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The purpose of this study was to examine the fate of activated sludge extracellular proteins in batch anaerobic and aerobic digestion using sodium dodecyl sulfate polyacrylamide gel electrophoresis (SDS-PAGE). We were particularly interested in studying the fate of proteins that are
bound with different sludge cations. Extracellular polymeric substances (EPS) extracted by three cation-specific extraction methods, the cation exchange resin (CER) procedure for extracting Ca2++Mg2+, sulfide extraction for removing Fe, and base treatment (pH 10.5) for
dissolving Al, were subjected to SDS-PAGE and the resultant protein profiles were compared before and after sludge digestion. The initial SDS-PAGE on undigested activated sludge showed different protein profiles by the three methods, suggesting that Ca2++Mg2+, Fe, and
Al are bound with different characteristic EPS and play unique physio-biochemical roles in floc formation. The analysis of SDS-PAGE post sludge digestion revealed that CER-extracted proteins remained intact in anaerobic digestion while they were degraded in aerobic digestion. While the fate
of sulfide-and base-extracted proteins in aerobic digestion was not clearly resolved, their changes in anaerobic digestion were well observed. Sulfide-extracted protein bands were reduced by anaerobic digestion, indicating that Fe-bound EPS were degraded under anaerobic condition. While parts
of base-extracted proteins disappeared after anaerobic digestion, others became more extractable along with extraction of new proteins, indicating that base-extractable EPS are dynamic in anaerobic digestion. All these results suggest that the metal content in influent sewage have a significant
impact on activated sludge characteristics and sludge digestibility. The potential application of SDS-PAGE and the three extraction methods for choosing a proper sludge pretreatment and digestion method are also discussed in this study.
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