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Anaerobic Digester

Anaerobic Digesters

Anaerobic digestion is the process through which organic materials break down in the absence of oxygen (the controlled, aerobic process is called composting). This produces a gas, sometimes called biogas, principally composed of methane and carbon dioxide.

This gas production occurs naturally in decomposition, but by controlling the process and collecting the biogas, it can be used for energy. When the biogas is not collected, it enters the atmosphere and becomes a greenhouse gas.

Ecology supports the development and use of technologies like anaerobic digestion that help us extract more value from waste and also reduce greenhouse gas emissions. Ecology assists local governments in overseeing anaerobic digesters and conducts inspections.

Anaerobic digesters have been around for hundreds of years and come in all shapes and sizes. Most are designed to process organic materials, extracting energy in the form of biogas, and producing by-products that can be land applied to improve soil health. Anaerobic digestion can reduce harmful pathogens in waste, making it safer to use as a beneficial soil amendment.

Anaerobic digester use around the world has been successful in rural areas of India, China, and Africa, as well as in urban European communities. Many of Washington's waste water treatment plants use anaerobic digestion as a method to reduce pathogens and stabilize biosolids. More recently, dairies in Washington have invested in digester technology as a way to generate energy and improve manure management.

What laws and rules govern anaerobic digesters?

The 2009 Washington Legislature passed into law (Chapter 70.95.330 RCW) that provides an exemption from solid waste permitting for dairy manure anaerobic digesters that meet certain conditions. In 2013, Ecology updated the Solid Waste Handling Standards, Chapter 173-350 WAC, to include permitting requirements for solid waste anaerobic digesters. You can find these regulations at the following links: What is the permitting process for anaerobic digesters?

The process for anaerobic digesters needing a solid waste permit is described on the Waste 2 Resources Permitting Process web page.

Some anaerobic digesters that process organic waste may qualify for a solid waste permit exemption provided operators meet certain conditions. The permit exemption conditions are detailed in Table 250-A (3) of WAC 173-350-250.

The conditions for operating most anaerobic digesters under one of the permit exemption include:
  • Comply with the restrictions on the types of acceptable organic wastes (see Table 250-A).
  • Notify Ecology 30 days before importing organic waste of the intent to operate under the exemption (submit this notification form to Ecology)
  • Comply with the requirements for onsite storage and handling of organics (see Table 250-A)
  • Submit an annual report to Ecology by April 1 for each calendar year (submit this annual report form to Ecology)
Dairy digesters operating under a conditional exemption must also meet the following conditions:
  • Codigest dairy manure (more than 50% by volume) and pre-consumer organic waste (not more than 30% by volume). Design and operate the digester in accordance with the Natural Resources Conservation Service's Conservation Practice Standard, Code 366
  • Comply with the requirements for digestate management (see Table 250-A), including Dairy Nutrient Management Program requirements (Chapter 90.64 RCW) of the Washington State Department of Agriculture.