Increase of microbial growth potential in municipal secondary effluent by coagulation

Microbial growth is a big issue of concern in the use of reclaimed water. In this study, the variation of microbial growth potentials of municipal secondary effluents after coagulation was evaluated by measuring assimilable organic carbon (AOC). Surprisingly, the AOC levels increased significantly (55-667%) after coagulation with poly-aluminum dosages of 60 mgL(-1) for the samples investigated in this research. By ultrafiltration membrane fractionation, the microbial growth potentials of the fractions with different molecular weight (MW) were measured. The results revealed that the maximum cell densities of microbial growth in secondary effluents were lower than those in their fractions with MW<10kDa. Meanwhile, the organic component with MW>10kDa in biological treated effluents was proved to have an inhibitory effect on microbial growth. Therefore, the removal of those high MW organic matters was the main reason for the increase of microbial growth potential in secondary effluents during coagulation. Furthermore, polysaccharides and/or proteins in secondary effluents were easily removed by coagulation and were thought to be the possible key organic substances affecting the microbial growth potential during coagulation. It is suggested that post treatments would be needed after coagulation to maintain the biological stability of reclaimed water.