Anaerobic Co-Digestion on Dairies in Washington State: The Solid Waste Handling Permit Exemption

This publication explains the role of co-digestion in dairy anaerobic digestion systems and outlines regulatory considerations for Washington state dairies. Co-digestion occurs when manure is processed together with additional organic materials, such as food-processing wastes, to increase biogas production, improve digester performance, and enhance project economics.

The publication focuses on Washington state legislation that allows certain dairy anaerobic digestion facilities to qualify for an exemption from a solid waste handling permit when co-digesting approved organic wastes. It describes the conditions required to maintain this exemption, including limits on feedstock composition, requirements for nutrient management plans, and procedures for handling digester byproducts such as solids and liquids.

Additional sections summarize broader regulatory requirements that may apply to digesters on dairies, including air quality permits, water quality protections, and local permitting processes. The fact sheet provides practical guidance to help dairy operators and digester developers understand how co-digestion can fit within Washington state regulatory frameworks.