PILOT PLANT INVESTIGATION OF THERMOPHILIC-MESOPHILIC DIGESTION FOR A FULL-SCALE RETROFIT

King County Wastewater Treatment Division has studied the use of temperature-phased anaerobic digestion at the West Point Treatment Plant (WPTP) in order to improve volatile solids destruction and/or to accommodate a reduced hydraulic residence time in the existing digesters. At WPTP, a specific objective is the elimination of one or more of the existing digester tanks. The pilot study has been conducted with two, 1900-L digesters, simulating the temperaturephased process with the first thermophilic stage operated at 55°C and the second mesophilic stage at 35°C. The pilot study was initiated in July, 1999, and operated for more than a year at loadings that were selected based on projected future loads and the removal of an existing digester. Specific loading conditions include normal average loading, average with one digester out of service for cleaning, and peak week and peak day conditions. The hydraulic retention times (HRT) tested include 12 day thermo-12 day meso, 8 day-8 day, 4 day-8 day, 2 day-12 day, a peak week of 5 day-5 day, and a peak day of 2.5 day-2.5 day. An initial phase tested the parallel operation of thermophilic and mesophilic digesters at a 20 day HRT.In all operating conditions, the pilot digesters were fed combined primary and waste activated sludge that had been prethickened to approximately 6 % total solids (TS) with WPTP's gravity belt thickeners. Feeding and effluent removal was performed in a draw and fill mode at a frequency of 1–4 times per day, depending on loading conditions. Each operating condition was maintained long enough to reach steady state and to obtain sufficient data to characterize operation and performance.Performance of the processes has been evaluated principally with conventional measures, including volatile and total solids (VS and TS), volatile fatty acids (VFA), gas production and composition, soluble and total COD, ammonia and total Kjeldahl nitrogen, pH, alkalinity, and fecal coliform and salmonella bacterial numbers. The results of the steady state tests conducted show stable performance at 12 day-12 day, 8 day-8 day, and 4 day-8 day operation.Overall volatile solids reduction for the coupled thermophilic-mesophilic process averaged between 68 and 74 percent. This compares favorably with the 62 – 68 percent volatile solids reduction observed when the thermophilic and mesophilic digesters were operating independently, i.e. prior to coupling, and 64 – 78 percent removal in the WPTP full-scale Digester 2. The thermophilic phase was responsible for approximately 80 percent of the VS destruction and gas production, with the mesophilic phase contributing about 20 percent.