

Above 5 per cent total solids, digestion of crops with high soluble carbohydrate content needed continual additions of alkali to maintain a pH of 7 ± 0.5. Volatile solid contents, volatile solids destruction, yield and composition of biogas from anaerobic digestion of various crops and organic wastes at 5 per cent total solids loading concentration and 37°C after 17-36 days of digestion are given in Table 7.6.

The composition of biogas varied with the material from 50 percent methane from newsprint to 68 per cent from cattle manure. The balance was carbon dioxide in all cases with less than 0.001 per cent hydrogen sulphide. In another similar study carried out at the Kasetsart University in Bangkok, S. Tumwasron reported biogas yield and gas composition from agricultural crop residues. S.A. Waksman presented chemical composition of select agricultural crop residues for assessing their biodegradability.

With growing age of agricultural plants, sugars, amino acids, proteins, minerals etc. all of which are soluble in water tend to decline whereas non-water-soluble components like cellulose, hemicellulose and lignin tend to grow. Actual nutrient content with possible exception of nitrogen is though seldom low enough to limit the rate of anaerobic fermentation, biological availability of organic matter is usually a rate-limiting factor.

The higher is proportion of lignin in substrate the less cellulose is available for bacterial action with the result that anaerobic fermentation becomes less efficient. In other words, presence of lignin largely controls the rate of anaerobic fermentation. Lignin is considered to be the main non-biodegradable component of the substrate. Lignin is insoluble in water, ether, alcohol, dilute alkali and is insusceptible to bacterial attack.

According to A.M. Buswell and W.D. Hatfield low availability of nitrogen is usually a rate-limiting factor for anaerobic fermentation of non-leguminous plants. Nitrogen content in agricultural crop residues depends on type of species, age of plant, environmental conditions and availability of nitrogen-based soil nutrients. Amount of crop residues available can be estimated by multiplying expected or available harvest weights by what is called residue coefficients.

If only a portion of available crop residues can be harnessed, only that part need to be multiplied with residue coefficients. These coefficients are ratios of weight of dry matter of crop residues to weight of harvest measured at field moisture level. For grains and straw, field moisture level can be assumed as 15 per cent.

Gliricidia macullata, a tree introduced in India from the West Indies, is grown for shade and green leaf manuring. It can be easily grown on wastelands and along roadsides. Based on experimental studies, V.N. Gunaseelam established the potential of Gliricidia leaves as feed for biogas production and the use of digested effluent as a rich fertiliser which is superior to fresh Gliricidia leaves.

Experiments were carried out in 3 litre batch digesters at room temperature (32 ± 3°C) and gas yield of 165-180 ml methane per g. VS added, and VS reduction of 37-39 per cent was reported. S .K. Sharma et al., studied the potential of Mirabilis leaves as feed for biomethanation.

They reported that under identical experimental conditions Mirabilis leaves produced nearly 400 litres of gas/kg of dry matter which is double the amount produced by cattle dung (200 litre/kg of dry matter). Methane content in biogas obtained from Mirabilis leaves was 69 per cent as against 62 per cent normally obtained from cattle dung. Total reduction of volatile matter in Mirabilis leaves was 42.6 per cent as compared to 22 per cent in cattle dung.

A.S. Traore carried out studies to analyse prospects of biogas produc­tion from Calotropis procera, a latex plant found in West Africa. This plant, which contains hydrocarbons as well, is found in great abundance in Burkino Faso with the highest density occurring in Sahelian and semi-desert zones. C. procera is a bush latex plant, 1-3 m high, its leaves are 10-13 cm wide and 17-19 cm long.

This plant is fast-growing, draught-resistant and grows throughout the Sahelian countries, notably in Burkino Faso. A high speed blade grinder was used to obtain crushed leaf powder of the plant. The fine particles thus obtained were suspended in water and used for fermentation test runs at 30°C in batch mode. It provided a valuable substrate for biogas production.

The highest yield obtained varied from 2.9 to 3.6 litres/day litre at initial pH of 7.5 with digester loading rate being 4 per cent (w/v) suspension of dry leaves. The acidogenic phase of the fermentation process was found to be very fast with 66 per cent of dry material loaded being degraded during the first two days of incubation at 30°C. The resulting biogas was found to contain 56-59 per cent methane by volume.

Studies have been carried out recently to examine the prospects of coir industry wastes, namely, coir pith for anaerobic fermentation. K.S. Jagadeesh carried out studies at the University of Agricultural Sciences, Dharwad, Karnataka by using coir pith as additive to cow dung in the ratio of 1 : 4 as feedstock.

In his experiment coir pith was soaked in calcium hydroxide for 24 hours and washed with water to remove the traces of alkali. This was followed by mixing with cowdung and water to obtain slurry with a total solid content of 9.2 per cent. The slurry was then fed to a 4 capacity KVIC biogas plant.

After a lapse of 40 days, the plant was fed with slurry once in 24 hours and digestion was continued for 80 days. In the first week there was no gas production but later it averaged around 2.5 m³. During the 50-55th day interval it went upto 2.74 m³/day which was equivalent to 0.152 m³ of gas/kg of dry matter/day. When compared the corresponding yield with cow dung alone-was 0.20-0.25 m³/kg dry matter/day.

K.G. Gollakota and K.K. Meher studied the effect of particle size, temperature, loading rate and stirring on biogas production from oil-expelled caster cake. Experiments were carried out in 5 litre capacity single-stage digesters protected from light at 30 and 37°C. Four particle sizes in the range less than 0.5 to 2 mm were used. Volumetric loading rates ranged from 4 to 12 g TS/1-day.

Stirring of cultures was continuous with a magnetic stirrer or intermittent with a platform shaker. The HRT was 15 days in all cases and the fermenters were fed once a day. Biogas yield was measured in terms of litre of biogas/g of TS. Both rate and yield were higher with particles 2-1.4 mm and 0.5 mm and less with particles of intermediate size, i.e. 1.4-1.0 mm and 1 to 0.5 mm.

Both rate and yield were higher at 37°C than at 30°C. W.J. Ng et al., carried out experiments for using Palm Oil Mill Effluent (POME) as feed for anaerobic fermentation which was claimed to be an effective means for POME treatment. At hydraulic retention time of 25 days and more, biogas yield during mesophilic stage was found to be more than during thermophilic stage.

Common Biogas Yield Norms Based on Several Studies:

Based on several studies carried out, United Nations attempted to stand­ardise norms of biogas yield from cattle, pig and poultry manure, night-soil, pretreated crop wastes and water-hyacinth. It also prescribed optimum proportions of night-soil, pig manure and crop residues in several mixed substrates for achieving maximum biogas yield.

National Academy of Science compiled biogas yield data of different waste materials. T.K. Ghose studied biogas from cattle manure, water-hyacinth, algae, bagasse, rice-husk and their different combinations. Biomass Energy Institute at Winnipeg in Canada provided estimates of equivalent energy from different animal wastes. Biogas yield from these wastes is given in Table 7.21.

Biogas yields as reported by R.D. Laura and M.A. Idnani from variety of mixed substrates comprising agricultural plant materials at ambient tempera­tures are given in Table 7.22.

Biogas Production from Dry Dung Cakes:

In view of the fact that dung cakes are more convenient to transport as compared to fresh manure and do not stink, D.R. Ranade, et al., examined the possibility of producing biogas from them. Experiments were carried out in two sets. In the first set, wet dung was diluted with an equal quantity of water to obtain slurry with 10 per cent total solids concentration. The same slurry with 10 per cent TS concentration was also used for preparing dung-cake samples.

These dung-cakes were later fermented by preparing their slurry at- (i) two months and, (ii) eight months intervals after they were formed. In another set of experiment about eight months old dung cakes purchased from market were used. Anaerobic fermentation was carried out separately with wet manure, two and eight month old cake samples prepared in laboratory and readymade dung cakes purchased from market, and the results of biomethanation compared.

Average gas yield with wet dung as feed was 3.68 litres/day, total gas generation over a period of 54 days being 204 litres with methane content as 55 and carbon dioxide content as 42 per cent. In the experiment with dry dung cakes purchased from the market, the average gas production was 2.78 litres/day and total gas produced over a period of 54 days was 150 litres with methane content as 47 and carbon dioxide also 47 per cent.

This means in relation to wet dung case, gas yield declined by 24.3 per cent and methane content less by eight per cent. In the experiment with two month old dry dung prepared in the laboratory, mean gas production was 3.2 litres/per day and total gas yield over a period of 54 days was 172.3 litres with methane and carbon dioxide content being 67 and 28.5 per cent, respectively.

When compared with wet dung case, this shows that mean gas production declined by 13 per cent and total yield was less by 32.3 litres but methane content went up from 55 to 66 per cent. In the experiment with eight months old dry dung cakes prepared in the laboratory, average gas yield was 2.72 litres/day with total gas production over a period of 54 days as 147.2 litres and methane content as 58.7 per cent.

This shows that in relation to the wet dung case, daily gas yield declined by 26 per cent and total yield for the same period was less by 57 litres. Comparative results of these experiments shows that one to two months old dung cakes can be harmlessly used for biomethanation without any appreciable decline in gas yield and also with some improvement in calorific value following somewhat higher methane content.