Page 338 -

LIME sOFrENING 11.17

Residuals Removal and Handling
Residuals from softening reactions are dense when compared with coagulation residuals,
with solids concentrations varying from 3% to 15% or greater by weight. Straight soft-
ening processes removing only calcium carbonate produce the highest-density residuals.
Residuals from excess lime processes are generally less dense because of the influence
of magnesium hydroxide, which is lighter and more gelatinous. For excess lime soften-
ing, an average residuals blowdown solids concentration of 5% by weight is often used
as a rough guideline. For split-treatment softening with coagulation of the bypass flow,
design consideration must be given as to whether to combine residuals from softening and
coagulation units.
Residual scrapers for softening basins require higher-torque units than for conventional
treatment basins because of the high density of residuals. The design of the load on scraper
mechanisms depends on the type of softening and the stage of the process. Equipment man-
ufacturers and experience with similar treatment facilities should be consulted for guidance.
Residuals blowdown from softening basins is usually controlled by a repeat cycle timer
that allows periodic, timed blowdowns at full pipe flow. Backflushing piping with clear
water after each blowdown must be provided to reduce the possibility of lines clogging
from settled residuals. Provisions for sampling the residuals blowdown should be included.
Some facilities have residuals blowdown viewing pits where the residuals can be observed
and sampled.
Residuals pumps for recirculation or disposal are typically solids handling centrifugal
units, although positive displacement pumping units may be required for extremely dense
residuals. Recirculation pumps should be provided with variable-speed drives.

Residuals Production

The quantity of residuals produced by the softening process can be estimated by per-
forming a mass balance on the residuals-producing calcium and magnesium, which rep-
resent the hardness removed in the process. One or both of the substances are present as
hardness in the water entering the plant. In addition, calcium is added to the plant flow
by the addition of lime.
Some hardness also leaves the plant in the finished water. The rest of the hardness
leaving the plant is in the residuals produced. The residuals are in the form of calcium
carbonate for the calcium hardness removed and in the form of magnesium hydroxide for
the magnesium hardness removed.
The calcium balance is as follows:
Dry weight CaCO3 residuals (lb/mil gal) = 20.9
x [(Ca in) + (Ca added by lime)
- (Ca out), mg/L] (11.22)
where Ca is calcium in milligrams per liter as calcium (calcium carbonate equivalent di-
vided by 2.5).
For quicklime (CaO), Ca added by lime is 0.71 x CaO (mg/L) x percent purity/100:
For hydrated lime [Ca(OH)2], Ca added by lime is 0.54 x Ca(OH)2 in mg/L × percent
purity/100.
The magnesium balance is
Dry weight Mg(OH)2 residuals (lb/mit gal) = 20.0
x [(Mg in) - (Mg out), mg/L] (11.23)