MIDLAND, ONTARIO EGG SHAPED DIGESTER A SMALL COMMUNITY SYSTEM THAT WORKS

In late 1996 the Town of Midland placed a 1,000 m3 (264,200 U.S. gal.) egg shaped digester (ESD) system in operation. The ESD system had been selected over a conventional American digester system to greatly increase reliability, predictability, and performance. A jet pump mixing system was selected for the installation. The mixing system provides at least 7.5 vessel turnovers per day. A high velocity and volume spray system is employed for foam control. The ESD system has operated continuously since start-up. The current hydraulic loading is 55 m3 (14,531 U.S. gal.). Raw sludge organic loading is reasonably high with the raw sludge averaging 4.36% total solids and 66% volatile fraction. At the current loading, the Hydraulic Retention Time (HRT) is approximately 18 days. High quality gas is produced and used for all necessary heat in the administration/lab building, large shop, and ESD. At no time has supplemental gas been necessary. The digested sludge is very stable at 48% volatile solids. The sludge shows good thickening potential and has an earthy odor. Volatile solids reduction for the period of operation data is 52.5%. When the project was initiated, the life cycle cost for ESD verses American conventional digestion was estimated to be approximately equal. The cost of ESD construction was estimated to be approximately 500, 0001 more than American conventional. The operation cost difference favored ESD by about 550,0001. After nearly four years of operation, the ESD system has clearly demonstrated stability and reliability. Actual operation costs show an ESD life cycle cost advantage of 908,000. Canadian dollar = 0.68 U.S. dollar.