Ecology's Anaerobic Digester Website

Anaerobic digestion (AD) 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.

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.

AD 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 AD 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.

Washington's Beyond Waste Plan Supports Anaerobic Digestion

Beyond Waste is the state plan for managing hazardous and solid waste. This 30-year plan has a clear and simple goal: Eliminate wastes and toxics whenever we can and use the remaining wastes as resources. This plan promotes economic, social, and environmental vitality.

Within Beyond Waste, the Organics Initiative supports research and development of processes and products to close the loop on organic materials. Keeping organics out of the landfill reduces greenhouse gas emissions by decreasing methane released during its decomposition. Turning organics into compost, bioenergy, biofuels and other products, promotes economic vitality in growing industries, and protects the environment by turning waste into resources. Anaerobic digestion is one of many important systems that will help us meet our closed loop organics recycling goals.

Ecology's Role in Organics Research

Ecology has partnered with Washington's research universities to help meet the Beyond Waste Organic Initiative and Washington's Climate Change Initiative goals. Several research projects completed by Washington State University (WSU) have helped guide anaerobic digestion development in our state.

In 2005, Ecology issued a report on Washington biomass - Biomass Inventory and Bioenergy Assessment, An Evaluation of Organic Material Resources for Bioenergy Production in Washington State (Publication 05-07-047). The inventory of 44 organic material sources and locations became the basis for additional research projects with the University of Washington and Washington State University.

Waste to Fuels Technology Project

The Waste to Fuels Technology Project is a partnership between Ecology and the Biological Systems Engineering Department, within Washington State University. The project has two main focal points. First, renewable fuels can be generated from organic materials in our municipal waste stream through several processes. Building on the Biomass Inventory, material characteristics were determined for 42 organic waste types, and cost curves determined for assessing transportation costs for three basic processing methodologies:

• Anaerobic digestion.
• Production of cellulosic ethanol.
• Thermo-chemical production of oils for fuel, and stable carbon char.

This research will help determine feasibility of the different technologies listed above. Results of the study are provided at: http://www.ecy.wa.gov/biblio/0707025.html (Publication 07-07-025).

Second, the project is also developing a municipal organics high solids anaerobic digester (HSAD) that will recover methane for fuel or energy, fertilizers to support our agricultural needs, and compost for enhancing and improving soil productivity. The project objective is to produce a computer model that will be used to test variables at a HSAD facility. These variables may include physical configuration and equipment needs for the facility, operating parameters, feedstocks mixtures, recoverable methane, fertilizers, and liquid and solids changes. Results of this project are highly encouraging. A world wide literature and technology review has been completed. Laboratory studies are complete, and computer code that models various digester configurations are written and calibrated. Preliminary results can be reviewed at: http://www.ecy.wa.gov/biblio/0707024.html (Publication 07-07-024).

The WSU research team is combining the Waste to Fuels research with findings from other funded research and has identified two distinct methods for recovering ammonium sulfate and phosphate fertilizers from digestate, while purifying the digester gases of acid compounds that damage generators and reduce engine life. These processes are in pilot scale testing by WSU and Andgar Corporation at the Vander Haak dairy in Whatcom County.

Organic Waste to Resources Project

The goal of the Organic Waste to Resources projects was to take further steps in the Beyond Waste Plan and increase organics recovery by focusing on new processing technologies. These projects incorporate sustainable, closed-loop, full cycle uses of organic materials to meet the following objectives:

• Produce renewable fuels to help replace current fossil fuels.
• Create carbon neutral and carbon negative solutions for fuels, energy and products.
• Recover valuable industrial and agricultural nutrients in process technologies.
• Support durable, secure systems through distributed production.
• Create sustainable economic vitality, social equity and environmental balance.
• Produce transferable research and technology that can be replicated around the state.

Over all, expanding and strengthening closed loop recycling and reuse of organic materials are priorities for the Beyond Waste Organics Initiative. Anaerobic digestion is a proven technology that meets the criteria of "closed loop recycling and reuse."