Encyclopedia of Environmental Science and Engineering, Volume I and II

142 BIOLOGICAL TREATMENT OF WASTEWATER

growth of the culture unaffected by the toxic substance. In

some cases, such as cyanide and phenol, the toxic substances

may be used as substrate. Rates of acclimation to lethal fac-

tors vary greatly. Thus, the toxicity to microorganisms may

result due to excess concentrations of substrate itself, the

presence of inhibiting substances or factors in the environ-

ment and/or the production of toxic by-products. 19 – 23

The influence of a toxicant on microorganisms depends

not only on its concentration in water, but also on its rate

of absorption, its distribution, binding or localization in the

cell, inactivation through biotransformation and ultimate

excretion. The biotransformations may be synthetic or non-

synthetic. The nonsynthetic transformations involve oxida-

tion, reduction or hydrolysis. The synthetic transformation

involve the coupling of a toxicant or its metabolite with a

carbohydrate, an amino acid, or a derivative of one of these.

According to Warren^19 , the additive interaction of two toxic

substances of equal toxicity, mixed in different proportions,

may show combined toxicity as shown in Figure 8. The com-

bined effects may be supra-additive, infra-additive, no inter-

action or antagonism. The relative toxicity of the mixture is

measured as the reciprocal of median tolerance limit.

Many wastewater constituents are toxic to microorgan-

isms. A fundamental axiom of toxicity states that all com-

pounds are toxic if given to a text organism at a sufficiently

high dose. By definition, the compounds that exert a delete-

rious influence on the living microorganisms in a biological

treatment unit are said to be toxic to those microorganisms.

At high concentrations, these substances kill the microbes

whereas at sublethal concentrations, the activity of microbes

is reduced. The toxic substances may be present in the influent

stream or may be produced due to antagonistic interactions.

Biological treatment is fast becoming a preferred option

for treating toxic organic and inorganic wastes in any form;

SUPRA-ADDITIVE INTERACTION

STRICTLY ADDITIVE INTERACTION

INFRA-ADDITIVE INTERACTION

NO INTERACTION

ANTAGONISM

SOLUTION COMBINATIONS

RELATIVE TOXICITY, 1/

TL

m

SOL. B

SOL. A

0

0

25

75

50

50

25

75

100

100

FIGURE 8 Possible kinds of interactions between two hypothetical toxicants, A and B.

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