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diagram of anaerobic disgestion

As a clean water utility, King County considers "HOW we do what we do" as important as "WHAT we do". In other words, in providing environmentally clean wastewater treatment, we focus our efforts on the most innovative and cost-wise technologies out there. Cleaning water is an energy-intensive business. The liquids and solids processing part of treatment offers the greatest opportunity for innovation in energy production. King County's Wastewater Treatment Division is investigating three primary technology areas as viable approaches toward producing energy from wastewater.

  • Biomass Energy - The conversion of the organic solids (biomass) produced by wastewater treatment into energy.
  • Hydraulic Energy - Recovering energy from the force of water as it passes through the wastewater treatment process.
  • Thermal Energy - Extracting heat from wastewater to warm buildings.

BIOMASS ENERGY: The general term “biomass-to-energy” refers to the use of organic material (typically plant materials and animal wastes) as a source of fuel. In the wastewater industry, the “biomass” produced are the solids recovered from the wastewater treatment process. For King County producing energy from wastewater solids involves "anaerobic digestion". Anaerobic digestion occurs in an environment with zero oxygen where beneficial microbes digest organic wastewater material (solids slurry). As the microbes process or biodegrade the organic solids they produce methane-rich biogas which can be used to generate electricity, produce heat, or can be sold to the local gas utility. The resulting digested solids are called “biosolids”, which are beneficially used to enhance agricultural crops and forest tree growth or produce rich planting compost. This process is shown in further detail in the diagram above.

While we believe anaerobic digestion is a fantastic biomass-to-energy technology, we continue to evaluate alternative biomass-to-energy technologies to determine if they can reduce costs, enhance energy production or provide other benefits to the county.

HYDRAULIC ENERGY: Hydraulic energy can potentially be recovered from the velocity of wastewater as it flows through a treatment plant. The energy can be generated as the water passes through specially-designed waterwheels or turbines. King County is investigating appropriate locations within its treatment plants for cost-effectively generating hydraulic energy.

THERMAL ENERGY: Extracting heat produced during industrial processes is not a new idea. In fact, heat recovery is a standard practice in industries to make use of heat energy produced by machinery or processes that would otherwise be wasted. Traditional heat exchangers capture and redistribute heat from existing industrial operations, such as boiler heat, engine heat and distribution pipe heat. The viable sources for this type of energy is expanding and includes recovering heat energy from sewage. This is a relatively new application of this technology. It has been demonstrated as effective, but is not yet common practice. This innovative heat recovery is often termed "District Heating" because the heat obtained from below grade sewer pipes has the potential to heat district-wide areas (such as, offices and other commercial space, public building and even wastewater treatment plants).

Visit our Research Library for information on completed research studies.

For more information about the Energy Program, please contact us at: