Residuals Management - Public Water Systems - page 767

with clay or lime have exhibited thickened concentrations of 3 to 6 percent, and 9
percent, respectively, at higher overflow rates than sludges without clay or lime ad-
dition. 10,39
Lime Sludge Thickening Results. Softening sludge can often be concentrated to
greater than 5 percent solids in the clarifier. Solids loadings of 20 to 40 lb of solids /
sq ft of thickener surface area / day (100 to 200 kg / m 2 / d) are commonly practiced.
Table 23-16 24 shows reported thickening results for lime softening sludges.
Thickening requirements increase if the softening sludge includes magnesium hy-
droxide, turbidity, or coagulants. Bench-scale thickening tests can provide a good es-
timate of sludge-thickening characteristics and design requirements.
Provisions to recycle the underflow to the thickener feed are sometimes provided
to prevent the solids from becoming too thick, causing subsequent handling problems.
In the design of a thickener, storage requirements, particularly when a dewatering
device is used, must be considered, along with the thickener area required to produce
a desired solids underflow.
Flotation Thickening Dissolved air flotation (DAF) thickening is a solids-handling
option for residuals concentrates that consist of low-density particles. Potential benefits
include lower sensitivities to changes in influent solids concentration and solids feed
rate.
Several plants have had success with DAF thickening in concentrating hydroxide
sludge to levels between 3 and 4 percent solids. 40,41 Loading rates for hydroxide
sludges vary from 0.4 lb / ft 2 /hr to 1.0 lb/ft 2 / hr for facilities achieving 2 to 4 percent
float solids concentration. The hydraulic loading of DAF units is reported at less than
2 gpm/ft 2 . 42
Survey of Thickening Methods A 1991 survey of water treatment plants in the
United States, ''Water Industry Data Base (WIDB),'' was developed by the American
Water Works Research Foundation (AWWARF) and the American Water Works As-
sociation (AWWA). The survey, which covered 438 utilities and 347 WTPs, is pre-
sented in Table 23-17 for thickening. 43
Dewatering
Either nonmechanical dewatering, such as lagoons or drying beds, or mechanical de-
watering, such as centrifuges, belt filter presses, or pressure filtration, can be used for
TABLE 23-16. Gravity Thickening Performance of Lime Softening Sludges
Location Solids Input (%) Solids Output (%)
Boca Raton, FL 1-4 28-32
Dayton, OH 2-4 15-25
Lansing, MI 12-16 20-25
Ann Arbor, MI 9 20
Miami, FL 30 40
Cincinnati, OH 5 15
Source: Reference 24. (From Water and Waste Engineering, ''Softener Sludge Disposal—What's Best?'' De-
cember 1974.)