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The following article will guide you about how to generate biogas from industrial wastes.
Introduction:
Biogas can be generated not only by anaerobic fermentation of animal wastes and agricultural plant materials but also host of industrial wastes. Wastes from sugar factories, textile-mills and food-processing industry are particularly appropriate for fermentation. Sugar-mill wastes like bagasse and pressmud can be gainfully utilised for biomethanation.
Bagasse is traditionally used for steam generation, preparing animal feed, biomanure, furfural, paper and fibre-boards which recently has also been used as feed material for biomethanation. Pressmud which is another sugar plant waste has also considerable potential as feedstock for biomethanation. Pressmud, in normal course, is used for extraction of wax and as fertiliser in agricultural field.
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Distilleries cause environmental pollution by their effluents. Anaerobic digestion of distillery-wastes not only helps to recover energy but also keeps the environment clean. Biogas can be recovered by digestion of distillery wastes, spent grams, beet pulp, root and other residues. Cotton-mill wastes which are used as main or supplementary boiler fuel are also valuable feedstock for biomethanation.
Willow-dust is one of the solid cellulosic textile-mill wastes which can be anaerobically fermented. Biogas can also be obtained from canteen and confectionery wastes. Wastes from food-processing industry are yet another valuable resource whose availability is going up from year to year with changing lifestyle.
Biogas Generation from Bagasse:
Bagasse a fibrous ligno-cellulosic residue resulting from sugar-cane following extraction of juice is used for variety of applications including biomethanation. Its composition varies from place to place and fibre size depends on cane variety and processing technology used. Bagasse in general comprises 45 per cent cellulose, 28 per cent hemicellulose (pentosans), 20 per cent lignin, and 2 per cent as sugar wax, minerals and other impurities. Pentosans include products like araban, galactan and xylan.
Due to variable composition, it is infeasible to estimate carbon, oxygen and hydrogen content in bagasse in precise terms. It comprises 5 to 7 per cent of hydrogen with balance equally divided between carbon and oxygen on dry weight basis.
Cellulosic content of bagasse is responsible for its fuel value. Pre-drying of bagasse helps to raise its calorific value which depends on cane variety, fibre size and external area. On dry and wet basis, it is estimated to be somewhere close to 8330 BTU/lb and 3250 BTU/lb, respectively.
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Moisture content in bagasse varies from region to region. For example, in northern India, it is found to vary from 33 to 36 per cent whereas in southern region it varies from 20 to 30 per cent. Chemical composition of bagasse as generated in different world-regions is given in Table 9.1.
In view of the fact that bagasse is primarily a fibrous material, its properties compare well with those of hard and softwood. Comparative characteristics of hardwood, softwood and bagasse are given in Table 9.2.
Bagasse undergoes change in characteristics with time. Comparative characteristics of fresh and one year old bagasse are summarised in Table 9.3. In past, studies were carried out at the National Sugar Institute in Kanpur to study biogas potential of bagasse. It was reported that 1 tonne of dry organic matter generated 200-300 m3 of biogas and 0.5 tonne of biomanure.
Biomanure was found to be rich in nutrients like potassium, calcium, phosphorus, nitrogen and humus and superior to ordinary farm manure in several respects. Biomanure contained 75 per cent nitrogen, 1-1.2 per cent phosphorus and 0.6-0.8 per cent potassium.
Based on experimental studies, G.S. Narsimhamurty and A. Purushothaman reported that it is possible to obtain higher biogas yield from bagasse if instead of one digester two digesters connected in series were used. By this arrangement corresponding to a total residence time of 20 days it was possible to convert 71.4 per cent of bagasse to biogas.
Biogas Generation from Pressmud:
Pressmud is a sugar factory waste which is rich in organic matter (20.3 per cent), nitrogen (0.4 per cent), phosphate (0.4 per cent), potash (0.02 per cent) and sterols (0.33 per cent) has considerable commercial potential as feedstock for biomethanation. In over 325 sugar mills in the country around 60 million tonnes of sugar-cane is crushed annually which generates about 2.5 million tonnes of pressmud.
One tonne of pressmud following anaerobic fermentation yields 50 m3 of biogas. If the entire pressmud available in the country is used for biomethnation, it can generate 120 million m3 of gas valued at 180 million rupees at a rate of Rs. 1.50 per m3 of biogas. B.G. Unni, et al., carried out laboratory-scale and pilot studies for producing biogas from pressmud separately during winter, spring and summer seasons the results of which are summarised in Table 9.4.
In view of the fact that pressmud normally has high soil, sand and grits content, formulation is somewhat less smooth on account of stratification inside the digester. The digester was fed at a rate of 2.1 kg VS/m3. d during winter season and at a rate of 1.05 kg VS/m3 . d during summer period causing biogas yield of 0.48 m3/m3. d and 0.59 m3/m3d, respectively with methane content ranging between 70 to 72 per cent. P. Rajasekran, et al., studied the effect of adding pressmud, groundnut shell and paddy-husk as additive to cowdung slurry as feedstock.
Gas output and methanogenic activity were maximum in pressmud mixed with cowdung (12, 173 ml and 222.3 x 10 /g) followed by groundnut mixed with cowdung (11,962 ml and 189.66 x 107g). Studies carried out, thus confirm the utility of mixing pressmud and groundnut shell as supplementary feedstock for added biogas yield.
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As with other feedstock’s, pressmud based plant sludge is a rich fertiliser but on comparative terms somewhat less in view of its low pH and acidic nature. The sludge can be used for making other products as well. It can be used for extraction of phytosterols (beta-sito-sterol) 69 per cent, stigmasterol 18.1 per cent, compesterol and brassicosterol 12.8 per cent which can be biodegraded with a particular bacterium to form precursors of steroidal drugs. The slurry after extraction of phytosterols can be used as a biomanure on account of its high NPK values.
According to MNES estimates, 4.2 m.t. of pressmud available from 104 m.t. of cane crushed during 1992-93 could generate 3.5 x 105 cum of biogas equivalent to about 700 mw of power. Existing 425 sugar mills in the country produce about 5 m.t. of pressmud annually. If the entire pressmud is used for biomethanation, it could yield 3215 kilolitres of biogas equivalent to 1.1 m.t. of wood, 0.2 million kilolitre of kerosene, 014 m.t. of LPG, or 386 million kwh of electricity.
Biogas Production from Canteen Wastes:
Canteen wastes particularly those from industrial canteens provide valuable feedstock for biomethanation. Canteen wastes mainly comprise vegetable and fruit wastes, food left-overs bread pieces, and cooked rice. Y.Y. Yeole, et al., carried out experiments for analysing digestibility of canteen wastes by fermenting them in 25 and 200 litre capacity laboratory-scale digesters of the KVIC design. The plants were separately fed with canteen wastes and cattle dung, and performance analysed.
Chemical composition of fresh canteen wastes and cattle dung as used in the experiments is summarised in Table 9.5 below:
Biogas obtained from 25 litre experimental plant for a 35 day retention period was 31.4 litre/kg per day with 71 per cent methane content as against a yield of 17 litre/kg per day with 62 per cent methane content from cattle dung. Biogas yield from 200 litre capacity plant while feeding kitchen wastes was analysed to be 54 litres/kg per day with 68 per cent methane content.
This shows that kitchen waste produces more gas as compared to cattle dung which can be attributed to easily biodegradable matters like carbohydrates, proteins, fats etc. present in the former as against more complex polymers like cellulose, hemicellulose etc. present in the latter.
Biogas Production from Distillery Wastes:
Distilleries need energy, both for the extraction of fermentable matter from cereal, sugar-cane, beet, cassava and other biomass and subsequent distillation. Distilleries also cause environmental pollution by their effluents. Biogas can be obtained through fermentation of distillery wastes, spent grams, beet pulp, root and other residues.
The amount of biogas produced depends on nature of feedstock. One cubic metre of distillery wastes following anaerobic fermentation can yield 14 m3 of biogas. S.N. Kaul, et al., made a comprehensive review of state-of-art of industrial waste water treatment including resulting methane recovery.
Biogas recovery from distillery wastes having COD concentration of 50,000 mg/l can go a long way in meeting at least part of the industrial energy needs. Rampur Distillery and Chemicals Co. Ltd. at Rampur in Uttar Pradesh set up a biogas plant in eighties for producing biogas and enriched fertiliser from distillery wastes.
According to Mr M.K. Gupta, Vice- President (Rampur Distillery), the company proposes to set up a biogas-based co-generation unit in near future. The plant is at present generating 40 m3 biogas/m3 of spent wash at N.T.P. through thermophilic fermentation (50- 52°C) of distillery effluent and achieving 85 to 90 per cent BOD reduction.
The resulting biogas (calorific value 5000 kcal/m3) contains 60 to 62 per cent methane, 2 to 3 per cent hydrogen sulphide, and balance carbon dioxide. As per broad estimates made by the Union Ministry of Environment and Forests, if 90 per cent of the existing distilleries in India install biogas recovery plants for reducing the spent-wash BOD, they have the capacity to contribute an equivalent saving of about 2,80,000 tonnes of coal per annum.
Biogas Production from Textile Mill Wastes:
There exists considerable scope for biomethanation from textile wastes in cities like Mumbai, Ahmedabad and Coimbatore where large number of textile mills are located. As per rough estimates about 5.28 million cubic metres of biogas can be produced from anaerobic fermentation of 30,000 to 33,000 tonnes of willow-dust produced every year. This is equivalent to three million litres of diesel or 17.4 kwh of electricity.
According to the results of studies carried out by the NEERI, Nagpur, 54 per cent of cotton wastes are finer than 0.707 mm, 13 per cent range between 0.707 to 2 mm and remaining more than 2 mm size. Comparative chemical analysis of typical raw cotton dust from textile mills and municipal refuse in India is summarised in Table 9.6. C : N ratio of cotton wastes is favourable for anaerobic fermentation.
Cotton dust is uniform in composition as compared to municipal refuse and free from problems of shredding and segregation. Furthermore, as textile mills are mostly located in big cities and as cotton dust is clean and homogenous waste material, it can be easily packed, retailed and carried to biogas plant site.
In past R.H. Balasubramanya, et al., carried out experimental studies at the Cotton Technological Research Laboratory (CTRL) in Mumbai to analyse the pattern of biogas yield from willow-dust. According to their findings dry anaerobic fermentation of willow-dust consumes less water, accommodates more fermentable material per unit space, and generates improved quality of readily usable biomanure.
A pilot plant for processing willow-dust for biogas production in a battery of six batch digesters was installed in a textile mill. The depth of loading of tank during anaerobic pretreatment was found to affect the biogas yield with solid to liquid ratio varying from 1 : 6 to 1 : 1.5. About 725 m3 of biogas/2 tonne of willow-dust was obtained in 90 days in three solid loading levels of 2,4 and 6 tonnes during pretreatment when the solid to liquid ratio was 1 : 6. A gradual decline in gas production was observed during anaerobic fermentation.
When the solid to liquid ratio was changed to 1 : 1.5 (dry fermentation) about 500 m3 of biogas/tonne of willow-dust was produced in 30, 60 and 120 days with 2, 4 and 6 tonnes of initial solid loading.
In this case gas was produced at more or less uniform rate during corresponding anaerobic fermentation. P. Rajasekran, et al., studied biogas yield from cellulose rich wastes such as willow – dust before and after predigestion. Cellulose wastes were predigested using 0.15 per cent sodium hydroxide along with 2 per cent lime for 72 hours.
These wastes were anaerobically fermented in batch process both with and without cowdung. Biogas yield was found to increase if willow-dusts were mixed with cattle manure after predigestion. Certain oil cakes like castor cake and neem cake when used in combination with industrial wastes such as willow-dust from cotton mills provided a valuable additive for biomethanation.
Biogas Production from Confectionery Wastes:
Biogas can also be generated from confectionery wastes. Considerable solid wastes are generated while producing confectionery items like biscuits and chocolate. Solid wastes obtained from mixing and packing units are rich in carbohydrates and easily biodegradable, and thus can be a good substrate for biogas production. Generally these wastes are mainly disposed of by incineration.
These wastes mainly consist of maida (fine wheat flour), ghee (milk fat), pieces and powder of biscuits with chocolate paste. D.R. Ranade, et al., studied biogas production from confectionery wastes generated by biscuit and chocolate manufacturing plant. These wastes were anaerobically fermented in a 180 litre capacity movable drum type biogas plant.
Biogas yield was analysed corresponding to three different hydraulic retention times of 20, 30 and 40 days with 10 per cent total solids in the influent slurry. It was found that confectionery wastes are easily amenable to anaerobic fermentation. The fermentation process was more stable at 40 days HRT as compared to 30 days HRT. Biogas yield at 40 days HRT was reported to be 261 litres per day with 57 per cent methane content; and at 30 days HRT as 140 and 42, respectively in the same units.
Biogas Production from Fruit and Vegetable Wastes:
Fruit and vegetable wastes provide valuable input for biomethanation. In the fruit and vegetable canning industry, part of the relevant waste can be used as feedstock for digestion. The remaining waste may be transported to a municipal landfill site. Biogas thus produced may meet significant part of a factory’s energy demand. Composition of some common fruits and vegetables for assessing their suitability for anaerobic fermentation is given in Tables 9.7 and 9.8.
Important characteristics of processed fruit and vegetable wastes for analysing their potential for is given in Tables 9.9 and 9.10. Wieger Knol, et al., studied anaerobic fermentation of wastes from fruit and vegetable industry at the Central Institute of Nutrition and Food Research (CINFR) at Zieist (the Netherlands) way back in 1978.
Wastes were divided into three groups depending upon carbohydrate content on dry weight basis as they affect rate of biomethanation. Spinach-waste and asparagus peels were placed in group 1 with their carbohydrate content in dry weight being 0.05 and 0.06 per cent.
The larger group 2 comprised apple pulp, apple slurry, carrot waste, French bean waste and strawberry slurry with a carbohydrate content varying between 0.21 and 0.29 per cent. Group 3 comprised green pea slurry only with carbohydrate content in dry weight being 0.44 per cent.
Wastes in these groups were subjected to anaerobic fermentation individually. The study confirmed that anaerobic digestion could be a suitable process for the treatment of waste materials from the fruit and vegetable industry.
C.M. Silverio, et al., carried out studies at the National Institute of Science and Technology (NIST) in Philippines on biogas production from banana peels. For this purpose ripe and unripe banana peels were sun-dried for 7-14 days which were later cut into smaller pieces and ground to enable homogenous slurry preparation.
After anaerobic fermentation, it was found that ground banana peels yielded more biogas following increased availability of surface area for microbial action. At low temperatures addition of 2 per cent ammonium phosphate was found to speed up anaerobic fermentation process.
In view of low C : N ratio of banana peels, addition of chicken manure was found to improve biogas yield. E. Elortegw, et al., studied prospects and potential of anaerobic fermentation of banana stem and identified critical operating parameters which help to maximise biogas yield.
M. Tanticharoen, et al., carried out experimental studies for producing biogas from solid pineapple wastes comprising shells and cores. Experiments were carried out using four 30 litre vessels, one 200 litre plug flow reactor without any mixing arrangement, and one 5 m3 tank with stirring facility. The two stage process was used to determine system efficiency at high loading rates and shorter retention periods.
Pretreatment of the wet solids with sludge effluent prior to loading the digester resulted in greater biodigester stability than without pretreatment. Average gas yield was found to be 0.3-0.5 litre corresponding to a loading rate of 2.5 g of dry solids added per day.
Some pioneering studies were also carried out at the University of California at Berkeley on anaerobic fermentation of tomato wastes in late seventies. The new plant is designed to yield twice the amount of gas per kg of fruit and vegetable wastes than a conventional plant that utilises cowdung as feedstock.
In India several studies have been carried out in past concerning anaerobic digestion of fruit and vegetable processing wastes for biogas production. Most fruits and vegetables are processed on seasonal basis for a period of 2-3 months and accordingly the resulting wastes vary in their physical and chemical characteristics.
Citrus wastes are rich in toxic constituents like limonin whereas mango and pineapple processing wastes are known to be deficient in nitrogen., Over 60 x 106 tonnes of fruits and vegetables are produced in India annually of which only a small proportion is utilised in the fruit and vegetable processing industries.
The solid wastes normally account for 40-50 per cent of the raw material processed and has a total solid concentration of 10-15 per cent. P. Viswanath, S. Sumitra Devi and Krishna Nand carried out important studies concerning anaerobic fermentation of fruit and vegetable processing wastes at the Central Food Technological Institute, Mysore.
In this study, mango, orange, pineapple and tomato processing wastes were collected from the fruit processing factories, namely, Kissan Products Ltd., Bangalore; Clear Foods, Madanpalle, A.P.; Globe Foods, Mysore, Karnataka; and Mysore Fruitin, Mysore, Karnataka. Jackfruit and banana wastes were obtained from restaurants and hotels located in Mysore city.
The effect of feeding different fruit and vegetable wastes, mango, pineapple, tomato, jackfruit, banana and orange was studied in 60 litre digester by cycling each waste every fifth day. The characteristics of the anaerobically digested fluid and digester performance in terms of biogas production were determined at different loading rates and at different hydraulic retention times (HRT).
The maximum biogas yield of 0.6 m3/kg. VS added was obtained at a 20-day HRT and 40 kg TS/m3 day loading rate. The hourly gas production was observed in the digesters operated at 16 and 24 days HRT. As much as 74.5 per cent gas was produced within 12 hours of feeding at a 16 day HRT whereas at a 24-days HRT only 59.03 per cent of the total gas could be obtained.