TM 5-814-3/AFM 88-11, Volume III
e. Mineral selection and dosages.
(1) Cost and availability. The choice of minerals should be based on cost of materials, availability of
materials, and process performance. Costs and availability will be determined for each particular situation.
Aluminum and iron salts are in the same cost range, with aluminum salts somewhat more expensive.
(2) Side effects of various minerals. Both aluminum and iron additives will produce soluble side
products in the form of chlorides, sulfates, and sodium compounds, as well as some free acids and alkalis due
to hydrolysis. These side products are seldom a serious problem, however, and can be controlled by adding
only the proper amount of chemical and through the use of the automatic monitoring instrumentation. Alum
and FeCl3 are the most commonly used mineral salts, and both cause an alkalinity drop which can lower pH
if the buffering capacity is not adequate. Iron tends to yield higher effluent residuals (around 6 milligrams per
liter as Fe) than aluminum (less than 0.5 milligrams per liter as Al). Aluminum addition produces 30 to 50
percent less additional solids than iron additives, and the sludge has greater dewaterability and sludge density.
Aluminate and alum produce about the same amount of sludge but the aluminate sludge is considerably less
dense. Alum addition tends to produce better effluent clarity than does aluminate.
(3) Determination of dosage. Mineral dosages will be determined by the weight ratios and, when
applicable, by pilot plant and laboratory studies and full-scale plant test runs. Alum dosages are usually in the
range of 150 to 250 milligrams per liter as Al2(SO4)3.18H2O for an average influent phosphorus concentration
of about 10 milligrams per liter. Typical iron dosages range from 10 to 25 milligrams per liter as Fe. These
typical dosages will accomplish 80 to 90 percent phosphorus removal. Greater phosphorus removal (down
to residuals less than 0.5 mg/L as P) can be obtained by using multi-media filtration techniques. Refer to the
EPA Process Design Manual for Phosphorous Removal.
f. Lime treatment.
(1) Process description and conditions. Lime addition accomplishes phosphorus removal through the
chemical precipitation of hydroxyapatite (Ca5OH(PO4)3). Although the solubility product theoretically
determines the amount left in solution, the actual determining factor in removal efficiency is the efficiency
of the clarifiers. The precipitation of phosphorus by lime requires a high alkalinity, with a pH of 11 being
optimum. Therefore, the lime dosage is not a function of phosphate concentration, but rather it depends on
the amount of lime necessary to attain the proper pH. This in turn is largely dependent on influent wastewater
alkalinity, which is illustrated in figure 15-2. Typically, a lime dosage of as much as 400 milligrams per liter
as CaO will be necessary to attain a pH of 11.
15-14
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