The Fate of Phosphorus in Waste
Treatment Processes:
The Enhanced Removal of
Phosphate by Activated Sludge
ARNOLD B. MENAR, Graduate Student
DAVID JENKINS, Assistant Professor
Department of Civil Engineering
University of Calif ornia
Berkeley, California
INTRODUCTION
In the considerable body of literature concerning the fate of phosphate in the
activated sludge process, there appear to be two distinct schools of thought. The first
viewpoint, represented in papers by Sawyer (1), Sekikawa etal. (2), Hall and Engel-
brecht (3), and Jenkins and Menar (4), can be summarized as follows: 1) The volatile
mass of activated sludge contains between two and three per cent by weight of biologically incorporated phosphorus; 2) the amount of phosphate incorporated biologically into activated sludge is not affected by the growth rate of the sludge, or by process operating parameters such as organic loading, MLSS concentration, aeration rate,
or mixed liquor DO concentration; and 3) phosphate removal is proportional to the
net sludge growth.
With activated sludge P contents of two to three per cent, domestic sewage P
contents of ten mg P/1, and substrate removal rates in the standard-rate range (0.4-
1.0 lb COD removed/lb MLVSS-day), biological removal of phosphate can only account for a maximum of 20 to 30 per cent of the influent phosphate or about two
to three mg P/l of an influent ten mg P/l.
The second point of view, expounded in papers by Levin and Shapiro (5),
Borchardt and Azad (6), and Connell and Vacker (7), embodies the idea that under
certain conditions activated sludge is capable of removing more than the two to three
per cent phosphate-P that it requires for growth. The biological uptake of excess
phosphate has been termed "luxury uptake." According to these authors phosphate
removal by this mechanism is enhanced by high DO concentrations and, conversely,
phosphate that has been incorporated into sludges by the luxury uptake mechanism
can be released easily under anoxic conditions and conditions of low pH, such as
those that might be encountered in the sludge blanket at the bottom of a secondary
clarifier or in an anaerobic digester. Further, luxury uptake of phosphate leads to
sludges with phosphate contents two to three times greater than "normal" sludges
and consequently, treatment of domestic sewage at standard organic loading rates can
lead to phosphate removals of 60 to 90 per cent of an influent ten mg P/l.
A mechanism for the enhanced removal of phosphate must be consistent with
these observations and in addition must provide satisfactory interpretation of the
following observations made at the Rilling Road Plant, San Antonio, Texas - a plant
that has experienced phosphate removals as high as 90 per cent:   1) At a point about
-655-