Aerobic heterotrophic conversion of organic matter
Toxic substances
Many substances act toxically on aerobic conversions even if aerobic processes in
treatment plants are rather robust. Complex formation, chemical precipitations
(sulphide precipitates metals) and biodegradation (cyanide, phenols etc.) make it
difficult to evaluate the toxic effects. Tests should be performed using the wastewater
concerned to obtain a reasonable possibility to evaluate the effects. For more detailed
information about the degradation and effect of toxic substances, see for example /36 I.
The mechanisms of inhibition include competitive and non-competitive reversible
inhibition /8/.
Competitive reversible inhibition influences the kinetics of growth by increasing the
saturation constant, Ks,
Ks1+ C1
Ks· = Ks . ---''':--Ks,I
where Ks'
is the saturation constant with inhibition,
Ks
is the saturation constant without inhibition,
Ks,I is the inhibition constant,
C1
is the concentration of inhibiting substance.
(3.19)
Non-competitive reversible inhibition influences the kinetics of growth by reducing
the maximum specific growth rate
Ks,1
llmax' = llmax · --=;.._
Ks,I+ CI
where llmax' is max. specific growth rate with inhibition
llmax is max. specific growth rate without inhibition
Nitrogen and phosphorus
(3.20)
Their effect on aerobic processes can be described in a double Monod expression
which conceals that the microbial growth is inhibited when the concentrations of
nitrogen and phosphorus are low
SNH4
Sp04
SNH4 + Ks,NH4 SPOI + Ks,POI
where SNH4 is the ammonia concentration
74
Spo4 is the phosphorus concentration (orthophosphate)
Ks,NH4 is the saturation constant, nitrogen
Ks,P04 is the saturation constant, phosphorus
(3.21)