Use of Polyelectrolyte Coagulants to
Enhance Settling Characteristics of
Activated Sludge
JOHN F. H. WALKER, Project Manager
JAMES H. DOUGHERTY, Project Engineer
Roy F. Weston, Inc.
Consulting Engineers
Newtown Square, Pennsylvania
The overall efficiency of the activated sludge type of biological treatment is
dependent, in great part, on the efficiency of secondary settling facilities. These
settling facilities are usually somewhat overdesigned, permitting a substantial degree of operational flexibility, and thereby maintaining effluent quality at a relatively high level.
On occasion, however, either municipal or industrial wastewater treatment
facilities may encounter difficulties in maintaining adequate concentration of active mixed liquor biological solids in the aeration tanks. This may be caused
either by loss of solids over settling tank weirs or by a combination of inadequate
return sludge solids concentrations and recycle pumping limitations. Such conditions can result in a reduction in the BOD removal efficiency of the system. Loss
of active solids, which may in themselves exert a significant BOD, can degrade
the quality of the effluent substantially and may ultimately result in complete
system failure. Such problems can often be attributed to significant changes in
the settling properties of the biological sludge, and/or increased hydraulic loadings.
Reductions in settling rates of biological sludge solids to values substantially
lower than those usually encountered in a given system may reflect inhibitory
effects, changes in treatability characteristics or substantial variations in wastewater organic content. Increases in flow rate over and above the hydraulic capacities of settling facilities can occur as a result of collection system expansion,
increases in discharge rate by individual contributors, short-term flow surging,
seasonal high rate flows, etc.
From the point of view of those responsible for meeting the costs incurred, it
is always economically desirable to construct treatment facilities, whether for
municipal or industrial wastewaters, to meet average conditions. In the case of
intermittent fluctuations in discharge quality or quantity, it is often not feasible
to construct additional settling or aeration facilities to operate on a "sometime"
basis. Even in the case of permanent increases in wastewater quantity or pollu-
tional characteristics, it is sometimes impossible to enlarge the scope of wastewater treatment facilities. Often, the only immediate solution to such problems
is either to discharge the undesired material without treatment or to treat it in the
currently undersized facilities at a much reduced efficiency.
The magnitude of the effects of such conditions as described above might be
considerably improved if an economically sound means of increasing the efficiency of secondary settling facilities could be found. Effluent suspended solids
concentrations might then be reduced and/or higher mixed liquor biological
sludge concentrations utilized to effect increased BOD removal efficiencies.
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