TM 5-814-3/AFM 88-11, Volume III
(4) Single-stage lime treatment. The general procedure for single stage treatment is as follows:
(a) Add lime slurry to wastewater as needed to obtain a pH of 9.5 to 10.5 and provide rapid mixing
for about 30 seconds.
(b) Provide flocculation and sedimentation.
(c) If recalcination is used, thicken the sludge to 8-20 percent solids, centrifuge to 30-40 percent
solids, and then recalcine to CaO and Ca(OH)2. This is usually not economical for the typical military-size
installation.
(d) After sedimentation, recarbonate for about 15 minutes to obtain the pH necessary for subsequent
treatment or discharge.
(5) Two-stage lime treatment. For two-stage treatment, the procedure is as follows:
(a) Add lime to wastewater as needed to obtain pH equal to 11-11.5 and provide mixing for about
30 seconds.
(b) Provide flocculation and settling (sludge can be recalcined as in single-stage).
(c) Provide recarbonation for 5 to 15 minutes to obtain a pH of 9.5 to 10.0.
(d) Provide second stage settling (sludge recalcined as required).
(e) Again recarbonate for about 15 minutes to obtain the pH required for following treatment or
discharge.
(6) Effectiveness. Lime addition at the primary treatment stage is effective in removing from 80 to 90
percent of phosphorus. It also reduces biochemical oxygen demand by 50 to 70 percent, suspended solids
by 85 percent, nitrogen by about 25 percent, and coliforms by as much as 99.9 percent. However, the igh pH
necessary for this type of treatment causes difficulties in downstream biological treatment processes.
Neutralization (recarbonation) may be required before biological treatment. Primary lime treatment will
reduce the organic load on the secondary treatment stage and will reduce secondary sludge by almost half;
the primary sludge, however, will increase about threefold. With lime treatment in the primary treatment
stage, recalcination is often impractical because CaCo3 may not precipitate in sufficient quantities. A more
effective and flexible technique is lime addition as a separate stage after secondary treatment. The advantages
of this are the flexibility of operation and the backup function of the secondary system. Either single-stage
or two-stage lime treatment can be used; however, two-stage treatment is preferred becaue it produces a
better clarified effluent, has more lime recovery potential, and provides higher phosphorus removal
efficiencies. The mixing, flocculation and settling units can be separate or integrated units. The integrated-
type unit (upflow clarifier) works well but sludge blanket problems are encountered. Integrated units that
work without sludge blankets and separate units are recommended.
(7) Lime addition treatment schemes. Lime addition in the primary treatment can make use of
existing process units, or separate units can be used. Modifications such as sludge recycle to the flocculation
chamber and polymer addition improve settleability of the sludge and thereby improve phosphorus removal
efficiency. Mineral addition can also be used (after primary treatment with lime) to improve overall
phosphorus removal efficiency. For additional information refer to the EPA Process Design Manual for
Phosphorous Remova.
(8) Performance and dosage criteria. The lime requirement can vary over a wide range, depending
on operating pH and water composition. Alkalinity affects the lime dose, as can calcium hardness. One part
by weight of CaO can react with from 0.89 to 1.79 parts of bicarbonate alkalinity (expressed as CaCo3); the
lower value applying to very soft waters and the higher value to very hard waters. In addition to the reaction
of lime with hardness, other competing reactions occur in lime treatment of wastewater. Also, there may be
incomplete reaction of the lime. These complications make calculation of lime dose difficult; consequently,
determination of lime dose is largely empirical. Some approximate values are given in the U.S. EPA Process
Design Manual for Phosphorous Removal. The lime dose will usually be in the range of 300 to 400
milligrams per liter as CaO for two-stage treatment, and from 150 to 200 milligrams per liter where single-
stage treat-ment is satisfactory.
15-16
Previous Page
