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Investigation Of Excess Sludge Reduction By An Anaerobic Side-Stream Reactor (ASSR): The Role Of EPS And Enzymes In Sludge Floc

Over the last two decades, minimization of excess sludge generation within the activated sludge process has been studied. Among several net sludge reduction technologies, the anaerobic side-stream reactor (ASSR) process is of particular interest because it has shown significant sludge reduction without causing negative effects on operational performances. This study focused on the verification of the ASSR process, the mechanisms of excess sludge reduction, and the development of a new process using high rate ASSR. The earlier part of this research found that a bench-scale ASSR with 10 day solids retention time (SRT) led to about 60% less sludge yield than conventional activated sludge, without causing negative effects on the main activated sludge process, i.e., sludge settling and effluent properties. This sludge yield result indicated that incorporation of an anaerobic side-stream reactor into activated sludge was a valid sludge reduction process and was much more effective than any other conventional methods (anaerobic digestion or aerobic digestion). New methods to estimate SRT and observed sludge yield for the ASSR process were also proposed during this stage of research. The later part of this research investigated the interaction between ASSR and activated sludge and the role of extracellular polymeric substances (EPS) and enzymes in sludge flocs to reveal the mechanisms of excess sludge reduction in the ASSR process. It was observed that much of the organic matter, particularly the EPS, was solublized in the ASSR and readily degraded in the main activated sludge reactor as the previous study showed. By accounting for the mass of sludge in the reactors, it was determined that half of the sludge reduction occurred directly in the ASSR while the other half of the sludge was degraded in the aeration basin. From an intensive side-by-side reactor study, it was found that the amount of the released material from ASSR was not proportional to overall sludge reduction, indicating that the success of the ASSR process is not solely dependent on the extent of hydrolysis or anaerobic sludge degradation in the ASSR but on the recirculation of the whole sludge between aerobic conditions (activated sludge) and anaerobic conditions (ASSR). This sludge recirculation reduced the accumulation of excessive EPS fractions within the flocs, allowing for balanced EPS fractions even under extremely long SRT conditions, and thus resulting in effective flocculation and sludge settling. Overall, the ASSR process kept the sludge refreshed in spite of the extremely long SRT due to the extremely minimal sludge wasting. This concept is proposed in this research as Sludge Refreshment . Preliminary research and examination of literature reviews during this doctoral research led us to develop a new 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 even more effective excess sludge reduction. To verify this hypothesis, an anaerobic batch study was conducted and various schemes of the ASSR process (different SRTs and temperatures) were operated side-by-side in the laboratory. The results from the anaerobic batch tests showed that maximum solubilization of key floc cations, extracellular polymeric substances, and enzyme activity occurred within 2 days of anaerobic digestion, regardless of temperature. The results from the reactor study showed that activated sludge with a 2.5-day-SRT ASSR, generated the lowest sludge yield among the studied systems. All these results indicate that an activated sludge process with a short-SRT (termed high rate) ASSR could result in greater solids reduction during wastewater treatment. In summary, this research found that the ASSR process is valid for effective sludge reduction in biological wastewater treatment. The study of a novel high rate ASSR process also expanded insight into sludge flocs and allow a better understanding of the fate of EPS in aerobic and anaerobic repeating conditions. This process should be considered to be a very effective method for sludge reduction which also maintains good operational performance for the activated sludge process.
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