NORTH AMERICA'S FIRST NEW TEMPERATURE PHASED ANAEROBIC DIGESTION SYSTEM – A SUCCESSFUL START-UP AT THE WESTERN LAKE SUPERIOR SANITARY DISTRICT (WLSSD)

The Western Lake Superior Sanitary District (WLSSD) is located on the western shore of Lake Superior in Duluth, Minnesota. The WLSSD has undertaken a major change in its biosolids program, shifting from sludge co-incineration with solid waste to temperature phased anaerobic digestion (TPAD) and land application. A majority of the loading to the plant's liquid stream treatment system, oxygen activated sludge, comes from paper mill wastewater making the waste activated sludge higher in poorly-degradable volatile solids than conventional municipal sludge for anaerobic digestion. All influent flow goes directly to the oxygen activated sludge system without primary treatment. Pilot studies conducted on the sludge at Iowa State University prior to commencing design indicated that the TPAD process could achieve significantly higher volatile solids destruction than conventional mesophilic digestion and produce a well-stabilized product suitable for land application.Following the initial positive results from the pilot work, the TPAD system was recommended. TPAD incorporates thermophilic and mesophilic digesters operated in series. The TPAD process biogas is captured and used for plant and digester heating. Biosolids are dewatered by new centrifuges, achieving greater than 27 percent solids prior to land application. Cake storage facilities have also been added as part of the project and are capable of storing up to 60 days of dewatered biosolids during adverse weather conditions and spring road restrictions.The TPAD process at WLSSD utilizes a single thermophilic digester that receives all raw waste activated sludge, thickened to 5 to 6.5 percent, followed by 3 mesophilic digesters operating in parallel. The nominal design hydraulic residence times are 5 days for the thermophilic digester and 15 days for the mesophilic digesters. The thermophilic digester is heated by the plant heating system, supplied by dual-fuel, low-pressure, steam boilers. All digester gas is used in the boiler system for plant and process heating. A heat recovery system is used to cool the thermophilic sludge prior to being fed to the mesophilic digesters. Recovered heat is used for building heating except during the few warm summer months when it is routed to a cooling tower.This paper presents the background and basic design elements of the system and focuses on the startup of the TPAD system and the initial months of operation. Startup began in May 2001 and all major facilities are now in operation. During startup, careful attention was given to the mechanics of equipment startup, operator training, safety issues, anaerobic biomass development, digester seeding, loading rate schedules, startup monitoring and process reactions, volatile acid speciation, ammonia concentrations, gas production and quality, and other basic digestion performance parameters. Information is also presented on pathogen destruction, biosolids dewaterablility, land application experiences, and energy issues.