Developing a Co-Digestion Program at the Tacoma, WA Central Treatment Plant

This paper reviews the development of a co-digestion program at Tacoma's CTP. Program development requires knowledge of the market to differentiate the co-digestion substrates that are available in the area that will eventually support the sustainable operation of the program. In order to target desired wastes an initial screening of the substrates is required and should consider chemical characteristics, logistical considerations and requirements for added infrastructure. Using this approach our initial list of 75 potential wastes that were identified in the region were narrowed down to16 potential wastes with 7 high priority substrates being identified. Having identified the regional high priority substrates, an analysis of the digestion system capacity identified an excess loading capacity of 10,000 lb-TS/day. This excess capacity would be divided between future loadings, existing contracts and co-digestion capacity. Along with process capacity an analysis of the existing process equipment capacity is required to determine choke points in the system which may supersede the identified process capacity. The analytical data collected during the substrate screening process was extended to understanding the impacts of the added substrate on the secondary system. High nitrogen substrates were identified as those of concern for the facility.