Anaerobic digesters

What is anaerobic digestion?

Anaerobic digestion is the process through which organic materials break down in the absence of oxygen (as opposed to composting, which is the controlled break down of material in the presence of oxygen). This process produces a gas, sometimes called biogas, primarily 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 escapes into the atmosphere, where it contributes to climate change.

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 digesters are in use around the world at many wastewater treatment plants and at dairies, where they are used to generate energy and improve manure management.

Anaerobic digester regulations

In 2013, we 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 law, Chapter 70A.205.290 RCW
• The rule, WAC Chapter 173-350-250
• For solid waste permit information and forms, visit our Solid waste permits web page.

Solid waste permit exemption

Washington law provides an exemption from solid waste permitting for dairy manure anaerobic digesters that meet certain conditions. The permit exemption conditions are detailed in Table 250-A (3) of WAC 173-350-250.

The conditions for a permit exemption:

• 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 percent by volume) and pre-consumer organic waste (not more than 30 percent 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

Additional information