Five Years of Successful Operation – A Report on North America’s First New Temperature Phased Anaerobic Digestion System at the Western Lake Superior Sanitary District (WLSSD)

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The Western Lake Superior Sanitary District (WLSSD) is located on the western shore of Lake Superior in Duluth, Minnesota. Five years ago, the WLSSD undertook a major change in its biosolids program, shifting from sludge co-incineration with solid waste to temperature phased anaerobic digestion (TPAD) and land application. The WLSSD wastewater treatment plant treats average flows of 162 million liters/day (43 million gallons/day). 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. In addition, all influent flow goes directly to the oxygen activated sludge system without primary treatment, contributing to the lower-than-typical digest-ability of the sludge. Pilot studies conducted on the sludge 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 initial positive results from pilot work, the TPAD system was recommended and the new facility was designed, built, and went on line in May 2001. 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 an average of 27 percent solids prior to land application.

The TPAD process at WLSSD utilizes a single, one-million-gallon (MG) thermophilic digester that receives all raw waste activated sludge, thickened to 5.0 to 6.5 percent, followed by three, one-million-gallon, 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, lowpressure, 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 warm summer months when it is routed to a cooling tower.

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