Drawing on examples from China and India, Betsy Otto, Xiaotian Fu, and Sahana Goswami from the World Resource Institute present the benefits of circular economy approaches in urban wastewater treatment.
Challenges of Wastewater and Sludge Management in China and India
Cities are at the centre of environmental and climate challenges, consuming 75 per cent of the world’s resources (including water). Increasing urbanisation lead to rising demand for water as well as a rapid increase in wastewater generation. Chinese cities generated 66 billion cubic metres of wastewater in 2019 – about 3.7 times the level of 2007. India’s cities generated about 22 billion cubic metres of wastewater in 2015, 1.5 times the level generated in of 1999.
While the volume of urban sewage is increasing rapidly, over 80 per cent of wastewater is still released to the environment without adequate treatment globally, leading to degradation of the environment and significant risks to human health. Where wastewater treatment occurs, a growing concern is the boom in sewage sludge production and its disposal – the by-product of most conventional treatment processes.
Wastewater: Ignored Asset
City leaders have begun to recognise the value of circular economy approaches that capture and reuse the water, energy, and nutrient value of wastewater flows. Israel and Singapore have made recycled wastewater a component of their water supplies, generating 40 per cent of water for irrigation and 30 per cent of daily water demand respectively.
In addition to boosting water supplies, wastewater-to-energy (WtE) and sludge-to-energy (StE) systems are critical circular economy strategies to recover and reuse waste products for environmentally beneficial uses (including energy, fertilisers, and building products). The interest in these approaches has further reinforced policymakers’ determination to find sustainable solutions for wastewater and sludge treatment that are both environmentally sound and economically viable.
In addition to reducing solid waste (sludge by-product), there is enormous untapped potential for converting sludge and waste into energy through biogas. The World Biogas Association estimates that the world’s available sewage flows could generate 210 to 300 terrawatt-hours of biogas energy – the equivalent of the electricity needs of 27 to 38 million people.
A Triple-Win Solution
Turning wastewater into energy and resources typically brings the following benefits:
- Energy benefits: capturing energy by-products (mainly CH4) from the treatment process and the waste heat to support the electricity demands of the treatment system itself. This can significantly reduce the consumption of fossil fuels and reduce strain on the electricity grid.
- Climate and environment benefits: WtE systems may reduce up to 99 per cent of the greenhouse gas emissions associated with treatment process by using renewable energy embedded in wastewater and sludge.
- Economic benefits: recovering energy and resources provides a model for lower energy and solid waste disposal costs, as well as new energy and resource value, creating a complete value chain. The biogas recovered may be purified into compressed natural gas or natural gas while biochar may be used as a soil amendment.
Practices in Chinese Cities
Cities in developing countries have started to implement practices of WtE or StE to mitigate wastewater and sludge challenges. In China, different cities have chosen WtE and StE to tackle their environment challenges.
Beijing planned and built five StE centres (with a total capacity of 6128 tons/day) to deal with the production of 6,000 tons (80 per cent water content) sludge per day. The captured biogas is used for powering the sludge treatment process and the wastewater treatment plants. Beijing is exploring further enhancing energy production and the possibility to provide energy to external users. Biochar (a remnant nutrient ash product from StE treatment) is used to plant trees for greening landscapes.
Zhenjiang – a mid-sized city in south China – is co-digesting sludge and kitchen waste from restaurants to produce biogas energy. The Zhenjiang project (total capacity 260 tons/day) closes the loop between the kitchen waste and wastewater, funnelling the biogas produced (7,500 cubic metres) to run project steam boilers and feeding excess energy into the municipal gas pipeline network. The digested sludge is dehydrated to be used as soil amendment for landscaping or planting trees while the leachate is used as liquid fertiliser. Similar practices are underway in other Chinese cities, including Qingdao, Xiangyang, and Ninghai.
Practices in Indian Cities
In India, up to 70 per cent of wastewater is released untreated into water bodies, swelling pollution of water sources and the risk of water-borne disease.
Some cities with larger sewer networks, such as Delhi, Chennai, and Bangalore have installed technologies which allow for the recovery of biogas then converted into electricity which is used to offset power needs internally in wastewater treatment plants (WWTPs).
Bangalore is experimenting with new technology installations that will allow higher electricity generation, which can be sold to the local grid. In 2017, WWTPs across India with a total capacity of 2,445 million litres per day (MLD) were producing more than 41 megawatt-hours of energy which is primarily used to meet internal energy demand in the WWTPs.
A pilot project in the city of Nashik offers another model to manage wastewater streams particularly useful for smaller Indian cities using a circular approach. This project manages about 10 to 15 tons of organic waste in municipal solid waste (MSW) and 10-20 tons of faecal sludge for this unsewered town. The process involves co-fermentation of the organic MSW and faecal sludge to generate energy and farm fertiliser. The energy is sold to the power grid and the farm fertiliser is sold in agricultural areas in the vicinity, creating an urban-rural circular economy loop.
In China, it is planned an annual biogas production of 14 billion m3 (equivalent to 3 million kw) by the end of 2020. In India, the Ministry of New and Renewable Energy estimate that a total of 5690 MWeq can be generated from various WtE biogas projects and already 330 MWeq installed capacity is available (of which 34 per cent is WtE generated from various urban waste streams).
Looking ahead, by 2050, 2.5 billion more people are projected to live in urban areas globally, 27 per cent of these in India and China. Helping these two countries to solve their wastewater and sludge challenges can bring local environmental benefits, improve water availability, and contribute to global climate change mitigation efforts. The experiences of the two countries will demonstrate effective wastewater and sludge management practices for other developing countries.