No abstract

To investigate the mechanism of sludge reduction in activated sludge (AS) with an anaerobic side-stream reactor (ASSR), four AS systems with different digestion schemes were operated in the laboratory. The four systems are: a) AS+ASSR; b) AS+aerobic digester; c) AS+anaerobic digester; and d) AS with no solids wastage. The average sludge yield of AS+ASSR from two phases was 0.14 mgVSS/mgCOD, which is 22-54% less than that from the three other systems. The accounting of biomass in AS+ASSR system revealed that 50% of sludge is degraded in ASSR while the other half is degraded in the aeration basin. Furthermore, both whole sludge and centrate from ASSR led to a significant oxygen uptake in AS, indicating the importance of aerobic biodegradation in AS+ASSR system. The extracellular polymeric substances (EPS) data showed that base-extractable EPS was much smaller for AS with ASSR than with no wastage. In contrast, cation exchange resin-EPS was similar for both systems. These results indicate that degradation of base-extractable EPS accounts for the lower sludge yield in AS+ASSR, and based on the literature this organic pool is believed to be aluminium and/or iron-bound EPS. The microbial profile data suggests that recirculation in AS+ASSR selects some unique microorganisms. Further research is warranted to study their role in sludge reduction.

The activated sludge (AS) system with a high rate anaerobic side-stream reactor (ASSR) has been developed and investigated in our laboratory. To study a main sludge reduction mechanism in this process, the anaerobic batch study was conducted and three AS systems with different schemes of ASSR were operated side-by-side in the laboratory. The three systems were: system #1; AS with 10-day-SRT ASSR, system #2; AS with 2.5-day-SRT ASSR, and system #3; control AS without ASSR. The anaerobic batch test showed that maximum solubilization of key floc cations, such as magnesium and calcium, and polysaccharides occurred within 2 days of digestion. Importantly, sludge that was treated under short anaerobic digestion also showed the highest protease activity. These results suggest that deflocculation and subsequent sludge hydrolysis occur more effectively for the short period of anaerobic digestion and that recirculation of this sludge back to the aeration basin could lead to more efficient excess sludge reduction for the AS system. Another great benefit of having a short-SRT ASSR will be a significant volume reduction for the side-stream reactor, thus, allowing for a simple tank reactor such as completely-stirred-tank reactor (CSTR) to be used in the side-stream treatment.The actual sludge yield data from system #1, #2, and #3 showed 0.25, 0.18, and 0.44 mg VSS/mg COD, respectively. These results indicate that AS with a short-SRT ASSR brought the greatest solids reduction. The reduction of sludge 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 activated sludge system. The study also found that the amount of released organic matter and actual volatile solids reduction (VSR) occurring in a short-SRT ASSR were much smaller than those from long-SRT ASSR, although the former system led to much lower sludge yield. These results indicate that the success of ASSR technology is not dependent on the extent of hydrolysis or actual anaerobic degradation occurring in ASSR but on "proper" combination of anaerobic hydrolysis (and partial degradation) and degradation of sludge in the main aeration basin. Overall, the study concludes that efficient sludge reduction within AS system can be achieved by interacting with a small anaerobic side-stream treatment. Furthermore, the amount of the released material and VSR in ASSR is not proportional to sludge reduction and ASSR works just like a conventional anaerobic digester.