BIOLOGICAL TREATMENT OF WASTEWATER 143
solid, liquid or gaseous. The application of biological pro-
cesses in degradation of toxic organic substances is becom-
ing popular because (i) these have an economical advantage
over other treatment methods; (ii) toxic substances have
started appearing even in municipal wastewater treatment
plants normally designed for treating nontoxic substrates;
and (iii) biological treatment systems have shown a resil-
iency and diversity which makes them capable of degrad-
ing many of the toxic organic compounds produced by the
industries.^24 Grady believes that most biological treatment
systems are remarkably robust and have a large capacity for
degrading toxic and hazardous materials.^25 The bacteria and
fungi have been used primarily in treating petroleum-derived
wastes, solvents, wood preserving chemicals and coal tar
wastes. The capability of any biological treatment system is
strongly influenced by its physical configuration.
As mentioned previously, the Michelis–Menten or
Monond equation, Eq. 5, has been used successfully to
model the substrate degradation and microbial growth in
biological wastewater treatment process. However, in the
presence of a toxic substance, which may act as an inhibi-
tor to the normal biological activity, this equation has to be
modified. The Haldane equation is generally accepted to be
quite valid to describe inhibitory substrate reactions during
the nitrification processes, anaerobic digestion, and treat-
ment of phenolic wastewaters. 24,26,27Haldane Equation
maxS
SKSK^2 i(18)where K i is the inhibition constant.
In the above equation, a smaller value for K i indicates
a greater inhibition. The difference between the two kineticequations, Monod and Haldane, is shown in Figure 9, in
which the specific growth rate, , is plotted for various sub-strate concentrations, S. The values for (^) max , K s and K i are
assumed to be 0.5 h– 1 , 50 mg/L and 100 mg/L, respectively.
Behavior of Biological Processes
The behavior of a biological treatment process, when sub-
jected to a toxic substance, can be evaluated in three parts:
- Is the pollutant concentration inhibitory or toxic
to the process? How does it affect the biodegrada-
tion rate of other pollutants? - Is the pollutant concentration in process effluent
reduced to acceptable level? Is there a production
of toxic by-products? - Is there an accumulation of toxic substances in the
sludge?
The above information should be collected on biological
systems that have been acclimated to the concerned toxic
substances. Pitter^28 and Adam et al.^29 have described the
acclimation procedures.
Generally, biological processes are most cost-effective
methods to treat wastes containing organic contaminants.
However, if toxic substances are present in influents, certain
pretreatment may be used to lower the levels of these con-
taminants to threshold concentrations tolerated by acclimated
microorganisms present in these processes. Equalization of
toxic load is an important way to maintain a uniform influ-
ent and reduce the shock load to the process. Also, various
physical/chemical methods are available to dilute, neutralize
and detoxicate these chemicals.FIGURE 9 Change of specific growth rate with substrate concentration
(inhibited and uninhibited).0 100 200 300 400
SUBSTRATE CONCENTRATION, S,mg/LHALDANE EQUATIONMONOD EQUATION0.1
0.2
0.3
0.4
0.5
SPECIFIC GROWTH RATE,
m
,
h
-^1
C002_001_r03.indd 143C002_001_r03.indd 143 11/18/2005 10:15:50 AM11/18/2005 10:15:50 AM