sanitation system. it produces a digested slurry (digestate) that can be used as a fertilizer, and biogas that can be used for energy. using anaerobic digestion technologies can help to reduce the emission of greenhouse gases in a number of key ways: replacement of fossil fuels reducing or eliminating the energy footprint of waste treatment plants reducing methane emission from landfills displacing industrially produced chemical fertilizers reducing vehicle movements reducing electrical grid transportation losses reducing usage of lp gas for cooking an important component of the zero waste initiatives. waste and wastewater treatment anaerobic digestion is particularly suited to organic material, and is commonly used for industrial effluent, wastewater and sewage sludge treatment. anaerobic digestion, a simple process, can greatly reduce the amount of organic matter which might otherwise be destined to be dumped at sea, dumped in landfills, or burnt in incinerators. pressure from environmentally related legislation on solid waste disposal methods in developed countries has increased the application of anaerobic digestion as a process for reducing waste volumes and generating useful byproducts. it may either be used to process the source-separated fraction of municipal waste or alternatively combined with mechanical sorting systems, to process residual mixed municipal waste. these facilities are called mechanical biological treatment plants. if the putrescible waste processed in anaerobic digesters were disposed of in a landfill, it would break down naturally and often anaerobically. in this case, the gas will eventually escape into the atmosphere. as methane is about 20 times more potent as a greenhouse gas than carbon dioxide, this has significant negative environmental effects. in countries that collect household waste, the use of local anaerobic digestion facilities can help to reduce the amount of waste that requires transportation to centralized landfill sites or incineration facilities. this reduced burden on transportation reduces carbon emissions from the collection vehicles. if localized anaerobic digestion facilities are embedded within an electrical distribution network, they can help reduce the electrical losses associated with transporting electricity over a national grid. power generation in developing countries, simple home and farm-based anaerobic digestion systems offer the potential for low-cost energy for cooking and lighting. from 1975, china and india have both had large, government-backed schemes for adaptation of small biogas plants for use in the household for cooking and lighting. at present, projects for anaerobic digestion in the developing world can gain financial support through the united nations clean development mechanism if they are able to show they provide reduced carbon emissions. methane and power produced in anaerobic digestion facilities can be used to replace energy derived from fossil fuels, and hence reduce emissions of greenhouse gases, because the carbon in biodegradable material is part of a carbon cycle. the carbon released into the atmosphere from the combustion of biogas has been removed by plants for them to grow in the recent past, usually within the last decade, but more typically within the last growing season. if the plants are regrown, taking the carbon out of the atmosphere once more, the system will be carbon neutral. in contrast, carbon in fossil fuels has been sequestered in the earth for many millions of years, the combustion of which increases the overall levels of carbon dioxide in the atmosphere. power generation through anaerobic digestors is best suited to large-scale operations, rather than small farms, as large operations have the volume of manure that is able to make the systems financially viable. biogas from sewage sludge treatment is sometimes used to run a gas engine to produce electrical power, some or all of which can be used to run the sewage works. some waste heat from the engine is then used to heat the digester. the waste heat is, in general, enough to heat the digester to the required temperatures. the power potential from sewage works is limited – in the uk, there are about 80 mw total of such generation, with the potential to increase to 150 mw, which is insignificant compared to the average power demand in the uk of about 35,000 mw. the scope for biogas generation from nonsewage waste biological matter – energy crops, food waste, abattoir waste, etc. - is much higher, estimated to be capable of about 3,000 mw. farm biogas plants using animal waste and energy crops are expected to contribute to reducing co2 emissions and strengthen the grid, while providing uk farmers with additional revenues. some countries offer incentives in the form of, for example, feed-in tariffs for feeding electricity onto the power grid to subsidize green energy production. in oakland, california at the east bay municipal utility district's main wastewater treatment plant (ebmud), food waste is currently codigested with primary and secondary municipal wastewater solids and other high-strength wastes. compared to municipal wastewater solids digestion alone, food waste codigestion has many benefits. anaerobic digestion of food waste pulp from the ebmud food waste process provides a higher normalized energy benefit, compared to municipal wastewater solids: 730 to 1,300 kwh per dry ton of food waste applied compared to 560 to 940 kwh per dry ton of municipal wastewater solids applied. grid injection biogas grid-injection is the injection of biogas into the natural gas grid. the raw biogas has to be previously upgraded to biomethane. this upgrading implies the removal of contaminants such as hydrogen sulphide or siloxanes, as well as the carbon dioxide. several technologies are available for this purpose, the most widely implemented being pressure swing adsorption (psa), water or amine scrubbing (absorption processes) and, in recent years, membrane separation. as an alternative, the electricity and the heat can be used for on-site generation, resulting in a reduction of losses in the transportation of energy. typical energy losses in natural gas transmission systems range from 1–2%, whereas the current energy losses on a large electrical system range from 5–8%. in october 2010, didcot sewage works became the first in the uk to produce biomethane gas supplied to the national grid, for use in up to 200 homes in oxfordshire. by 2017, uk electricity firm ecotricity plan to have digester fed by locally sourced grass fueling 6000 homes vehicle fuel after upgrading with the above-mentioned technologies, the biogas (transformed into biomethane) can be used as vehicle fuel in adapted vehicles. this use is very extensive in sweden, where over 38,600 gas vehicles exist, and 60% of the vehicle gas is biomethane generated in anaerobic digestion plants. fertiliser and soil conditioner the solid, fibrous component of the digested material can be used as a soil conditioner to increase the organic content of soils. digester liquor can be used as a fertiliser to supply vital nutrients to soils instead of chemical fertilisers that require large amounts of energy to produce and transport. the use of manufactured fertilisers is, therefore, more carbon-intensive than the use of anaerobic digester liquor fertiliser. in countries such as spain, where many soils are organically depleted, the markets for the digested solids can be equally as important as the biogas. cooking gas by using a bio-digester, which produces the bacteria required for decomposing, cooking gas is generated. the organic waste like fallen leaves, kitchen waste, food waste etc. are fed into a crusher unit, where it is mixed with a small amount of water. the mixture is then fed into the bio-digester, where the archaea decomposes it to produce cooking gas. this gas is piped to kitchen stove. a 2 cubic meter bio-digester can produce 2 cubic meters of cooking gas. this is equivalent to 1 kg of lpg. the notable advantage of using a bio-digester is the sludge which is a rich organic manure. products the three principal products of anaerobic digestion are biogas, digestate, and water. biogas typical composition of biogas compound formula % methane ch4 50–75 carbon dioxide co2 25–50 nitrogen n2 0–10 hydrogen h2 0–1 hydrogen sulfide h2s 0–3 oxygen o2 0–0 source: www.kolumbus.fi, 2007 main article: biogas biogas holder with lightning protection rods and backup gas flare biogas carrying pipes biogas is the ultimate waste product of the bacteria feeding off the input biodegradable feedstock (the methanogenesis stage of anaerobic digestion is performed by archaea, a micro-organism on a distinctly different branch of the phylogenetic tree of life to bacteria), and is mostly methane and carbon dioxide, with a small amount hydrogen and trace hydrogen sulfide. (as-produced, biogas also contains water vapor, with the fractional water vapor volume a function of biogas temperature). most of the biogas is produced during the middle of the digestion, after the bacterial population has grown, and tapers off as the putrescible material is exhausted. the gas is normally stored on top of the digester in an inflatable gas bubble or extracted and stored next to the facility in a gas holder. the methane in biogas can be burned to produce both heat and electricity, usually with a reciprocating engine or microturbine often in a cogeneration arrangement where the electricity and waste heat generated are used to warm the digesters or to heat buildings. excess electricity can be sold to suppliers or put into the local grid. electricity produced by anaerobic digesters is considered to be renewable energy and may attract subsidies. biogas does not contribute to increasing atmospheric carbon dioxide concentrations because the gas is not released directly into the atmosphere and the carbon dioxide comes from an organic source with a short carbon cycle. biogas may require treatment or 'scrubbing' to refine it for use as a fuel. hydrogen sulfide, a toxic product formed from sulfates in the feedstock, is released as a trace component of the biogas. national environmental enforcement agencies, such as the u.s. environmental protection agency or the english and welsh environment agency, put strict limits on the levels of gases containing hydrogen sulfide, and, if the levels of hydrogen sulfide in the gas are high, gas scrubbing and cleaning equipment (such as amine gas treating) will be needed to process the biogas to within regionally accepted levels. alternatively, the addition of ferrous chloride fecl2 to the digestion tanks inhibits hydrogen sulfide production. volatile siloxanes can also contaminate the biogas; such compounds are frequently found in household waste and wastewater. in digestion facilities accepting these materials as a component of the feedstock, low-molecular-weight siloxanes volatilise into biogas. when this gas is combusted in a gas engine, turbine, or boiler, siloxanes are converted into silicon dioxide (sio2), which deposits internally in the machine, increasing wear and tear. practical and cost-effective technologies to remove siloxanes and other biogas contaminants are available at the present time. in certain applications, in situ treatment can be used to increase the methane purity by reducing the offgas carbon dioxide content, purging the majority of it in a secondary reactor. in countries such as switzerland, germany, and sweden, the methane in the biogas may be compressed for it to be used as a vehicle transportation fuel or input directly into the gas mains. in countries where the driver for the use of anaerobic digestion are renewable electricity subsidies, this route of treatment is less likely, as energy is required in this processing stage and reduces the overall levels available to sell. digestate digestate is the solid remnants of the original input material to the digesters that the microbes cannot use. it also consists of the mineralised remains of the dead bacteria from within the digesters. digestate can come in three forms: fibrous, liquor, or a sludge-based combination of the two fractions. in two-stage systems, different forms of digestate come from different digestion tanks. in single-stage digestion systems, the two fractions will be combined and, if desired, separated by further processing. acidogenic anaerobic digestate the second byproduct (acidogenic digestate) is a stable, organic material consisting largely of lignin and cellulose, but also of a variety of mineral components in a matrix of dead bacterial cells; some plastic may be present. the material resembles domestic compost and can be used as such or to make low-grade building products, such as fibreboard. the solid digestate can also be used as feedstock for ethanol production. the third byproduct is a liquid (methanogenic digestate) rich in nutrients, which can be used as a fertiliser, depending on the quality of the material being digested. levels of potentially toxic elements (ptes) should be chemically assessed. this will depend upon the quality of the original feedstock. in the case of