Environmental Engineering FOURTH EDITION

Wastewater Treatment 189

I I I I I I

0 50 100 150 200 250 300

Solid loading kg/d-rn2

Figure 9-17. Allowable solids loading for final clarifiers increase for better-settling
sludge.

The yield is usually expressed as kilograms of SS produced per kilogram of BOD
used.
The growth rate of microorganisms is

dxldt = pX, (9.13)

where p is a proportionality constant (growth rate constant). Monod6 demonstrated
that for different substrate concentrations, the growth rate constant can be estimated as

p=- fis

Ks + S

and therefore

_- dS X

(9.14)

(9.15)

where

p = growth rate constant (in s-l),

jl = maximum growth rate constant (s-l), and

K, = saturation constant (in mg/L).

The two constants jl and Ks must be evaluated for each substrate and microorganism
culture.
Figure 9-18 illustrates the application of biological process dynamics to the acti-
vated sludge process. The reactor has volume V and effluent flow 0. The reactor is

%etailed discussion of the empirical Monod model of enzyme kinetics is beyond the scope of this
text, but can be found in modem textbooks on biochemistry and on wastewater processing.