Environmental Engineering FOURTH EDITION

Sludge Treatment and Disposal 215

Feed

Underflow

Solids concentration -

Figure 10-6. Gravity thickener. (Photo courtesy of Dorr Oliver Inc.)

severalfold. Further, for high levels of suspended solids, the SVI reaches a maximum
value, above which no additional settling will occur. Take, for example, a lime sludge
with a suspended solids concentration of 40,000 mgL and suppose that it does not
settle at all (the sludge volume after 30 min is 1000 mL). Calculating the SVI gives us

= 25.

(1000) (1000)

SVI =

40,000

How can a sludge that does not settle at all have an SVI of 25, when we originally

noted that an SVI of less than 100 indicated a good settling sludge?