Development, Implementation, and Demonstration of the Co-digestion of Source-Separated Food Waste by the City of Tacoma

The City of Tacoma is taking a systematic approach to understanding the benefits and drawbacks of co-digesting food waste and sludge at its Central Treatment Plant. As part of this process laboratory analysis indicated that both food waste and the liquids that accumulate in food waste collection trucks are good substrates for digestion. The food waste from the City's pilot diversion program is very clean relative to other source-separated materials, but still requires preprocessing to pulp the material and meet state regulations for manufactured inerts. A screening process developed to meet the City's requirements resulted in a hammer mill technology being selected. A substrate storage study conducted by the City, while not completely successful, identified two critical design considerations: low pH acid conditions and the potential for expansion of tank contents. Additional testing has been conducted and the City is now moving into facility construction.