On-Site Wastewater Treatment and Disposal Systems - Forced ...

6.6.3.3 Chemical Precipitation
Phosphorus in wastewater may be rendered insoluble by a selected number
of metal salts, including aluminum, calcium, and iron (62). Although
the reactions are complex, the net result is the precipitation of an
insoluble complex that contains phosphate. Phosphorus precipitation
methods normally include the addition of the chemical, high-speed
mixing, and slow agitation followed by sedimentation.
There has been little long-term experience with phosphorus removal of
wastewaters onsite (21170). Precipitation of phosphates is less easily
accomplished for polyphosphates and organic phosphorus than for ortho-
phosphate. Therefore, precipitation within the septic tanks, although
simpler to manage, may not remove a significant portion of the phos-
phate, which is in the poly and organic form. Substantial hydrolysis of
these forms may occur in the septic tank, however, producing the ortho-
form. Thus, precipitation following the septic tank may achieve higher
overall removals of total phosphorus.
Performance is dependent on the point of chemical addition, chemical
dosage, wastewater characteristics, and coagulation and sedimentation
facilities. Dose-performance relationships must be obtained through
experimentation, but one should expect phosphorus removals between 75
and 90%. Improvement in this performance may be achieved if intermit-
tent sand filters follow the precipitation/sedimentation process. Side
benefits are achieved with the addition of the precipitating chemicals.
Suspended and colloidal BOD and solids will be carried down with the
precipitate, producing a higher quality effluent than would otherwise be
expected.
Chemical precipitation of wastewaters generates more sludge than do
conventional systems due to both the insoluble end product of the added
chemical and the excess suspended and colloidal matter carried down with
it. Estimates of this increased quantity are very crude at this time,
but may range from 200 to 300% by weight in excess of the sludge nor-
mally produced from a septic tank system.
a. Process Features
The chemicals most often used for phosphate precipitation are aluminum
and iron compounds. Calcium salts may also be used, but require pH ad-
justment prior to final discharge to the environment. Aluminum is gen-
erally added as alum (A12S04'n H20). Ferric chloride and ferric sulfate
are the most commonly used iron salts.
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