TM 5-814-3/AFM 88-11, Volume III
(b) Velocity gradients. The velocity gradient will be determined using equation 15-1.
G values greater than 75 sec-1 will cause some floc disintegration and this is usually exceeded in typical
aeration basins. To achieve better settling and therefore more effective phosphorus removal, gentle mixing
will be provided toward the end of the aeration basin or in a flocculation chamber.
(c) Weight ratio
. For a combined chemical-biological
phosphorus removal system, the weight ratio
of the net volatile solids in the aeration basin to the aluminum added to it must exceed the Al:P weight ratio
(i.e., not less than 3) to prevent the occurance of non-settleable suspended solids in the effluent from the
aeration basin. The more biological solids produced from the system, the greater the aluminum dosage that
can be used without effluent suspended solids problems.
(d) Mineral precipitates. In general, sludges containing mineral precipitates of phosphorus are stable
in sludge digestion and heat treatment. The phosphates, as well as the insoluble hydroxides of excess
minerals, do not resolubilize and they have no detrimental effects on the digestion process.
(4) Addition at final settling basin. At the final settling basin, phosphorus removal is very effective
because most of the soluble phosphates are in the orthophosphate form, which is the easiest to precipitate.
The general procedure for mineral addition is essentially the same as in the primary stage. A surface overflow
rate of 500 gallons per day per square foot should be used to size the final settling basin.
c. Mineral addition using iron.
(1) Iron requirements. The theoretical requirement for iron in phosphorus precipitation in terms of
mole ratio of iron to phosphorus (Fe:P) is 1:1 for the ferric ion and 3:2 for the ferrous ion. Actual plant
results indicate that the mole ratio for the ferrous ion is closer to 1:1. With the same mole ratio for ferrous
and ferric ions, the weight ratio (Fe:P) is 1.8:1. As with aluminum, however, experience has indicated that
the weight ratios are higher. The optimum pH range for ferric iron precipitation of phosphorus is 4.5 to 5.0,
and for ferrous iron about 8.0. Ferrous salts cause a lowering of pH and may necessitate addition of alkali;
however, alkali addition is not necessary when there is a subsequent aeration step.
(2) Effectiveness. Addition of ferric forms of iron tends to yield a fine, light floc which does not settle
well, but subsequent addition of lime and/or a polymer aids flocculation and settling. Ferrous iron addition
may present residual problems in that excess ferrous ions may not hydrolyze and settle out at a pH lower than
8.0; lime addition will raise the pH and alleviate this problem. Ferrous salts yield good results when oxygen
is available, such as in the activated sludge process. Ferric and ferrous iron addition, together with lime or
polymer flocculation aids, is particularly applicable to primary treatment without a subsequent activated
sludge step because there is little effluent floc carryover. However; for military installations, it is preferred
that chemical addition follow the biological reactor.
d. Mineral addition treatment schemes. Pilot plant study and full-scale plant operation will determine
the most effective and practical treatment scheme for a particular situation. This most often involves multiple-
point chemical addition with recycle of mineral sludges. In the case of trickling filter plants, mineral addition,
with a split of about 20 percent at the primary stage and 80 percent at final clarification with sludge recycle
from the final settler to the primary settler; provides very effective phosphorus removal and good clarification.
When removal requirements permit 5 milligrams per liter or more of phosphorus in the effluent stream,
required treatment will follow the trickling filter. For very high phosphorus removal efficiencies, multi-media
filtration is added after secondary settling (see a. above).