Biopolymer and Cation Behavior during WAS Digestion with an AnMBR Process and Their Contribution to Membrane Fouling

This paper addresses a study that was conducted to characterize the behavior of biopolymers and cations and their impact on membrane performance in a pilot AnMBR stabilizing waste activated sludge (WAS). Three AnMBR operating conditions consisting of 15/30 (run 1), 7/15 (run 2) and 7/30 (run 3) day HRT/SRT respectively were evaluated with regard to generation, consumption and/or accumulation of solution and floc associated biopolymers and cations, membrane performance and composition of the membrane fouling layer. The study showed an overall decrease in bound biopolymers, an increase in floc associated Fe, an increase in colloidal biopolymers, and a decrease in soluble carbohydrate upon anaerobic digestion of WAS. Bound protein and carbohydrates decreased by 60±3 and 64±2% respectively in the AnMBR digester operated at a 15 day SRT. Increasing the SRT to 30 days but maintaining a similar HRT resulted in a greater reduction of bound protein and carbohydrate (73±1 and 74±3% respectively). These trends were similar to those observed for volatile solids destruction (41% and 49% for15 and 30 days SRT respectively). The study also showed an overall decrease in soluble calcium and iron concentrations. However an increase in soluble protein concentration was observed with an increase in SRT indicating release and preferential accumulation of soluble inert protein material. Declines in the critical flux from 21 (run-1) to 17 (run-2) LMH with an increase in SRT and from 40 (run 1) to 17 (run 3) with an increase in HRT and/or SRT: HRT ratio was observed. A comparison of biopolymer composition with the sustainable flux indicated negative correlations between colloidal protein (r2=-0.89), soluble carbohydrate (r2=-0.91), bound carbohydrate (r2=-0.32) and bound protein (r2=-0.42) concentrations with flux. An examination of the foulant layer confirmed the presence of proteinaceous and carbonaceous materials and significant amounts of iron, calcium and sulfur on the membrane surface. Overall the results indicated that increasing the degree of digestion and thickening resulted in the generation of colloidal biopolymers, accumulations of Fe, Ca and S based precipitates and an increase in concentration of floc associated materials that negatively affected membrane performance.