216 ENVIRONMENTAL, ENGINEERING
The answer is that the maximum SVI for sludge with 40,000 mg/L of suspended
solids concentration is 25. At 20,000 mg/L the maximum SVI is 50; at 10,OOO mg/L
the maximum SVI is 100; and so on. These maximum values can be plotted as in
Fig. 10-7, which also shows how the SVIs for two different types of sludges might
vary as their solids concentrations increase toward the maximum SVIs.
A better way to describe the settling characteristics of sludge is by using a flux
plot. Flux plots are developed by running a series of settling tests at various solids
concentrations and recording the sludgewater interface height with time. The velocity
of the settling is calculated as the slope of the initial straight portion of the curve and
plotted against the solids concentration. Multiplying a velocity by its corresponding
solids concentration yields the solids flux, which is plotted against the concentration
(see Fig. 10-8). Note that for concentration Ci, the interface settling velocity is vi, and
the solids flux at the concentration is CiVi (kg/m3 x m/h = k@m2 x h). The flux curve
is a “signature” of the sludge thickening characteristics and may be used to design
thickeners.
- Sludge 4
I I I 1 I 1
ob 2000 4000 6000 8000 10,000
Suspended solids concentration (rngL)
Figure 10-7. Maximum values of SVI.Solids fluxc,
kg/tn2-hGLYSolids concentration, mg/L
Figure 10-8. Calculation of limiting solids flux in gravitational thickening.