ANALYSIS OF FLOC STRUCTURE AND PREDICTING SLUDGE DIGESTIBILITY USING DIFFERENT CATION-ASSOCIATED EPS EXTRACTION METHODS

Extracellular polymer substances (EPS) extracted from both full and bench scale activated sludges by three different extraction techniques were examined in order to analyze the structure of activated sludge floc and EPS digestibility under anaerobic and aerobic conditions. The extraction methods used in this study are the cation exchange resin (CER) procedure, extraction under high pH (˜10.5) conditions, and extraction using sulfide. It was hypothesized that each extraction method targeted different cation-bound EPS in floc. It is proposed that flocs are composed of different biopolymeric factions bound with different cations, so that the CER procedure extracts Ca2+ and Mg2+-bound EPS, base treatment with NaOH extracts Al-associated EPS, and addition of sulfide dissociates Fe-linked biopolymer from floc. The data have shown that different extraction methods resulted in a different quantity of EPS protein and polysaccharide and also distinct molecular weight distributions of these biopolymers, indicating that EPS extracted by different extraction methods were primarily composed of different types of EPS. In addition, the data associated with CER related procedures (CER alone and CER following sulfide or high pH pre-extractions) clearly illustrated that the EPS that was extracted by CER is not removed by other extraction methods. These data show that there are different EPS pools that are associated with different cations in activated sludge floc and a single extraction method cannot extract all the extractable EPS from floc. The fate of three different extraction associated-EPS in sludge digestion was studied using anaerobically digested sludge and feed sludge collected from the field. Comparison of EPS extraction data before and after anaerobic digestion revealed that most difference occurred with the degradation of feed sludge protein that was extracted in presence of sulfide. This result further suggested that Fe-associated protein might be the primary organic matter that is degraded under anaerobic conditions.