Occurrence, source, and fate of dissolved organic matter (DOM) in a pilot-scale membrane bioreactor

In this study, the fate of carbohydrates, proteins, and humic substances in feedwater, sludge supernatant, and permeate of a pilot-scale membrane bioreactor (MBR) was investigated. Over 10 months, carbohydrates were observed to have a lower bioelimination degree (45%) and higher rejection degree (79%) than those of proteins (81% and 44%, respectively), which led to a high carbohydrate/protein ratio of dissolved organic matter (DOM) in sludge supernatant. The batch tests showed that DOM derived from feedwater and bound extracellular polymeric substances (EPS) was eliminated by activated sludge via biosorption and biodegradation. The proteins in bound EPS and feedwater were also found to have much higher biosorption potential (27% and 31%, respectively) than humic substances (11% and 17%, respectively) and carbohydrates (16% and 14%, respectively), indicating that proteins had a high affinity with sludge flocs. The results also showed that carbohydrates and humic substances in bound EPS were more difficult to be eliminated by activated sludge. In addition, the batch tests confirmed that feedwater was mainly composed of readily biodegradable matter, and bound EPS was mainly composed of slowly biodegradable matter. Size exclusion chromatography with continuous organic carbon and UV(254) detection (LC-OCD) showed that large-size substances (i.e., carbohydrates and macromolecular proteins) in sludge supernatant might originate from bound EPS, which can be rejected by membranes. The LC-OCD analysis, together with the results obtained from batch tests, suggested bound EPS might be the most important source of DOM in the sludge suspension.