Investigation of a new anaerobic side-stream reactor (ASSR) process for sludge reduction in biological wastewater treatment

Preliminary research on the anaerobic side-stream reactor (ASSR) process and examination of literature led us to develop the hypothesis that deflocculation and subsequent sludge hydrolysis occur more effectively under the short period of anaerobic digestion and that recirculation of this sludge back to the aeration basin could lead to more effective excess sludge reduction. To verify this hypothesis, an anaerobic batch study was conducted and three activated sludge (AS) systems with different schemes of ASSR were operated side-by-side in the laboratory. The anaerobic batch test showed that maximum solubilization of key floc cations, extracellular polymeric substances, and enzyme activity occurred within 2 days of anaerobic digestion. For the operation results for three laboratory systems, the observed sludge yield from system #1, AS with 2.5-day-SRT ASSR, was the lowest (0.252 mg TSS/mg COD) among the three systems. These results indicate that AS with a short-SRT ASSR brought the greatest solids reduction. In addition, this sludge reduction did not cause negative effects on sludge settling and effluent quality, indicating that sludge recirculation between a short-SRT ASSR and the aeration basin provides the most favorable condition for sludge reduction in the AS in spite of extremely long solid retention time (SRT). The study also found that the amount of released organic matter and actual volatile solids reduction occurring in a short-SRT ASSR were much smaller than those from long-SRT ASSR, although the ASSR system led to much lower sludge yield. These results indicate that the success of the ASSR process is not solely dependent on the extent of hydrolysis or anaerobic sludge degradation in ASSR but on the interaction between AS and ASSR.