FLOC BREAKUP IN ACTIVATED-SLUDGE PLANTS

This investigation, conducted at 24 full-scale activated sludge wastewater treatment plants, quantified the extent of floc breakup resulting from the manner in which mixed liquors in aeration basins and conveyance systems are handled. Results showed that the concentration of dispersed solids increases with increasing aeration basin mixing intensity in the case of diffused air aeration systems. Also, the beneficial effect of the tapered aeration mode of operation on the extent of flocculation was shown. In mechanically aerated basins, the location of the aerator nearest the basin discharge point was found to be critical with respect to floc integrity since the degree of solids dispersion was shown to vary inversely with distance from a mechanical aerator. The presence of elbows and free falls of less than 0.5 m in mixed liquor transport systems was found to slightly increase solids dispersion. Free falls greater than 1.2 m however, were observed to cause significant floc breakup. Shear rates normally present in conveyance pipelines resulted in net floc aggregation.