TM 5-814-3/AFM 88-11, Volume III
appropriate locations such as manholes, Parshall flumes or pre-aeration tanks. For maximum phosphorus
removal, the mineral addition will be downstream of return streams such as digester supernatant. The required
procedure for mineral addition at the primary stage is as follows:
--
Add mineral to raw sewage and mix thoroughly;
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Add alkali (if necessary for pH adjustment) 10 seconds later;
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Allow reaction for at least 10 minutes;
--
Add anionic polymer and flash mix for 20 to 60 seconds;
--
Provide mechanical or air flocculation for 1 to 5 minutes; and
--
Deliver flocculated wastewater to sedimentation units.
The advantages of removing phosphorus at the primary stage are the flexibility of chemical feeding, the
adequate detention times and mixing conditions available, the reduced suspended solids and biochemical
oxygen demand loading on the ensuing secondary treatment stage, and the ease of process instrumentation.
The principal disadvantage is that a significant portion of the phosphates is not in the orthophosphate form
at the primary treatment stage and therefore does not precipitate easily. Mineral addition also causes
increased solids production (1 .5 to 2.0 times the weight of normal primary solids), and the solids density
increases with increasing aluminum dosages. Solids increases attributable to aluminum addition are about 4
pounds per pound of aluminum added, up to stoichiometric proportions, after which the weight gain is less.
The addition of alum to the primary stage generates large quantities of metal hydroxide sludge, which is
difficult to dewater. Therefore, alum addition to the primary clarifier shall be implemented with prior approval
from HQDA (CEEC-EB) WASH DC 20314-1000 for Army projects and HQ USAF/LEEE WASH DC
20332 for Air Froce projects.
(3) Addition at secondary treatment stage. The advantages of mineral addition to the activated
sludge process are enhanced sludge removal properties, shorter residence times, more effective phosphorus
removal because of sludge recycle, relatively small additional solids production (which improves sludge
density and dewaterability), and flexibility to changing conditions. The extra solids production, however, does
involve additional sludge handling just as when alum is added at the primary stage. Therefore, alum addition
to the aeration basin shall only be implemented with prior approval from HQDA (CEEC-EB) WASH DC
20314-1000 for Army projects and HQ USAF/LEEE WASH DC 20332 for Air Force projects. In the
activated sludge process, the chemical is added near the discharge point(s) into the aeration basin(s). Mixing
of the chemical and wastewater must occur in the basin but premature precipitation must be prevented.
Phosphorus capture will occur primarily in the sedimentation units following aeration in the basin. For
trickling filter plants, it has been demonstrated that precipitation in the final clarifier can be both effective and
controllable, as per EPA Manual 670/2-73-060. Orthophosphate predominates at this point and it precipitates
readily; also, biodegradable detergents, which can interfere with precipitation, are largely absent. If underfiow
solids from a dosed final clarifier are returned to the primary clarifier, it will result in unusually effective
clarification there. If the operation is not done effectively, poorly treated effluent can escape into the receiving
water. In order to allow for incorporating the advantages of chemical treatment in both the primary and
secondary clarifiers, provisions for future installa-tion of chemical addition and mixing facilities will be
provided at the primary clarifier. Facilities or provisions for mineral addition to both primary and secondary
clarifiers will provide flexibility of operation. Chemical treatment in both clarifiers can then be used if
experience shows it to be the most effective technique at the particular plant involved. Initially, equipment
will be provided only at the secondary clarifier unless bench studies or pilot plant operations show a more
effective performance at another location or show the necessity of chemical feed at the primary clarifier also.
(a) pH. The optimum pH for precipitation by aluminum is about 6.0, which agrees with the operating
pH range for activated sludge processes. While this makes phosphorus removal during activated sludge treat-
ment very effective, it interferes slightly with nitrification, which has an optimum pH range of 7.0 to 8.0.
When both nitrification and phosphorus removal are desired to be accomplished in a single process such as
extended aeration, nutrient removal is effectively accomplished at a pH of about 7.0.
15-12
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