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Sanitary landfills, municipal refuse, anaerobic lagoons and night-soil provide valuable source for biogas recovery. Landfills are essentially dumping ground for municipal wastes. Anaerobic conditions generally exist in top 10 feet of a landfill resulting in production of gases and leachates. Landfill gas is directly usable as a principal fuel or a supplementary fuel after removing impurities in industrial units, fertiliser plants, utilities and gas-based power plants. It can also be liquefied for being used as a liquid fuel.
Much of the work concerning biogas recovery from landfill si tes has been carried out in the USA and UK and in recent years considerable headway has been made in India as well. Municipal refuse generated in most cities can also be directly used as a valuable feedstock for anaerobic fermentation without landfilling.
These wastes comprise 20 to 25 per cent of kitchen and household wastes and 40 to 50 per cent of some kind of paper wastes. Biogas can also be recovered from anaerobic lagoons which exist in large numbers in USA and several European countries. Biogas from lagoons has more methane concentration than obtainable from mesophilic digesters.
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Night-soil digestion which is traditionally regarded as a waste treatment process is equally valuable as an energy recovery process. It provides energy and biomanure, keeps environment clean and minimises groundwater contamination by percolation and leaching.
Source # 1. Biogas Recovery from Sanitary Landfills:
Biofuels can be recovered from sanitary landfills and gainfully utilised. Within a landfill organic components of refuse breakdown under microbial attack resulting in production of various gases like carbon dioxide and methane. Biodegradation of domestic refuse in a landfill is an anaerobic process involving fermentation of polymeric carbohydrates into carboxylic acids, carbon dioxide and methane.
Landfill gases have been located as deep as 600 feet at one site. Landfill gases while being formed tend to escape by choosing the path of least resistance which may entail diffusion upwards through the surface or laterally through the sides of the site. This route depends on permeability of top surface which in turn depends on top cover used and lateral permeability of refuse and permeability of surrounding rock structure.
Thus three escape routes can be identified viz., diffusion through top surface, diffusion through permeable rocks from sides, and if both the surface and sides are impermeable then through cracks which will usually form at the interface between them. Rates of their outwards movements are generally found to be inconsistent.
Biogas recoverable from landfill has a typical composition of 50 to 55 per cent methane with remainder as carbon dioxide, traces of hydrogen sulphide, nitrogen mercaptans and some chlorinated hydrocarbons. It has calorific value 530 BTU/ft3 which is nearly half that of natural gas (1075 BTU/ft3).
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Appreciable quantities of this gas can be obtained from large sites with theoretical norm that 1 lb of refuse can produce 2.7 ft3 of carbon dioxide and 3.9 ft3 of methane. D.R. Anderson, et al., reported that a maximum of 121 ft3 of gas with 60 per cent of methane can be obtained from a cubic yard of refuse. K. Richards made a detailed account of fuel supplies from landfill gas in different parts of the world.
A survey of landfill gas recovery projects and utilisation facilities throughout the world showed that by 1988 there were at least 146 commercial schemes in operation in a total of 15 countries with a further 32 trials underway. At a very conservative estimate, these projects could save 8,25,000 tonnes of coal equivalent (tee) annually of non-renewable sources.
Period of gas formation varies from site to site and depends on nature of organic material in refuse, proportion of moisture content and extent of rainfall percolating down into a site. According to F.R. Bowerman, et al., maximum and nearly half of total gas production occurs within 2 to 3 years of landfill completion. R.C. Merz and R. Stone found that surface irrigation of landfill affects gas generation rate.
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Stages of biogas production in relation to the age of a landfill can be divided into four phases. The initial aerobic phase is short one leading into second phase of high carbon dioxide production at approximately a molar equivalent to oxygen consumed so that little nitrogen is displaced. F.R. Bowerman provided estimates of carbon dioxide percentages at varying intervals such as after 11 days 70 per cent, after 23 days 50 per cent and after 40 days 90 per cent of carbon dioxide was formed.
This is accompanied by increases in hydrogen and decreases in nitrogen production. In third phase, methanogenesis activity begins causing methane concentrations to increase with reduction in carbon dioxide and hydrogen levels. J.N. Ramaswamy reported that this steady state phase normally occurs about 180 days after the disposal in landfill begins. F.A. Rovers, et al., estimated this period as 250 days whereas R. Beluche estimated it as high as 500 days.
According to C.E. West and E. Ashare, the fourth steady-state phase of methane production occurs much later producing 50 to 70 per cent methane. Microbial activity can be intensified by recycling leachate through the decomposing refuse.
According to G.F. Farquhar and F.A. Rovers, biogas generation increases with increased moisture content although methane production is found to decrease during periods of excessive water infiltration following inhibition of methane forming bacteria. Maximum production of gas occurs at 30-35°C. Optimum value of pH for gas production was found to be around 7. Composition and quantity of gas formed was dependent on the nature of city wastes.
For determining the rates of gas production, it is necessary to know area and depth of landfill, quantity and composition of refuse dumped, local geology and hydrology, age of site and other related parameters. This information helps to determine number of wells and monitoring probes required for testing, their spacing and desired withdrawal rates during testing.
After installation of test wells and monitor probes, an operating crew visits site with a test rig and instrumentation for carrying out flow measurement. Various parameters of the landfill are first measured under static conditions. Flow tests are then initiated to record variation in values of these parameters under different flow rates.
Tests are conducted on a twenty-four hours-a-day and seven days-a-week basis. Though total time required for production testing varies from site to site, it has however been in general found to vary between two to four months.
India’s first major attempt to produce biogas from landfill was made during 1986 at Timarpur in Delhi. The project was a joint undertaking by the then DNES and the Delhi Administration. Biogas was recovered by drilling a series of wells. In 1986 eight wells at a spacing of 200 ft3 from one another were drilled. The recovered gas was used for power generation.
As per the design criterion, one well was found adequate to produce 100 kilowatts of electricity from a landfill area of one acre. In other words, landfill covering an area of 10 acres can produce 1000 kw or equivalently one megawatt of electricity. Considering that the landfill at Timarpur covers an area of 80 acres, it has the potential to produce 8 mw of electricity.
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In 1986, cost of boring a single well was estimated as Rs. 10 lakhs which was inclusive of the cost of installation of generators, compressor, pumps and pipes. The cost of electricity generation from landfill gas was estimated as 10 paise per unit which is nearly one tenth of the power generated by conventional sources. In Delhi alone by 1986 there were over 50 landfills in existence under control of various agencies like the Municipal Corporation of Delhi, Delhi Development Authority, New Delhi Municipal Committee and the Cantonment Board of Delhi.
In India as a whole as per rough estimates, about 8000 tonnes of garbage is generated per day which is likely to be doubled by the turn of the century. If this entire garbage is available for being dumped into different landfills, it can produce about 60 mw of electricity which can meet about 8 per cent of the total peak demand of the area covered. Electricity so produced can be fed into the power grid. A single landfill is sufficient for generating electricity for a minimum period of 10 years.
After the entire gas produced during this period is taken out, spent refuse can be removed and refilled with fresh urban refuse. Cycle can thus be maintained to generate electricity continuously. Generation of electricity from landfills is also eco-friendly. In the absence of any such gainful project, gas formed in landfills tends to diffuse into environment causing pollution. Electricity generation from landfill gas thus helps to meet power needs and also helps to keep the environment clean and unpolluted.
As in many other fields, biogas recovery from sanitary landfills too has been pioneered in the United States. A number of landfill sites in California, New Jersey, Washington D.C., New York and Chicago have been used in past to extract gas with commercial success. The U.S. interest originally began with containment of hazard posed by gas migration but later on it shifted into a full-scale gas recovery programme with its uses in several utilities, industrial units, and gas-based power stations.
The world’s one of the first gas recovery facilities at the Palos Verdes landfill sites in Rolling Hills Estates, California met the household energy needs of over 3500 households. The Palos Verdes facility is owned and operated by the Getty Synthetic Fuels Inc., California, and a wholly owned subsidiary of the Getty Oil Company. The company is regarded as a pioneer in recovery, processing and sale of methane gas from sanitary landfills.
In 1981, the plant capacity was expanded to produce some 2 million cubic feet of gas per day (2 MMCFD) with heat value of 500 BTU/ft3 which on purification produced 1 MMCFD of gas with a heat value of 1000 BTU/ft3. Purified gas is supplied to the Southern California Gas Company’s natural gas grid. The Operating Industries landfill at Monterey Park (California) is another major landfill in the U.S.A. where gas recovery facility began operations in 1979.
The gas is supplied to the southern California Gas Company which in turn meets the energy needs of more than 15,000 households. Based on American and British experience, it is to be emphasised that energy recovery from landfills is economically viable normally for large and deep sites only because in shallow sites, refuse undergoes aerobic fermentation instead of anaerobic which results in low gas yield.
Source # 2. Biogas Recovery from Municipal Wastes:
Municipal wastes generated in most cities provide valuable feed material for anaerobic fermentation. Carbohydrates present in paper, grass, leaves and other cellulosic material provide important ingredient for anaerobic fermentation. These wastes at times on account of high lignin content pose problems for fermentation. Lignin accounts for 20 to 25 per cent of natural fibres and is non-biodegradable.
When lignin content is less than 5 per cent, fermentation is greatly inhibited. The pH of the digester content tends to affect these limits. For instance, alkaline substrate allows a greater tolerance for residual lignin to achieve a given level of digestibility. Sodium hydroxide has been one of the more popular agents for delignification and has been applied to a variety of materials to improve their digestibility for use as a ruminant feed.
Improvement in digestibility results from increased availability of insoluble residual cell as well as formation of soluble matter. It has been analysed that a high conversion efficiency is possible at much shorter digester retention times with pretreatment than without pretreatment. However, a more efficient conversion process needs a system to solubilize or remove lignin and break the lignin-carbohydrate complex.
Among others, lignin can be removed by chemical reaction as is done in wood pulping or by physical action as is done in fine milling. An important indicator to reflect changes in organic fibre characteristics is the ratio of cellulose to lignin. This ratio in municipal wastes before digestion is around 8 which following fermentation reduces to about 3 or so.
D.L. Wise, et al., described a low capital cost batch type digestion process which can convert all of a town waste such as garbage, sewage, food processing waste, crop residues and animal wastes into biogas. Based on landfill gas recovery process, the scheme involves an in-ground reactor lined with an impervious material such as clay, a liquid collector pond situated at the base of the digester, a liquid recirculation pipe, and a gas removal well and pipeline.
J. Mata-Alvarex, et al., carried out laboratory simulation of municipal wastes fermentation by using five test cells operated at different temperatures under enhanced digestion conditions with leachate recycle, with water spiked with added buffer, nutrients and inoculum. They also reported that 90 per cent of digestion can be completed in 25-27 days.
R.A. Kumar described a pilot project undertaken by the Tata Electric Co., Mumbai on recycling of sewage, garbage and garden waste and its final conversion into biogas, organic fertiliser and distilled water through anaerobic fermentation process.
In this pilot plant, 1 to 2 m3 of sewage and 1 to 2 tonnes of processed garden waste and garbage are fed into the digester daily. It produces around 100 m3 of gas per day, 100-200 kg of sludge (fertiliser) and 500-1000 litres distilled water per day. Laboratory-scale studies on being scaled up by about 2000 times can provide the level of output from a pilot project.
French engineers combined anaerobic digestion with direct combustion for maximum recovery of energy from municipal wastes. Called the Valorga process, the scheme first sorts the organic matter from the metal and inert solids and then passes all organic wastes into the digester.
Downstream of the digester, a combustion unit burns the undigested matter comprising generally wood, rags and plastics for steam and hot water production. Resulting biogas from the digester is purified by removing hydrogen sulphide and carbon dioxide, dehydrated and finally compressed prior to distribution.
Digester effluent is separated into fertiliser, and combustible material is subjected to pyrolytic combustion to produce heat, a part of which is used for obtaining added gas yield. Almost 50 per cent of initial organic material are converted to biogas in a retention time of 15-20 days.
Source # 3. Biogas Production from Anaerobic Lagoons:
Anaerobic lagoons are widely used in the United States and many European countries. As with other types of anaerobic treatment, methane and carbon dioxide are the two principal by-products of biodegradation. Anaerobic lagoon biogas production rates on volumetric basis are much lower than those from mesophilic digesters.
However biogas production from lagoons on volatile solids added basis holds promise of being greater than for mesophilic digesters. On surface area basis of lagoon, biogas yield was found to vary from 0.02 to 0.50 m3/m2/day whereas on volumetric basis of lagoon, it is reported to range from 0.03 to 0.23 m3/m3/day.
As with other types of anaerobic treatment, methane and carbon dioxide are the two principal by-products of biodegradation. Lagoon biogas is found to have higher methane concentration than obtainable from mesophilic digesters. Lagoon temperatures fluctuate depending on liquid depth and ambient temperature. Loading rates for lagoons which normally vary from 0.056-0.14 kg VS.m3/day are low as compared to those for biogas plants.
By contrast, most anaerobic digesters are operated at either constant mesophilic or thermophilic temperatures and loading rates varying from 2.7-17.7 kg. VS/m3 of digester volume per day. It is found that biogas yield from normally loaded lagoon (less than 0.06 kg VS/m3/day) is not of sufficient quantity nor is production rate consistent enough throughout the year to be considered as a significant source of energy.
Biogas production from lagoon has been studied by several researchers. For a swine manure loading rate of 0.36 kg VS/m3 day on a pilot-scale lagoon, F.J. Humenik and M.R. Over-cash measured biogas yield as 0.21 m3/m2 day (0.113 m3/m3.day). J.B. Allen and B. Lowery studied biogas production from a full-scale swine lagoon and dairy and poultry pilot-scale lagoons. For the swine lagoon, they reported a mean biogas production rate of 0.006 m3/m2.day for a mean lagoon temperature of 14°C.
However, the biogas yield was 0.55 m3/m2.day for a 3-day period when the lagoon temperature was 27.5°C. Biogas production from the poultry and dairy pilot-lagoons ranged from 0.001 to 0.02 m3/m2.day for a 3-day period when the lagoon temperature was 27.5°C.
Biogas production from the poultry and dairy pilot-lagoons ranged from 0.001 to 0.02 m3/m2.day. J.A. Chandler, et al., studied biogas production from a California swine lagoon where a 1070 m2 floating cover was used to collect biogas. Gas yield ranged from 0.66 to 0.92 m3/m3.day).
The lagoon was 6.1 m deep and the liquid temperature varied from 11 to 22°C throughout the year. L.M. safely and P.W. Westerman reported lagoon biogas production rates ranging from 0.02 to 0.50 m3 /m2 .day (0.03 to 0.23 m3/m3.day). These rates were determined during summer months from swine, poultry and dairy lagoons with varying loading rates. Lagoon loading rates, lagoon depth, lagoon temperature and position of the collection unit on the lagoon surface were found to affect gas yield.
Biogas production dropped appreciably when lagoon liquid temperatures dropped below 15°C but again increased once the lagoon liquid temperature increased. Methane content in biogas from lagoons was found to range from 62 to 65 per cent for lagoon temperatures above 15°C.
Source # 4. Biogas Generation from Night-Soil:
Night-soil digestion is traditionally regarded as a waste treatment process with recovery of bioenergy and biomanure viewed only as by-product. In most cities night-soil is disposed off through water carriage system (sewerage) but most rural and semi-urban areas in the country even do not have this basic facility. Night-soil scattered in fields is disposed off either by dumping or trenching.
In many cases it remains unremoved making it breeding ground for flies and insects. Within this backdrop anaerobic fermentation of night-soil is an attractive proposition which not only keeps the environment clean but also provides energy and biomanure. H.B. Gotass reported that per capita production of human faeces and urine lies within range 35-70 g and 50-70 g per day, respectively.
The COD : BOD ratio of 2.4 indicates that night-soil is readily biodegradable. For efficient biodegradation BOD : N : P ratio of 100 : 5 : 1 is considered to be optimum, and for a stable anaerobic fermentation process a ratio of 100 : 2 : 0.5 is regarded as optimum.
Night-soil contains nitrogen and phosphorus much in excess of that needed for efficient aerobic or anaerobic treatment. Excessive nitrogen present in digester content causes ammonia toxicity which adversely affects fermentation. However, excessive phosphorus present has no harmful effect on biomthanation.
In past the NEERI (Nagpur) helped in setting up night-soil based biogas plant at the Nagpur Central Prison. Feed rate was kept as 1.5 to 3.5 kg VS/m3/day, digester temperature varied from 20 to 32°C and ambient temperature ranged between 10 to 45°C. When the feed rate was increased beyond 3.5 in usual units, dewatering ability dropped substantially causing excessive delays in drying-up of sludge.
Even at a feed rate of 2.6 kg VS/m3/day filterability of the sludge was poor and it took 10 days to obtain a spadable cake. This also added to storage space problems as space requirement increases with increasing feed rate. Digested slurry from the plant is transferred to the sludge bed where it gets dewatered and dries up.
The filtrate from the drying bed is further treated in a stabilisation pond. A flow diagram of the integrated bioenergy system indicating all major components is shown in Fig. 10.1. The dry sludge is an effective biomanure and soil-conditioner on account of its being rich in nitrogen, phosphorus and potassium content.
S. Iwai, et al., carried out studies on biogas production from night-soil. Based on experimental studies, it was reported that there occurred greater gas output during thermophillic digestion as compared to mesophillic digestion but this had no differential impact on gas composition which remained unaffected in the two cases. Heat requirement for thermophillic digestion was 2.5 times more than needed for mesophillic digestion and thus former can be economic only if inexpensive heat source such as solar energy is harnessed.
Besides increased gas output, thermophillic digestion also ensures environmental improvement by destroying parasitic ova, pathogenic bacteria and other pollutants. J. Rammohan Rao carried out-experimental studies concerning thermophillic digestion of night-soil individually and in combination with cowdung in proportions of 2 : 1 at a temperature of 55°C.
For a feed rate of 2.4 kg/VS/m3/day, a volatile solid reduction of 45 per cent was reported for both the cases, and biogas yield of 0.45 m3/kg. VS/day for night-soil and 0.35 m3/kg. VS/day for night-soil and cowdung combination was observed.
It was argued by the researchers that cowdung as a feedstock had a potential to suppress ammonia toxicity as the quantity of ammoniacal nitrogen released during mixed feed digestion was low compared to when night-soil was used alone, S. Shanta carried out experimental studies concerning survival of helminthic ova during night-soil digestion. It was reported that on 20th day of detention period there occurred a reduction of 48 and 63 per cent in ascaris and hookworm population which rose to 70 and 93 per cent, respectively on 30th day of residence period.
C. Polprasert, et al., described results of experiments conducted on four 3.5 m3 ferro-cement digesters equipped with manually operated mixers. Digesters, fed with a mixture of night-soil, water-hyacinth and rice-straw were operated on a semi-continuous basis at hydraulic retention times of 30, 50 and 70 days. The corresponding organic loading rates were 1.2, 0.75 and 0.53 kg m-3 day-1 of total volatile solids. Biogas production rate [m3 (kg TVS)-1 day-1] was 0.183,0.210 and 0.164 at 30,50 and 70 days of retention time, respectively.
The respective volumetric production rates [m3/m-3 day-1] were 0.220, 0.158 and 0.086. During this study, common problems of excessive scum formation and accumulation were also faced. In another experiment 20-litre aspirator glass bottles were used for digestion in batch mode. Based on results of the experiment, at the end of 30 and 50 days biogas production rate was found to be 0.400 and 0.406 m3 (kg TVS)-1, respectively.
Night-soil-based biogas plants (NBPs) have become very popular and socially acceptable in Maharashtra in recent years. S. Kumar, et al., carried out a survey of 40 NBSs installed under the aegis of the Maharashtra Energy Development Agency (MEDA). The survey of 8 to 85 m3 capacity NBPs revealed that 75 per cent of them were working satisfactorily.
In total about 715 m3/day biogas was being produced which met cooking fuel needs of 1790 persons every day saving 1100 tonnes of fuelwood annually. Uninterrupted running of plant was found to largely depend on regular cleaning of slurry pit, maintenance of toilets and carrying out minor repairs locally from time to time.
Failure of the plants were analysed to be due to underutilisation or poor upkeep of toilets leading to inadequate slurry and negligence of the owner towards occasional minor repairs. The institutional NBPs were found to be operating more efficiently following availability of improved infrastructural facilities with them as compared to those with gram panchayats and municipal councils.
Performance of a fixed dome type Krishna biogas plant of 6m capacity using night-soil as feedstock was evaluated by Shivasadan Sahakari Society, Sangli, and Maharashtra a type of which is owned and run by it. The plant is connected to three latrines and two urinals which are each provided with water-tap connected to an overhead water reservoir.
The latrines were visited 6 days per week, 21 times per day and the urinals 32 times per day. This corresponded to hydraulic retention time of 7 days. The pH of the influent, BOD of the effluent and methane content of the gas were estimated intermittently. Gas production rate was measured as 0.427 m3 per kg of faecal matter. Gas generation rate corresponding to a retention period of 7 days/week was found to be 0.727 m3/day.
Dr. Y.P. Anand, former Chairman, Railway Board, Govt., of India, has done pioneering work on low-cost sanitation systems which are very appropriate for India and developing countries at large.
He has evolved designs for individual and community biogas plant-related sanitation systems based on extensive field work in railway complexes in Gorakhpur and Moradabad. According to him, system of bio-latrines based on onsite biogas digesters is very economic and appropriate for a country like India.