Properties of extracellular polymer having an effect on expression of activated sludge

Extracellular polymer in activated sludge was found to affect expression of these sludge by raising the value of the average specific resistance (α_av) during filtration and lowering the value of the modified consolidation coefficient (C_e) during consolidation. Further, the addition of extracellular polymer to the sludges caused higher values of α_av and lower values of C_e. The logarithmic plots of α_av and C_e against the quantity of extracellular polymer was linear. When these two values (α_av and C_e) of each activated sludge were plotted against the quantity of extracellular polymer extracted from each sludge, they were correlated by a single straight line with two exceptions. These were the return sludge of a textile factory and the anaerobically digested sludge of a sewage treatment plant. These exceptions indicate that the values of α_av of these sludges are higher than those of other sludges containing the same amount of extracellular polymer and that of C_e are lower than those of other sludges. It was confirmed that the extracellular polymer of these two sludges contains a lot of polysaccharides of the molecular weight 600–100,000 (with a standard of polyethylene glycol) by gel chromatography. The effects of the flocculant on dewatering were investigated. The effects of the cationic polymer flocculant addition to the extracellular polymer were analyzed using an ultrafiltration cell (molecular weight cut-off 10,000). The ultrafiltration rate of extracellular polymer was much improved by adding a cationic polymer flocculant. The removal of extracellular polymer by adding cationic polymer flocculant was ascertained by measuring the molecular weight distribution of the polysaccharides. On the other hand, no flocculations of extracellular polymer and sludge by adding anionic and nonionic polymer flocculant could be observed.