Energy recovery - creating resources from wastewater

King County is committed to recovering and reusing the products of the wastewater treatment process at its regional clean-water facilities. The capability to beneficially reuse products increase the efficiency of the wastewater treatment plants, offer environmental sustainability and save the ratepayers money.

The treatment plants have three main products: treated water, treated solids known as biosolids, and digester gas. All biosolids are beneficially reused in agriculture and forestry. Some highly treated reclaimed water is reused for irrigation and plant processes.

Digester gas is released from the solid matter removed from wastewater during the treatment process. The solid matter is collected and thickened. It then goes to large digester tanks. In the digesters, the solid matter is heated and mixed. And during that process, the solid matter begins to decompose and create gas.

The digester gas is produced when naturally occurring microorganisms break down the solid matter. Containing carbon dioxide and methane, the digester gas can be used as fuel for several different "co-generation" systems at the county treatment plants.