BACKGROUND This study aimed to clarify how a variation in the carbon to nitrogen (C/N) ratio in influent wastewater affects the production and composition of extracellular polymeric substances (EPS) in activated sludge. In the literature such a comparative analysis is rarely presented, most studies on EPS focus on simultaneous organic carbon removal and nitrification whereas little attention is paid to the case of low organic load or separate‐stage nitrification. RESULTS Among the very loosely bound, loosely bound and tightly bound EPS fractions, tightly bound EPS prevailed in semi‐continuously fed batch activated sludge reactors that were fed at a C/N ratio of 10, 5 and 0, respectively. The C/N ratio had no clear effect on total EPS production, however it changed the proportion of proteins and carbohydrates. In each sludge proteins constituted the dominant part of EPS, but the EPS of the nitrifying sludge (C/N = 0), had a definitely higher protein content. As evidenced by HPSEC fingerprints, in all EPS fractions most proteins had MWs ≤16 kDa. Depending on the C/N ratio in a reactor, high MW proteins in EPS shifted from tightly bound fractions to others. CONCLUSION The study is among the few that highlight how the protein and carbohydrate content in EPS changes at different C/N ratios. Since wastewater composition in a treatment system affects the chemical composition of EPS, each sludge might have different responses to uptake of xenobiotics such as metals. Further, this difference in composition might affect sludge settling, dewatering and membrane fouling. © 2014 Society of Chemical Industry

The study aims to clarify how the type of organic substrate in a wastewater affects the production and composition of extracellular polymeric substances (EPS) and hydrophobicity and surface charge of activated sludges. For this purpose, three activated sludge reactors were operated in parallel with feeds composed of the organics (i) peptone, glucose, and acetate and (ii) peptone and (iii) glucose. EPS extracted from sludges were fractionated into very loosely bound, loosely bound, and tightly bound fractions and analyzed for protein and polysaccharide. Also, molecular weight distribution of proteins was determined by using high-pressure size exclusion chromatography (HPSEC). Regardless of the type of organic substrate, in each sludge, tightly bound EPS (TB-EPS) prevailed. The type of organic substrate affected the relative proportion of protein and polysaccharide and had an impact on hydrophobicity and surface charge. The sludge fed with peptone was distinctly more hydrophobic and had a lower negative surface charge than others. HPSEC fingerprints revealed that the variety and size of proteins were dependent on the type of feed. HPSEC also pointed to a shift of high molecular weight (MW) proteins from TB-EPS to others. In addition, results of a parallel study examining the inhibitory effect of Ag(+) on three sludges were interpreted along with feed composition, EPS, and surface measurements. The response of each sludge to toxic Ag(+) ion seemed to change with the type of feed.

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