Preliminary research on the anaerobic side-stream reactor (ASSR) process and examination of literature led to the hypothesis that subsequent sludge hydrolysis occurs more effectively under a short period of anaerobic digestion and that recirculation of sludge back to the aeration basin
could lead to more effective excess sludge reduction. To verify this hypothesis, five activated sludge (AS) systems with different ASSR setups were operated side-by-side in the laboratory. The observed sludge yield from system #3, AS with mesophilic 2.5-day-SRT ASSR, was the lowest (0.17
mg TSS/mg COD) among the five systems. These results indicate that AS with a short-SRT ASSR created the greatest solids reduction. In addition, the sludge reduction did not cause negative effects on sludge settling and effluent quality even in an extremely long solid retention time (SRT).
It was found that the amount of released organic matter and actual volatile solids reduction occurring in a short-SRT ASSR was much smaller than those from long-SRT ASSR, despite the fact that 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 reduction in the ASSR but also on degradation of the anaerobic sludge in the AS system. The study also found that methanogens were detected even in the short-SRT ASSR identified through PCR. This work indicates
that methanogens could be selectively enriched in the ASSR system by the continuous sludge recirculation between AS and ASSR.
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