Basic Biological Processes
f(S)
describes the growth kinetics
(for example zero, first order or Monad kinetics)
XB
is the concentration of biomass
(dimension Mx · L- 3 , unit for example kg COD(B)/m 3 or kg SS(B)/m~
The substrate consumption corresponding to the biological growth can be found
from
rv;s = ( rv;xa) /Y max
Y max
is the maximum yield constant
(dimension MxB · Ms- 1 , unit for example kg COD(B)/COD(S) or
kg VSS(B)/kg COD(S))
Monod kinetics are frequently used, and hence the substrate consumption for
growth attains the following description:
llmax
S
rvs= - - · - - · X a
'
Ymax S+Ks
(3.2)
It should be noted that the expression applies in a situation where only the substrate, S, is a limiting factor for the growth. Alternatively, J.Lmax can be seen as the
maximum specific growth rate under given environmental conditions (temperature, pH, oxygen, nutrients, toxic substances). Variations in the environmental
factors may be adjusted as shown in the section about aerobic conversions.
The biomass, Xa, is only a fraction of the organics in the sludge. Figure 3.4 shows
the influence of the sludge load (see chapter 4) on the sludge fractions.
XI
Xs
Xs
Sludge
load:
High
Low
Fig. 3.4 Biomass distribution versus sludge load.
3.2.2 Hydrolysis
The hydrolysis process converts larger molecules into small, directly degradable
molecules. It may be a degradation of both particulate and dissolved solids. The
hydrolysis processes are normally slow compared to the biological growth processes. Hence, in terms of rate of reaction, the hydrolysis will often be the rate
limiting step in biological wastewater processes.
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