Sludge Treatment and Disposal 217Designing a sludge thickener involves choosing a desired sludge solids concen-
tration, C, (estimated in laboratory tests), and drawing a line from this value tangent
to the underside of the flux curve. The intercept at the ordinate is the limiting flux, GL,
or the solids flux that controls the thickening operation. In a continuous thickener it is
impossible to pass more solids than GL (at the stated concentration) through a unit area.
The units of solids flux are kg/m2-h (or other comparable units), and are defined as
G=- Qoco
A’where
G = solids flux
Qo = interflow rate (m3/h),
Co = interflow solids concentration (mgL), and
A = surface area of thickener (m2).If G is the limiting flux, the necessary thickener area can be calculated as
QoCo
GLAL = -,
(10.2)(10.3)where the subscript L specifies the limiting area and flux.
This graphical procedure may be used to develop an optimal process. If the calcu-
lated thickener area is too large, a new, less ambitious underflow solids concentration
may be selected and a new required area calculated.
EXAMPLE 10.3. What would be the required area for a thickener if in Fig. 10-8 C, was
25,000 mg/L, GL was 3 kg/m2-h, and the feed was 60 m3/h of sludge with 1% solids?
The limiting area is calculated as
QoCo (60m3/h)(0.01)(1000kg/m3)
AL=-- - = 200m2.Suppose the C, was 40,000 mgL and the GL was 1.8 kg/m2-day. Now what is the
required area?
GL 3 kg/m2-h= 333 m2.
(60 m3/h)(0.01)(l~0 kg/m3)
1.8 kg/m*-hAL =
Note that the area requirement increases as the desired underflow concentration
increases. In the absence of laboratory data, thickeners are designed on the basis of
solids loading, which is another way of expressing the limiting flux. Gravity thickener
design loadings for some specific sludges are shown in Table 10-2.
A flotation thickener, shown in Fig. 10-9, operates by forcing air under pressure to
dissolve in the return flow, releasing the pressure as the return is mixed with the feed.