Digestion Process Development for the Egg-Shaped Digesters Facility at DCWASA'S Blue Plains AWTP

The Washington DC Water and Sewer Authority (DCWASA) is developing its Egg-Shaped Digesters Facility at the 1400 megaliter/day-capacity Blue Plains Advanced Wastewater Treatment Plant. Preliminary Design evaluations included analysis of several anaerobic digestion process options, sludge heating (and cooling) options, and assessments of pre-digestion and post-digestion processes. The temperature phased anaerobic digestion (TPAD) process has received major attention in the design because of its ability to destroy maximum solids and create a dewatered cake product with the lowest odor level. Additional thermophilic digestion stages or batch thermophilic vessels are being designed to add to the TPAD configuration to meet standards for Class A (pathogen-free) digestion. Class B mesophilic anaerobic digestion is also retained as a process option for the plant. About 300 dry tonnes/day (330 dry tons/day) of raw sludge will be processed in the new digestion facility on an average basis.The engineering team for this project has utilized a wide variety of evaluation techniques to assess the various alternatives and subsystems for this large, new digestion facility on a small footprint at the Blue Plains AWTP. Work included pilot testing of digestion process options, site visits and investigations of other facilities in North America and Europe, conducting new research on controlling odor levels and fecal coliform densities in centrifuge-dewatered cake, and development of economic and non-economic comparisons of alternatives and subsystems. Preliminary Design and Final Design has encompassed the period of 2002 to 2005.