Wastewater Sludge: A New Resource For Alternative Energy

Wastewater treatment plants are net users of energy. In the U.S. they consume an estimated 21 billion kilowatt hours per year. There are important reasons for this energy use, as society demands increasingly intensive treatment to remove nutrients and chemicals from wastewater before it is discharged back into water bodies or is reused. But energy use is coming under increasing scrutiny, with the financial cost of energy and the environment cost of energy generation driving new interest in the conversion of sewage sludge to energy.

Researchers are exploring sustainable wastewater treatment with a reduced carbon footprint. The view of municipal sewage has shifted, from a waste to be treated and disposed of, to a resource that can be processed for recovery of energy, nutrients, and other constituents.

Research has demonstrated that sewage actually contains 10 times the energy needed to treat it, and it is technically feasible to recover energy from sludge. As renewable energy, it can be directly used for wastewater treatment, reducing the facility's dependency on conventional electricity. The greater the quantity of energy produced by the industry, the more the industry can help reduce emissions of greenhouse gases.

Using solids as a resource rather than a waste may help stressed public budgets as well. Wastewater solids must be processed prior to disposal, and solids handling accounts for as much as 30 percent of a wastewater treatment facility's costs.

According to State of the Science Report: Energy and Resource Recovery from Sludge, published by the Global Water Research Coalition with support from WERF, converting solids to energy is feasible and desirable, from a treatment perspective. The challenge is finding a process that meets social, economic and environmental objectives, as well as being affordable and cost effective. For instance, chemical use may be required in certain processes, but it may not always be the best option in terms of health protection and life cycle impacts (energy use and emissions during production and transportation).

There are about 2,000 central sludge processing facilities in the U.S. As of 2004, 650 of those facilities used anaerobic digesters to process its sludge. When sludge is digested, it produces methane gas. The Water Environment Research Foundation developed the Life Cycle Assessment Manager for Energy Recovery (LCAMER) model to helps wastewater agencies determine the feasibility of recovering energy from anaerobic digestion of wastewater solids.

Other examples of energy conversion:

• The city of Watsonville, California uses restaurant grease to increase sewage sludge digester gas production by over 50 percent.
• The use of methane as a source of hydrogen has been demonstrated at King County (WA) South Treatment Plant.
• In 2005 in the U.K., waste (including sewer sludge) combustion and biogas production accounted for 10.8 percent and 4.2 percent respectively of all UK renewable energy.
• A German plant produces excess energy. In 2005, an average of 113 percent of the electricity consumed in the operation of the plant was generated onsite by gas engines.
• A Swedish treatment plant produces and sells biogas to Stockholm's bus company, which uses it to run at least 30 buses.
• Stockholm's energy company uses heat recovery pumps to extract heat from treated sewage to provide hot water and heating to 80,000 apartments.
• The Sewerage Bureau of Tokyo Metropolitan Government turns dewatered sewage sludge into fuel charcoal for thermal power generation.

While the current technology is promising, none of the processes can fully extract all the energy available in wastewater. The exploration of new technological developments, or improvements of current technologies, will be necessary to get the maximum energy out of sewage and sludge.

In addition to the State of Science Report, WERF has two fact sheets that summarize the current situations. Wastewater Sludge: A New Resource for Alternative Energy and Resource Recovery, is a basic primer for the non-technical audience, public officials and consumers. A Technical Look at Energy and Resource Recovery from Wastewater Residuals provides wastewater treatment professionals with an overview of some of the specific processes.

SOURCE: Water Environment Research Foundation (WERF)