REFLOCCULATION OF SEWAGE ACTIVATED-SLUDGE DISRUPTED BY ULTRASONICATION

The flocculating properties of activated sludge were investigated using sludge samples from a municipal sewage-treatment plant. Ultrasonication caused disruption of the sludge flocs, releasing a high concentration of deflocculated sludge particles (DSP) and relatively low concentrations of extracellular polymers (ECPs) such as protein, RNA, DNA, and polysaccharides. The extent of deflocculation after 2, 5, and 10 min of ultrasonication was 59, 68, and 79%, respectively. The DSP returned nearly to the original good settling of flocs after 24 h of stirring. This reflocculation process was divided into two steps: a fast step in which the DSP reflocculated regardless of soluble ECPs, followed by a slow step in which a constant DSP/ECP ratio was maintained for each ECP component during the DSP reflocculation. Correlation coefficients (r=0.94-0.97, n=12) between each ECP and DSP were obtained by linear regression analysis for the second reflocculation process. In addition, the DSP showed good flocculation in the sewage filtrate and in NaCl solution above 5 mM, but poor flocculation in distilled water.