The effect of soluble microbial products on membrane fouling in a fixed carrier biological system

The fixed carrier biological system (FCBS) has been developed for treatment of wastewater with an intent to achieve high simultaneous removal of carbon and nitrogen in a single through-put process. It is believed that a longer sludge retention time (SRT), high dissolved oxygen (DO), low suspended solids and good contact with the bio-balls in an FCBS possibly enable less production of extracellular polymeric substances (EPS) and soluble microbial products (SMP). The present study relates the role of SMP in membrane fouling and the characteristics of SMP in two different biological processes, a fixed carrier biological system (FCBS) with 300 d SRT and an activated sludge process (ASP) with 15 d SRT. Unfiltered effluent was directly taken from both reactors and filtered effluent was also produced to eliminate the role of suspended solids, and both were subjected to the membrane module for fouling studies. Results of this study indicated that suspended solids and bound EPS unexpectedly played a negligible role in membrane fouling and the fouling was actually controlled by SMP. The results also showed that the FCBS achieved an approximately 25–30% higher permeate flux than that of the ASP because of the presence of higher SMP (71%), total carbohydrate (60%) and total protein (77.5%) in the ASP as compared to the FCBS. Studies on gel filtration chromatography (GFC) revealed that smaller molecular weight fractions (<1 kDa and 1–10 kDa) from both the FCBS and the ASP might be responsible for most of the membrane fouling. Further, the findings with l-tyrosine and glucose as the model foulants for protein and carbohydrate respectively showed that protein (l-tyrosine) caused more severe permeate flux decline than carbohydrate (glucose). The conclusions suggest that the use of an FCBS could hamper membrane fouling by producing less SMP.