PRODUCING EXCEPTIONAL QUALITY BIOSOLIDS THROUGH DIGESTION PASTEURIZATION AND REDIGESTION

The AGF process is the anaerobic activated sludge process using biogas flotation for liquid solids separation (Burke 1991, Burke 1992). The AGF pasteurization process (Burke 2000) includes the additional step of pasteurizing and re-digesting the thickened waste solids to convert an even greater portion of the biosolids to gas.The AGF pasteurization process was installed at Southwest Suburban Sewer District's 3.0-mgd (1136 m3/d) Salmon Creek plant in 1998. Installation of the process involved converting an unused 18-foot (5.5 m3) diameter thickener to a gas flotation (AGF) unit and installing a 220-gallon (0.83 m3) pasteurization tank. This paper presents the results of the AGF operation over the first 60 weeks of operation. This paper also presents the pasteurization pilot results using waste solids from the AGF float concentrate. Full-scale pasteurization results will be reported at the Biosolids 2001 Conference. Table 1 compares the solids handling process at Salmon Creek before and after AGF installation.The process stabilized the digester and eliminated digester foaming. Volatile solids conversion to gas increased from 57 to 73% while the total solids removed from the plant decreased 26%. Operation of the belt press decreased from an average of 4 days per week to 1 day per week. The process increase digester capacity by a factor of 3 times its existing capacity (SRT/HRT = 3.04). The process also consumed less energy than that required to mix and heat an existing mesophillic digester.A six month pilot investigation confirmed that concentrated float recycle could be pasteurized and redigested to achieve an additional 10 to 12% volatile solids destruction. The final biosolid product was essentially odorless, pathogen free, and fully stabilized.