Reduced sludge production in a membrane bioreactor by uncoupling metabolism and its effect on phosphorus accumulation in the biomass.

Due to its limited recycling or reuse, treatment and disposal of excess waste activated sludge has been a major challenge. As a preemptive method, therefore, uncoupling metabolism for reduced sludge production has been investigated recently. In this study, we operated a pilot-scale A2O-membrane bioreactor (MBR) system incorporating an anaerobic sludge holding tank (SHT) in a sludge recycling line to induce uncoupling metabolism, and investigated sludge production and treatment efficiency. After operation for ≥1 year, the Yobs value was estimated to be 0.041 g mixed liquor suspended solid (MLSS)/g chemical oxygen demand with 198.7 days of solids retention time (SRT). This Yobs value was markedly lower than those reported previously. Since MBR can be operated with a relatively high MLSS and prolonged SRT, the greatest reduction was achieved by combination with uncoupling metabolism. Phosphate fractionation experiments of the MLSS from the pilot MBR suggested the total phosphate content of microorganisms was 47.0 mg P/g mixed liquor volatile suspended solid; 83% higher than that of the activated sludge process and 49% higher than that of the conventional A2O process. Of the increased phosphate contents, that of the acid-insoluble polyphosphate (AISP) fraction was greatest, suggesting that growth inhibition by uncoupling metabolism stimulates AISP synthesis, which can be utilized under growth-limiting conditions. ABBREVIATIONS: AISP: acid-insoluble polyphosphate; ASP: acid-soluble polyphosphate; BNR: biological nutrients removal; EPS: extracellular polymeric substance; LMH: L/m2 h; MBR: membrane bioreactor; OST: oxic-settling-anaerobic; PAO: phosphate accumulation organism; PCA: perchloric acid; SBR: sequencing batch reactor; SHT: sludge holding tank; SRT: solids retention time; TN: total nitrogen; TP: total phosphate; WAS: waste activated sludge.

Analysis of extracellular polymeric substance (EPS) release in anaerobic sludge holding tank and its effects on membrane fouling in a membrane bioreactor (MBR).

Amongst sludge reduction strategies, the anaerobic side-stream sludge holding tank (SHT) is of particular interest because it has shown significant sludge reduction efficiency. However, due to the anaerobic and starving environment of the SHT, the release of extracellular polymeric substance (EPS) may be stimulated, and it may hamper the application of the SHT to the membrane bioreactor. In order to investigate the effect of sludge storage on EPS release, sludge samples from a pilot-scale sequencing batch reactor coupled with SHT was incubated in a series of bench-scale SHT reactors for different periods of time (0-24 h). The increase in EPS was not significant until 12 h of incubation (9.3%), while 40.9% of the increase was observed in the sample incubated for 24 h. The rapid increase in EPS concentration after 12 h indicates a greater rate of cell lysis than that with EPS consumption as substrate. Since inducing the initial stage of the endogenous phase within microorganisms is a key factor for the successful operation of the SHT for sludge reduction, the retention time for the SHT should be shorter than the time for the sudden increase in EPS release.