Environmental Engineering FOURTH EDITION

Wastewater Treatment 195

where

X, = the expected return suspended solids concentration (in m&),

X = MLSS (ma),

H = height of cylinder (m), and

h = height of settled sludge (m).

The mixed liquor solids concentration is a combination of the return solids diluted by

the influent, or

X= Qtxt + QXO

Qt+Q.

If we again assume no solid is in the influent, or Xo = 0, then

X=- Qtxt

Qt+Q'

(9.34)

(9.35)

The performance of an activated sludge system often depends on the performance

of the final clarifier. If this settling tank cannot achieve the required return sludge

solids the MLSS will drop and the treatment efficiency will be reduced. Final clarifiers

act as settling tanks for flocculent settling and as thickeners. Their design requires

consideration of both solids loading and overfiow rate. Solids loading, more fully

explained in the next chapter, is expressed in terms of kilograms of solids per day

applied to a surface area of square meters. Figure 9-17 shows some commonly used

solids loadings for final clarifiers as a function of the SVI.

Secondary treatment of wastewater usually includes a biological step like acti-

vated sludge, which removes a substantial part of the BOD and the remaining solids.

The typical wastewater that we began with now has the following approximate water

quality:

Raw After primary After secondary

wastewater treatment treatment

BOD (mg/L) 250 175 15

ss (mi&) 220 60 15

p (ma) 8 7 6

The effluent from secondary treatment meets the previously established effluent stan-

dards for BOD and SS. Only phosphorus content remains high. The removal of inor-

ganic compounds, including inorganic phosphorus and nitrogen compounds, requires

advanced or tertiary wastewater treatment.

TERTIARY TREATMENT

Both primary and secondary (biological) treatments are incorporated in conventional

wastewater treatment plants. However, secondary treatment plant effluents often are