Role of extracellular protein in the formation and stability of aerobic granules

The extracellular proteins extracted from the sludge samples had molecular weights in the range of 31.0–97.4 kDa. Several new protein bands were observed with the aerobic granulation. The quantitative data on extracellular polymeric substances (EPS) indicated the secreted extracellular protein contents increased from 49.4 mg g−1 (VSS) in the seed sludge to 148.3 mg g−1 (VSS) in aerobic granules on day 45, while the extracellular polysaccharides remained almost unchanged. The quantitative data on protein was consistent with the sodium dodecyl sulfonate-polyacrylamide gel electrophoresis (SDS-PAGE) analysis. Sludge surfaces were more hydrophobic and less negatively charged after aerobic granulation than the flocculent seed sludge. The ratio of proteins to polysaccharide within the sludge EPS increased with the granulation from 2.3 to 4.9 corresponding to the changes in the surface properties of the sludge. A more hydrophobic and less negatively charged surface corresponded to a higher ratio of the extracellular proteins to polysaccharides. After the complete aerobic granulation, the increase of the influent COD loads from 4 to 12 g L−1 d−1 was accompanied with a decrease of the secreted extracellular protein in types and contents and an increase of polysaccharide concentration in sludge EPS. A less hydrophobicity of sludge surface was associated with the changes in the composition of EPS. This might explain why the disintegration of granules would occur at the high influent COD loads employed (10 and 12 g L−1 d−1) from a standpoint of microbiological surface properties. It was proposed that that extracellular protein might be strongly involved in the aggregation of suspended flocs into granules, and also associate with the stability of the granules through affecting the surface properties of sludge.