A new nationwide survey of American wastewater treatment plants (WWTPs) by the American Biogas Council (ABC) reveals opportunities to expand the production of renewable energy from wastewater, make greater use of existing digesters, and put more organic waste to productive use.
The American Biogas Council’s new report, “Anaerobic Digestion at U.S. Water Resource Recovery Facilities“, is the first comprehensive assessment of anaerobic digestion (AD) at wastewater treatment plants in more than a decade.
Anaerobic digestion breaks down organic waste such as sewage sludge (aka “biosolids”) to produce biogas that can be used to generate electricity and heat or processed and upgraded into biomethane, aka renewable natural gas (RNG).
The 18-month study included more than 33,000 calls and 3,271 conversations with facility managers, operators, and other key personnel.
The survey identified 1,213 wastewater resource recovery facilities using anaerobic digestion, representing about 24 percent of the roughly 5,000 facilities surveyed that treat at least one million (US) gallons (c. 3.8 million litres) of wastewater per day.
A national overview of wastewater digestion
The report provides new data on where biogas is being captured at wastewater plants across the country and how facilities are using the energy it produces, including:
- More than 75 percent of facilities treating more than 30 million gallons (c. 113.5 million litres) per day use anaerobic digestion.
- About 50 percent of facilities treating 5 to 30 million gallons (c. 18.9 million to 113.5 million litres) per day use digesters, while fewer than 30 percent of facilities treating less than five million gallons (c. 18.9 million litres) per day do.
- 103 facilities with digesters treat less than one million gallons (c. 3.8 million litres) per day, representing nearly 9 percent of all wastewater facilities with digesters.
- Anaerobic digestion has been part of US wastewater treatment for more than a century. The earliest facility identified by the survey began operating in New York in 1914.
- 89 of the electricity generated from biogas at WWTPs is used on-site.
Many wastewater plants are already producing energy, but opportunity remains untapped
The survey also found that wastewater facilities with anaerobic digesters currently capture about 100.8 billion cubic feet of biogas each year – enough energy to generate electricity for about 1.1 million average American homes.
More than 80 percent use the biogas they produce for energy onsite, with 55 percent using it to generate electricity or combined heat and power (CHP) and another 26 percent using it for heat.
The report also identifies the potential to capture an additional 70 billion cubic feet annually, bringing the total to nearly 172 billion cubic feet – enough to generate electricity for about 1.9 million homes.
Co-digestion can divert more organic waste
There is also an opportunity to make better use of the digesters already operating at wastewater facilities. Some facilities accept organic material from outside sources – a practice known as co-digestion.
Food processing waste, post-consumer food scraps, and other organic materials can be added to wastewater solids in an existing digester, producing additional biogas and putting some of the 24 million tonnes of food waste currently landfilled each year to productive use.
Today, 139 wastewater facilities accept outside organic material, including food waste and fats, oils, and grease (FOG).
America has over a thousand wastewater facilities managing organic waste. Our survey shows there are opportunities to do more — by expanding anaerobic digestion where it makes sense and getting more value from the digesters already in place, said Patrick Serfass, Executive Director of the American Biogas Council.
