Economics of Anaerobic Sludge Digestion in Cylindrical Digesters at the Joint Water Pollution Control Plant

The Joint Water Pollution Control Plant (JWPCP) provides centralized solids processing for a network of seven wastewater treatment plants which treat a combined flow of 22 m3/s (500 MGD). Blended primary and waste activated sludge is anaerobically digested prior to dewatering and reuse. The current digester design at the JWPCP is a simple cylindrical tank with a flat roof supported by columns with a capacity of 14,200 m3 (3.75 MG). The JWPCP has seventeen such tanks and will soon construct seven more. The tanks are mixed by injection of compressed digester gas into draft tubes and heated by direct injection of low pressure steam into the draft tubes. No sludge recirculation or other form of supplemental mixing is employed. The digesters are taken out of service for cleaning on a 8 year cycle. Operational performance of these digesters compares favorably with that of egg-shaped digesters now being constructed and operated at many wastewater treatment plants. The total cost to construct and operate these digesters is significantly lower than the cost for egg-shaped digesters.A state-of-the-art egg-shaped digester facility was recently constructed at the City of Los Angeles' Hyperion Treatment Plant (Hyperion). Due to severe space constraints at Hyperion, an egg-shaped or ‘German Conventional’ digester design was necessary to provide the required capacity within the available land area. The facility includes 20 new tanks of welded steel construction with steep conical bottoms and spherical tops, each with a capacity of 9460 m3 (2.5 MG). Hyperion's egg-shaped digesters are mixed by axial flow mixer/draft tube combination devices and heated by direct steam injection. The Hyperion facility was chosen as a source of representative information on egg-shaped digester construction and operating costs for comparison with the costs of JWPCP's cylindrical digesters.