Phosphate Removal by Activated Sludge
JACK L. WITHE ROW, Chief
Agricultural-Mining Wastes Section
Robert S. Kerr Water Research Center
Federal Water Pollution Control Administration
U. S. Department of the Interior
Ada, Oklahoma
INTRODUCTION
Phosphate removal has exceeded 80 per cent during treatment of domestic-
industrial wastes in diversely located activated sludge plants. Unusually high phosphate reductions were correlated with design and operation of the activated sludge
process. Determination of the design and operational conditions resulting in consistent high-level phosphate removals was the goal of a series of field investigations
which are summarized in this paper.
Initial field investigations at three plants operated in parallel (Rilling, West, and
East) in San Antonio, Texas, were ten weeks in duration. Initially, the Rilling Plant
was studied most intensively; later considerable effort was expended on the West
Plant. The investigation at San Antonio showed that, for two years prior to the
studies, phosphate removal had averaged 80 per cent in the Rilling Plant and that
higher removal was possible to determine the applicability of the process to other
wastes and locations, 13 additional plants were monitored from one to two weeks
each. During plant monitoring, bench-scale batch studies showed that three-fourths
of the wastes and activated sludges were amenable to phosphate removal under
improved operating conditions.
In addition to the Rilling Plant, four plants were accomplishing high phosphate
removal. Additional ten-day investigations, carried out on these four plants, included
a total phosphate balance and round-the-clock monitoring. Data from the five removing plants demonstrate that the activated sludge process can result in high phosphate removal efficiencies on diverse domestic-industrial waste mixtures. The designs
and operating conditions of these five plants have striking similarities.
GENERAL CASE OF PHOSPHATE REMOVAL
Figure 1 is presented to illustrate the most common conditions in an activated
sludge plant removing phosphate. Figure 1 is a combination of an activated sludge
schematic diagram and a phosphate (Phosphate and orthophosphate are used as
synonyms; total phosphate is specified) concentration curve. Orthophosphate concentration versus location in the schematic is presented by using a common abscissa.
Figure 1 presents a summary of information gathered from the field investigations. There is little, if any, change in orthophosphate through the primary clari-
fier. (A decrease in total phosphate has been measured in some clarifiers.) The mixing
of return sludge and primary effluent results in an orthophosphate concentration in
excess of either the components or a calculated mixture concentration. This results
from a release of phosphate from the sludge to solution.
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