Effects of pH on extracellular polymeric substances compositions of biofilm in Integrated Fixed Film Activated Sludge process

Integrated fixed film activated sludge system typically integrates biofilm in the aerobic zone. The proteins-to-polysaccharides ratio of extracellular polymeric substances in the biofilm matrix is related to the strength of biofilm and, thereby, involves with the reliability and performance of this system. This study evaluated the effects of pH on proportions of both components in the extracellular polymeric substances of biofilm installed in the aerobic zone of this system. The experiments were conducted in four identical pilot-scale activated sludge systems, by which three systems were installed with Bioweb® media in the aerobic zones and fed with different wastewater pH values of 6.5, 7.5 and 8.5, respectively. Another system without fixed film media installed was fed with influent pH of 7.5 and then used as a control system. It was found that the proteins-to-polysaccharides ratios of the extracellular polymeric substances in biofilm were 0.12, 0.10 and 0.08 (w/w) at the influent pH values of 6.5, 7.5 and 8.5, respectively. The ratios decreased as the influent pH increased from 6.5 to 8.5, suggesting that extracellular polymeric substances were loosely structured in biofilm layers. Less proteins with more polysaccharides were found at high pH of 8.5, leading to resulting in the easily detachment of biofilm and viscous bulking of suspended flocs. Loosely structured biofilm decreased the mass transfer resistances, resulting in higher denitrification and nitratation in aerobic zones; therefore, the highest biological nitrogen removal was achieved at the pH of 8.5.