Anaerobic Thermophilic/Mesophilic Dual-Stage Sludge Treatment

Conventional anaerobic mesophilic (AnM) digestion coupled with anaerobic thermophilic (AnT) pretreatment (AnTAnM system) and anaerobic thermophilic posttreatment (AnMAnT system) of mixed sludge (thickened waste activated sludge and primary sludge) was investigated. The main objectives were to investigate the ability of AnTAnM and AnMAnT systems to produce a product sludge that can meet Class A sludge requirements and to enhance sludge treatment in terms of volatile solids (VS) destruction, gas production, sludge supernatant chemical oxygen demand (COD) reduction, and sludge dewaterability. Lab-scale AnTAnM and AnMAnT systems were operated at a system sludge residence time of 15 days and temperature of 62°C in AnTAnM and AnMAnT thermophilic reactors. A lab-scale control anaerobic digester was operated at a system sludge residence time of 15 days and temperature of 37°C. The AnTAnM and AnMAnT systems and control achieved VS reductions of >38% (Class A sludge vector attraction reduction requirement). Average VS reductions by the AnTAnM (61%) and AnMAnT (63%) systems were significantly higher than VS reduction by the control (50%). The fecal coliform densities in the AnTAnM and AnMAnT system product sludges were below 1,000 most probable number (MPN) per gram total solids (TS) (Class A sludge fecal coliform density limit) compared to 106 MPN/g TS in the control product sludge. The product sludge from the AnTAnM and AnMAnT systems and the control anaerobic digester met the Class A sludge Salmonella density limit (<3 MPN/4 g TS) when fed with feed sludge containing 2-12 MPN/g TS. Average methane production by the AnTAnM mesophilic digester (0.66 ± 0.10 m3/kg VS destroyed) was higher than those of the AnMAnT (0.51 ± 0.06 m3/kg VS destroyed) and the control anaerobic mesophilic digesters (0.52 ± 0.03 m3/kg VS destroyed). The average supernatant CODs in the AnTAnM system product sludge (10,500 ± 200 mg/L) and AnMAnT system product sludge (10,200 ± 150 mg/L) were approximately the same and were significantly lower than the supernatant COD in the control anaerobic digester (14,100 ± 350 mg/L). All three systems were fed with feed sludge containing an average supernatant COD of 22,500 mg/L. Dewaterability of the product sludges, measured as time to filter, was 244 and 207 s for AnTAnM and AnMAnT systems, respectively, whereas it was 364 s for the control anaerobic digester product sludge.