Aerobic Thermophilic and Anaerobic Mesophilic Treatment of Sludge

The objective of this research was to investigate the effectiveness of aerobic thermophilic treatment in enhancing conventional anaerobic mesophilic digestion in terms of pathogen reduction. vector attraction reduction, volatile solids (VS) reduction, gas production, and product sludge dewaterability. Lab-scale two-stage experiments were conducted with the aerobic thermophilic stage as pretreatment (AerTAnM) or as posttreatment (AnMAerT) to mesophilic anaerobic digestion. The lab-scale AerTAnM and AnMAerT systems were operated at system sludge residence times (SRTs) of 15 and 15.5 days, thermophilic reactor temperature = 62°C, and mesophilic reactor temperature = 37°C. The control anaerobic digester was operated at a system SRT of 15 and 15.5 days and temperature = 37°C. The AerTAnM and AnMAerT systems and control anaerobic digester operated at a system SRT of 15 days were able to achieve VS reductions of >38% (Class A sludge vector attraction reduction requirement). The VS reductions by the AerTAnM and AnMAerT systems (~65%) were higher than the VS reduction in the control (~51%) by 14%. The AerTAnM and AnMAerT systems reduced fecal coliform density in the feed sludge from 108 most probable number (MPN) per gram of total solids (TS) to <103 MPN/g TS (Class A sludge fecal coliform density limit), whereas the control reduced the same feed sludge fecal coliform density to about 106 MPN/g TS. The AerTAnM and AnMAerT systems and control can reduce Salmonella density in the feed sludge from 5 to 12 MPN/4 g TS to <1 MPN/4 g TS. Average methane gas production by the AerTAnM system anaerobic mesophilic digester (0.61 m3/kg VS destroyed) was higher than those of the AnMAerT system (0.50 m3/kg VS destroyed) and control (0.52 m3/kg VS destroyed) anaerobic mesophilic digesters. Average H2S content of the AerTAnM [133 ppm volume-to-volume ratio (v/v)] system anaerobic thermophilic digester gas was significantly lower than those in gas from the AnMAerT system (249 ppm v/v) and control (269 ppm v/v) anaerobic mesophilic digesters. The dewaterabilities of the product sludge (measured as time-to-filter, s) from the AerTAnM system (237 s) and AnMAerT system (203 s) were significantly better than that of the product sludge from the control (346 s).