An earlier paper by the authors (1988) introduced a correlation between the Vesilind settling parameters, Vo and K, and SVI performed in a stirred, 1‐L graduated cylinder even though SVIs also were determined in more commonly used containers such as an unstirred, 1‐L graduated cylinder and an unstirred, 2‐L Mallory settlometer. Only the stirred graduated cylinder relationship was given because stirring increases the precision and decreases the variability of the SVI test, and Standard Methods (1975) has included stirring in the SVI test since 1975. Nonetheless, because the SVI test continues to be performed in unstirred containers, the correlations for the other SVI test methods measured during the study are presented herein so that they can be used by the wastewater treatment plant operations community.

The success of gravity separation of activated sludge from a treated effluent depends on the flocculent nature of the mixed liquor entering the secondary clarifier. Despite its importance to the overall effectiveness of the activated sludge process, flocculation phenomena are not routinely considered in the design and operation of the process. Further optimization of the activated sludge process to meet higher performance demands requires that the competing reactions of floc aggregation and breakup be maximized and minimized, respectively. Accordingly, the goal of this study was to develop an improved understanding of activated sludge flocculation.
A theoretically based and easily performed batch flocculation procedure was developed. The procedure enabled the quantification of the flocculation characteristics of activated sludges. The procedure was field applied, testing 30 activated sludges obtained at 21 full‐scale facilities. Results obtained during the field study indicated that the equilibrium concentration of supernatant suspended solids following batch flocculation and settling is comparable for a wide variety of activated sludges regardless of the initial aggregative state of the mixed liquors or the aeration device employed. The results indicated that flocculation of activated sludge cannot be used to reduce supernatant suspended solids below a certain limit. Moreover, the results indicated that attainment of equilibrium is rapid; the activated sludge flocculation reaction in batch reactors was 99% complete within 10 minutes for all but six of the activated sludges studied. Field‐determined estimates of activated sludge flocculation characteristics can be used to predict the performance of flocculators placed either upstream of or in secondary clarifiers. These estimates also can be used to determine the impact of altering process variables on flocculation, thereby affording a procedure for optimizing an activated sludge's flocculation potential.

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.

Historically, secondary clarifiers have contributed to instability in plant performance. The flocculator-clarifier development has demonstrated in a variety of plant situations that stable secondary process operation is achievable. Consistently low effluent SS levels (average values of 10 mg/l) are realistically achievable. This performance enhancement has been obtained at lower costs than for conventional designs. The clarifier incorporates some special design features including a large flocculator centerwell to incorporate dispersed solids into settleable floc. Other features include rapid sludge removal, inboard weir placement and deeper sidewater depths than conventionally used (5.5 to 6m).

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