Phosphate Removal by Mineral
Addition to Secondary and
Tertiary Treatment Systems
LEON S. DIRECTO, Project Engineer
ROBERT P. MIELE, Head
Research Section, Technical Services Department
Los Angeles County Sanitation Districts
Los Angeles, California
ARTHUR N. MASSE, Chief
Municipal Treatment Research Program
Advanced Waste Treatment Research Laboratory
Environmental Protection Agency
National Environmental Research Center
Cincinnati, Ohio
INTRODUCTION
As is well documented in the literature, phosphorus is a key nutrient responsible for
accelerating the cultural eutrophication of surface waters. There are several sources from
which phosphorus may ultimately gain entrance into surface waters; however, in most
instances, the single largest contribution comes from domestic wastewaters. As a result,
control and removal of phosphorus are directed principally toward wastewaters.
The potential of secondary treatment processes; specifically, the activated sludge
process, to remove phosphate by biological uptake has been studied extensively by several
investigators (1,2,3,4). Except for a few activated sludge plants reporting unusually high
phosphate removal, most conventional activated sludge plants treating domestic
wastewater normally remove only about 15-30 percent of the incoming phosphate. To
achieve a consistently high phosphate removal, supplemental methods must be
incorporated into the conventional sewage treatment scheme. Evidence in the literature
suggests that the most economical and practical approach is by chemical precipitation, in
either the secondary system or the tertiary system (5,6,7).
The most commonly used phosphate precipitant-coagulants have been lime and the
salts of aluminum and iron. In this field study, the chemicals evaluated were alum and ferric
chloride. The objectives were to evaluate the efficacy and economics of aluminum sulfate
and ferric chloride in producing low phosphate residuals in a plant scale conventional
activated sludge plant and in a pilot scale tertiary treatment system operated alone and in
series with the secondary system; to determine the effects of mineral addition on the
biological treatment plant operation and to establish desired operating conditions for a
high degree of removal of phosphate, suspended solids and COD.
The study, which was conducted at Pomona, California, was a joint project of the
Environmental Protection Agency and the Los Angeles County Sanitation Districts.
EXPERIMENTAL APPROACH AND PROCEDURES
Study Plan and Process Schemes
The plant scale experimental program was conducted in three separate phases; baseline
operation period, the alum addition phase, and the ferric chloride addition phase. The first
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