With the objective of recycling and reusing sewage, Sulabh International Social Service Organisation set up a decentralised effluent treatment plant and biogas production facility in Adalatganj, Patna, in 1982. After the plant was deemed a success, Sulabh set up more than 200 such facilities in rural and urban areas across India. These treatment facilities recycle and reuse the human excreta discharged from public toilets.
At the above-mentioned facilities, the effluent collected from a society’s toilet complexes is treated using technologies like filtration and ultraviolet (UV) rays. The treatment process involves four stages. First, the effluent is pumped into a sedimentation tank where all the solid particles get accumulated at the bottom. Then, the residual liquid passes through a sand filter, which further reduces the amount of total suspended solids present in it. The next levels of treatment involve processing the effluent in aeration tanks and finally passing it through an activated carbon filter for enhanced treatment.
At the end of this process, the effluent is converted into a colourless, odourless and pathogen-free liquid manure. The filtration unit gives it a colourless and odourless quality that is free from organic practices while the UV rays eliminate bacteria. The treatment procedure drastically reduces the biological oxygen demand (BOD) and chemical oxygen demand levels of wastewater. After treatment, the BOD level of human waste is reduced from around 200 mg per litre to less than 10 mg per litre. This makes the treated sewage safe for aquaculture and agriculture use as well as for discharge into any waterbody.
A major benefit of this system is the production of biogas, which can be used for meeting the electricity requirements of a small community. The Ministry of Non-Conventional Energy Resources has approved the biogas plant set up at the effluent treatment plant for being implemented by various state agencies. Inside the digester, which is built underground, biogas is produced by anaerobic fermentation with the help of methanogenic bacteria. On average, the human excreta of one person produces 1 cubic foot of biogas per day, which contains 65-66 per cent methane, 32-34 per cent carbon dioxide and 3 per cent hydrogen sulphide and other gases. The biogas produced is stored in inbuilt liquid displacement chambers and used for purposes like cooking, lighting, electricity generation and heat production. It can also be used as fertiliser as it contains nitrogen, potassium and phosphate.
The methane gas produced by the plant is the only combustible constituent, which can be utilised in different forms of energy. Its caloric value is 24 MJ per cubic metre or about 5,000 kCal per cubic metre. This caloric content is high in energy production as 1,000 feet (30 cubic metres) of biogas is equivalent to 600 cubic feet of natural gas, 6.4 gallons of butane, 5.2 gallons of gasoline or 4.6 gallons of diesel oil. The volume of biogas thus produced can be utilised for cooking, lighting and heating purposes, of which cooking is the most efficient and feasible use. A wide range of biogas burners are available in capacities ranging from 8 cubic feet to 100 cubic feet of biogas consumption per hour. The gas is odourless and does not produce any soot.
Another use of biogas is for lighting mantle lamps. A typical biogas mantle lamp requires biogas in the range of 2 to 3 cubic feet per hour after which it can provide illumination equivalent to a 40 W electricity bulb at 220 V. The plant is also equipped with a dual-fuel internal combustible engine for generating motive power through biogas. Air mixed with biogas is pumped into the engine and the mixture is then compressed, raising its temperature to about 350 degrees Celsius. This is still less than the ignition temperature of biogas (600 degrees Celsius). Thus, a small quantity of diesel is mixed with biogas to initiate combustion. The optimal ratio of diesel and biogas at which the ignition process can be smoothly carried out is 20:80. To eliminate the need for diesel, Sulabh has developed a new technology that can operate dual-fuel gensets solely on biogas. The ignition of compressed biogas is done through a battery-operated spark system, thus eliminating the need for any other fuel and making it an entirely sustainable form of energy.
With environmental concerns on the rise, biogas offers great potential as an alternative source of energy. As a by-product of the effluent treatment process, Sulabh’s combined facility ensures the treatment of sewage and promotes sustainable energy production. Studies have also revealed that a public toilet facility used by 2,000 people a day has the capacity to generate 60 cubic metres of biogas per day. This is sufficient for operating a 10 kVA genset for eight hours a day and produ-cing 65 units of electricity. Meanwhile, the increased focus of urban local bodies like the Delhi Jal Board on setting up decentralised effluent treatment plants indicates a huge potential market. These initiatives, along with the growing acceptance by the public, can transform the outlook of various stakeholders regarding the treatment of sewage and its reuse. Given the in-numerable benefits of such systems across all regions and sections of society, their installation is expected to grow manyfold.