Chennai: The city’s sewage treatment plants may soon become a source of compressed biogas (CBG). Metrowater will be appointing a consultant to examine how biogas generated from sewage sludge can be purified, compressed and sold.Officials said Metrowater wants to develop the project under a public-private partnership (PPP) model. The proposal covers 11 sewage treatment plants (STPs) with a combined capacity of 844 million litres a day (mld).The board operates 20 sewage treatment plants with a combined treatment capacity of 957.8mld and treats around 650mld of sewage. These plants generate significant quantities of sludge, which in turn produce biogas through anaerobic digestion. Much of the gas is either flared or used to generate power for the treatment plants.Metrowater officials said that about 50kg of biogas is generated for 1mld of STP capacity. “The gas can be captured, purified, compressed and sold as CBG. For now, the biogas is used for power generation, which captures about 65% of the methane in the generated biogas,” said an official.The proposed study will cover 11 wastewater treatment plants where biogas generation facilities are available. These include plants at Kodungaiyur, Koyambedu, Nesapakkam and Perungudi. The study will involve wastewater and sludge sampling at all 11 plants, including analysis of wastewater quality, sludge generation and sludge quality. It will compare theoretical biogas generation with actual generation and conduct treatability studies to assess the potential for increasing gas output.“One of the tasks is to compare two options — continuing to use biogas for electricity generation or selling it through a pipeline. The consultant will examine the infrastructure needed for thickening the sludge, anaerobic digestion, biogas purification, gas holders, flaring and bottling the gas,” said the official. The consultant will study whether carbon dioxide, separated during biogas purification, can itself be purified and sold, if financially feasible. Possible revenue from digested sludge, including options such as composting, will be examined too.“The consultant shall identify potential CBG off-takers, including oil marketing companies, city gas distribution entities, CNG retail outlets, public and municipal transport fleets, commercial establishments and industrial consumers, and assess the applicable mechanisms, market demand, logistics, infrastructure needs and regulatory framework. The study shall recommend the most suitable PPP structure, revenue-sharing or concession mechanism and implementation strategy, based on techno-economic viability,” said the official.
