Methane contributes approximately 20 per cent to the annual increase in global warming. And landfills and sewers contribute to approximately 15 per cent of global methane emissions (West et al., 1998).
If methane is allowed to leak into the atmosphere before being used, it effectively traps the sun’s heat and contributes to global warming. In fact, methane traps 20 times more heat in the atmosphere than carbon dioxide (CO2) and there is enough empirical evidence to prove that methane is 25 times more potent than carbon dioxide as a greenhouse gas. The IPCC Fifth Assessment Report listed methane in the earth’s atmosphere with a global warming potential (GWP) of 34 compared to carbon dioxide over a 100-year period. GWP values compare the impacts of emissions and reductions of different gases and carbon dioxide is chosen as the reference gas and its GWP is set equal to one (IPCC, 2013; Gillenwater, 2010).
Although the earth’s atmosphere consists mainly of oxygen and nitrogen, neither plays a significant role in enhancing the greenhouse effect because both are essentially transparent to terrestrial radiation. The greenhouse effect is primarily a function of the concentration of water vapour, carbon dioxide, methane, nitrous oxide (N2O), and other trace gases in the atmosphere that absorb the terrestrial radiation leaving the surface of the earth. Changes in the atmospheric concentrations of these greenhouse gases can alter the balance of energy transfers between the atmosphere, space, land and the oceans. This is called radiative forcing, or the ability of a gas to force climate change (IPCC, 2013). Holding everything else constant, increases in greenhouse gas concentrations in the atmosphere will produce positive radiative forcing a net increase in the absorption of energy by the earth (IPCC, 2013).
In India, the problem of methane is a lot more serious, owing to rapid urbanisation. As compared to the global average of 15 per cent, CH4 constitutes 29 per cent of total Indian greenhouse gas (GHG) emissions. Emissions from solid waste in India is also higher (6 per cent) compared to the global average of 3 per cent (Parvathamma, 2014).
Methane from solid waste
The world today has to find ways to reduce waste however the notion that this organic waste is a resource is unthinkable and unbelievable to many. Methane, which is the simplest hydrocarbon, is produced by breaking down organic waste by bacteria. This raw biogas, if purified, can yield pipeline quality bio-CH4, which could be piped into homes for cooking. Besides, as a major component of compressed natural gas (CNG), it can be used as an eco-friendly vehicle fuel.
Most significantly, the CH4 from sewage can yield electricity, which in turn could help run sewage treatment plants. In Renton, in Seattle (USA), the electricity generated from sewage is being used to run the world’s largest sewage treatment plant.
The Renton Plant
The largest project of its type in the world, the Renton plant has biodegradable solid waste sent to large tanks, called digesters, where it is retained for three to four weeks. Once the bacteria eat away the waste, methane gas is released.
Around 700,000 people send 86 million gallons of sewage daily, mostly toilet and kitchen waste, to the King County treatment plant in Renton, a Seattle suburb. Around 30 million gallons of sewage are used to produce enough CH4 gas to run the 1 megawatt (MW), fuel-cell power plant, which needs about 7.5 MW per hour a day on average.
Unlike most treatment plants that flare off the methane, the Renton Plant captures the gas and sends it to a fuel cell system, where it is broken down into hydrogen and carbon dioxide. The carbon dioxide is recirculated to produce carbonate, which then combines with the hydrogen to produce electricity, water, carbon dioxide and heat (Lanos, 2004).
Methane plants around the world
In New York, about half the methane produced by the city’s plants is already used to meet about 20 per cent of the energy demands of the city’s 14 sewage plants, whose electric bills run to a total of about 50 million USD a year. Britain has been generating green energy since 2010 from human waste. In Manchester, a biogas conversion plant built at Britain’s second biggest sewage works generates enough power for 5,000 homes (Derbyshire, 2009).
In India, the Tambaram Municipality in Tamil Nadu has launched a bio-methanation plant to convert solid waste into methane (EAI, 2013; Manikandan, 2013 June 15). The bio-methanation programme followed the success of the Municipality’s Namma Toilet Project, meant to end open defecation by putting up toilets in public places. The methane gas, generated naturally from the sewage, is directed through an overhead pipe which is then routed through smaller pipes to 12 conventional stoves in a separate kitchen located within the complex. The plant is a boon for housewives in the adjoining slum who now benefit from the cooking fuel they get as a result.
Tamil Nadu has also taken the lead in converting garbage into power. Venkatamangalam village under Pallavapuram Municipality acquired a new INR 100 crore power plant to convert garbage into fuel. Designed to handle 300 tonnes of solid waste every day, it will generate 3 MW of power every hour. Essel Pallavapuram and Tambaram Municipal Solid Waste Private Limited—specially created for the purpose will operate and maintain the facility, including landfill sites, for 20 years.
The garbage that will feed the project will be collected from the Pallavaram and Tambaram Municipal areas, and brought to the plant site for segregation into degradable, recyclable and other inert waste. The wet waste, dried with blowers, will then be sieved, shredded and subjected to a thermalisation process to generate a syngas, which will be used to drive dynamos and generate power supply.
The waste generated as a byproduct will be either used for making eco-bricks or dumped in scientific landfills (One World South Asia, 2013 May 1).
Methane for power generation
It is ironical that most countries do not have programmes to capture methane given the uses it can be put to and the tremendous threat it poses to our planet if allowed to escape as a greenhouse gas. The importance of not flaring off the methane from large sewage treatment plants (STPs) has not gained the kind of attention it deserves. This is in spite of pilot projects yielding good results in India so far. For instance, a pilot plant by Western Paques found that 150 tonnes per day of municipal solid waste could yield 14000 cubic metres (m3) of biogas with 55-65 per cent of methane, which could be converted to 1.2 MW of power (Sharholy et al., 2007). Yet, very few systems recover any value from sewage in terms of electricity and heat generation, although the surplus energy from a 200 million gallon per day (MGD) STP can easily be used as a fuel for cooking for at least 200-300 households nearby.
In countries such as India, plagued by erratic power supply, STPs don’t run to their optimum efficiency or optimum capacity for various reasons. Since the energy requirements of STPs are huge—for pumping, aeration and the like, the full power requirements for running STPs are rarely met. Under such circumstances, using the methane generated for meeting the ever increasing energy costs of STPs, would mean huge power savings. Even if a 20 per cent reduction in energy bills is achieved, the savings in terms of energy cost would be substantial. Besides, the discharge of sewage directly into drains because of non-availability of power would be significantly reduced.
The change in mind-set towards treating methane as a useless byproduct to a major resource can be the beginning of a virtuous cycle that can change the way STPs function and the way people view STPs.
Given the emphasis on construction of public toilets as well as toilets in schools under the Swachh Bharat Mission, it seems logical that our next step ought to involve harnessing the huge amounts of methane to be generated out of sewage, as is being done by the Tambaram Municipality. It would also prove the best method to teach children both chemistry and biology, if they get to see bio-digesters within their own school compounds or nearby STPs to produce methane for cooking their mid-day meals.
Although isolated and fragmented efforts have been made in Jaipur and Delhi and in smaller municipalities like Tambaram, the utilisation of methane will have to be scaled up by way of statutory legal provisions to reap the huge untapped potential of this energy resource.
Hurdles in Implementation:
- Firstly, as simple as it may seem, many STPs built in the last two decades have never been built with the intention of utilisation of this resource. The useless byproduct was just flared off. However, retrofitting and redesigning these STPs could be easily achieved for methane recovery.
- Secondly, some STPs still continue to run on highly inefficient physio-chemical treatment processes and not on biological processes, rendering production of methane virtually impossible.
- Thirdly, STPs in India often receive storm water as sewage since most storm water drains are not segregated. This renders STPs inefficient and anaerobic digestion ineffective.
- The most significant problem, though, is the lack of adequate experience in the treatment of solid waste, except for sewage sludge and animal manure (Sharholy et al., 2007).
Given these drawbacks, there is little thrust towards generating methane from STPs at the moment, notwithstanding some important exceptions. The inertia in this respect could be easily overcome if green energy generation from methane is encouraged and mandated by law. If utilisation of fly ash bricks within a particular radius of thermal power plants can be made compulsory, there is no reason why STPs can’t be mandated to capture this resource and supply fuel to households.
Derbyshire, D. (2009, June 16). Human waste to heat thousands of homes in £4m plan to recycle it into gas. Mail Online UK. Retrieved from http://www.dailymail.co.uk/news/article-1193169/Human-waste-heat-thousands-homes-4m-plan-recycle-gas.html
EAI. (2013, June 15). Tambaram municipality, TN, launches bio methanation plant that produce cooking gas from sewage. Energy Alternatives India (EAI). Retrieved from http://www.eai.in/360/news/pages/9981#sthash.QNhyvqd0.dpuf.
FICCI. (2009). Survey on the current status of municipal solid waste management in Indian cities and the potential of landfill gas to energy projects in India. Retrieved from https://www.globalmethane.org/Data/292_2_ficci_survey_09.pdf.
Gillenwater, M. (2010). What is a global warming potential? And which one do I use? Greenhouse Gas Management Institute. Retrieved from http://ghginstitute.org/2010/06/28/what-is-a-global-warming-potential.
Intergovernmental Panel for Climate Change. (2013). Climate Change 2013: The Physical Science Basis. Retrieved from www.ipcc.ch/report/ar5/wg1/
Lanos, M. (2004, July 19). Poop power?-Sewage turned into electricity. NBC News. Retrieved from http://www.nbcnews.com/id/5335635/ns/us_news-environment/t/poop-power-sewage-turned-electricity/#.V1GC-dR97Mp
Manikandan, K. (2013, June 15). In Selaiyur, methane from sewage becomes cooking gas. The Hindu, Retrieved from http://www.thehindu.com/news/cities/chennai/in-selaiyur-methane-from-sewage-becomes-cooking-gas/article4815131.ece.
OneWorld South Asia. (2013, May 1). Chennai power plant to use garbage as its fuel. Retrieved from http://southasia.oneworld.net/news/chennai-power-plant-to-use-garbage-as-its-fuel#.V02zyfNJnIU.
Parvathamma, G.I. (2014). An Analytical Study on Problems and Policies of Solid Waste Management in India-Special Reference to Bangalore City. IOSR Journal of Environmental Science, Toxicology and Food Technology, 8(10), pp2319-2402
Sharholy, M., Ahmad, K., Mahmood, G., & Trivedi, R.C. (2007). Municipal Solid Waste Management in Indian cities. Waste Management 28(2008), pp459–467. Retrieved from https://www.unc.edu/courses/2009spring/envr/890/002/readings/SolidWasteIndiaReview2008.pdf.
West, M.E., Brown, K.W., & Thomas, J.C. (1998). Methane production of raw and composted solid waste in simulated landfill cells. Waste Management Research, 16 (5), pp430-436.