By Leah Schleifer and Xiaotian Fu
Growing cities are generating higher volumes of wastewater and putting a strain on clean water supplies, calling for solutions that extract value from “waste” and ensure the sustainability of resources — with the added bonus, or imperative, of protecting the environment.
From Cape Town to São Paulo, many cities around the world are struggling to meet the water demands of growing populations. In 1960, 34 percent of the world’s population lived in cities. That number is 54 percent today; by 2050, cities are projected to hold closer to three-quarters1 of all people.
Rapid migration into urban areas, and population growth within the areas themselves, have strained resources like water and energy. Cities receive a finite amount of water from sources — like upstream watersheds and dams — that must be shared with agricultural and industrial sectors. There’s a growing need to make urban water supply resilient to droughts and other disruptive forces.
At the same time, cities are struggling to manage the waste produced by these growing populations. In China, for example, cities collectively generate over 40 million metric tons of sewage sludge — enough to fill six Great Pyramids of Giza annually2. But treating wastewater is expensive, and it can take time to build support for investments.
When local institutions can’t afford to properly manage wastewater, it’s often dumped directly on land or in nearby waterbodies. Eighty percent of the world’s wastewater is discharged back into nature without further treatment or reuse3, endangering public health and sanitation. And sludge — the solid byproduct of wastewater treatment — is often spread on land, or landfilled. Even where wastewater is treated, sludge is often ignored and wasted. Beyond nutrient pollution and the potential spreading of pathogens, disposing of wastewater and sludge without treatment releases methane, a powerful greenhouse gas, that furthers climate change.
Tapping Wastewater For Energy
There is a way to tackle water resources and waste management simultaneously. Sludge-to-energy systems separate, capture, and utilize the methane gas from sewage sludge for energy, instead of releasing it into the atmosphere. Treating and utilizing wastewater as a valuable resource — rather than discarding it as waste — can create a new stream from which water, nutrients, and renewable energy can be harnessed.
Wastewater is 99 percent pure water and 1 percent wastes — primarily in the form of organic matter and nutrients such as phosphorus and nitrogen4. During wastewater treatment, the solid sludge (the organic matter and nutrients) is separated from the combined, liquid wastewater. Many sludge-to-energy systems then treat this waste at a high temperature and pressure in a process known as thermal hydrolysis, to maximize the amount of methane that can be produced. Thermal hydrolysis is gaining popularity, as treatment plants in the U.S., Europe, China, Brazil, Argentina, Singapore, and many other countries in the world are recognizing the benefits of this step for increased energy production.
Next, the treated waste enters an anaerobic digester, which finishes breaking down the waste, separating it into a methanerich biogas, and a solid, nutrient-rich “digestate” byproduct. The biogas is then used for energy onsite, or can be purified and compressed and used as natural gas to fuel vehicles. The solid digestate can be used as soil for planting trees or for land restoration.
Sludge-to-energy systems can often be economically, environmentally, and energetically advantageous solutions for cities. Wastewater treatment is expensive: The electricity costs to power a completed wastewater treatment plant can account for up to 30 percent of the total operating cost for water and wastewater utilities5. These costs can be even higher in some developing countries, such as India and Bangladesh6. Utilizing sludge-for-energy allows utilities to use the waste they receive to power the wastewater treatment process — making them energy-self-sufficient, reducing or eliminating additional electricity costs. This alleviates a major cost for wastewater treatment plants — electricity — potentially making them financially sustainable options in areas that otherwise could not afford treatment. The energy may also be supplied to external users, such as households, or to vehicles.
In addition, using methane for energy — instead of wasting this potential resource — can slash a plant’s emissions. The city of Xiangyang, China, which treats sewage and kitchen wastes together for energy, reduced 95 percent of CO2 equivalent emissions compared to if these wastes had been landfilled. Xiangyang’s plant is also able to sell the solid digestate, as well as the excess natural gas produced by the sludge-to-energy process, generating more than $1.5 million for the plant annually7.
After the success of using sludge for energy in Xiangyang, 100 additional Chinese cities will pilot the use of waste-to-energy systems for kitchen wastes. An outcome of this large scale required not only action at the local level, but buy-in from the national government.
“The Chinese government requires that each city should solve the problem of sludge pollution in the next few years,” said Yue Zhang, former director general of the Urban Water Management Office at the Chinese Ministry of Housing and Urban-Rural Development. “This is a political requirement that has been incorporated into the assessment index of local officials.”8
Municipalities around the world are also recognizing the value of utilizing wastewater for energy, rather than wasting it. As research continues to uncover the benefits of such systems in cities, sludge-to-energy technology is emerging as an exciting innovation that can help tackle water, waste, and energy crises.
About The Authors
Leah Schleifer is the communications specialist for the World Resources Institute’s Water Program. Her work includes outreach and communications surrounding the Water Program’s various reports and projects, such as Aqueduct. Leah holds a bachelor’s in environmental science and policy, and a master of public policy with an environmental policy specialization, both from the University of Maryland, College Park.
Xiaotian Fu joined the WRI China office in May 2012 as an associate to the Water Program’s Sustainable and Livable Cities Initiative. She provides analytical support to identify options to reduce water pollution and increase water availability in China. She also has a double bachelor’s degree in materials science and engineering, and international trade and economics from Beijing University of Technology, Beijing, China.