Nitrifiers are slow-growing organisms and they are accordingly particularly susceptible to toxicants. Certain heavy metals and organic compounds are toxic to nitrifiers. The presence of toxic compounds causes a change in the environmental conditions for the nitrifying population, and they are therefore, a threat to any nitrification plant.
Tomlinson etal. (1966), however showed that nitrifiers are capable of adapting to almost any toxic substances, when the toxic compound is consistently present at a concentration higher than the concentration of the toxic compound that would cause sludge discharge of the plant. Most toxic compounds in municipal systems stem from industrial dumps or urban storm water inflow.
The possibility of a toxic inhibition must be recognized in every design of nitrification systems. Either implementation of source control programs or inclusion of toxicity removal processes upstream may be required, particularly in cases where significant industrial discharges are tributary to the collection system.
It is therefore important to understand the difference between long-term and short-term toxic inhibition. Figure 3.10 shows the difference in nitrification efficiency, applying a long-term or a short-term inhibition with a toxic substance. This difference is brought about because nitrifying bacteria are capable of developing adaptation to most toxic compunds especially during a long-term contact.
Any inhibition of the nitrification process results in a decrease in the maximum specific reaction rate of the nitrifying organisms. A change in the maximum specific reaction rate can be compensated for by a longer solid retention time in a waste water plant. If we suppose that for a specific plant an SRT (solids retention time) of 8 days were required for an efficient nitrification and carbonaceous removal in a single process; and if, after the plant was built, a new waste flow containing an inhibitory compound were added; and if the maximum specific reaction rate of the nitrifying organisms was reduced by 40%, it would be necessary to increase the SRT to 8 days/0,40 = 12 days. Such a large increase in SRT might not be possible without extensive plant modifications, and when carried out, it might harm the heterotrophic population.
Today, unfortunately only very little is known about the influence of different groups of toxic substances on nitrifiers. Almost nothing is known about the consequen ces, when two or more toxic substances are present at the same time. It is, therefore difficult to predict how a toxic compound or a number of toxic compounds will change the biomass concentration in a plant. Investigators should In future study this field carefully, because it would be of benefit to and facilitate the daily maintenance of any type of nitrifying plant.
Fig 3.10 Differences in nitrifying efficiency, comparing long- and short-term effects of a toxic substance.
The reduction of maximum specific growth rates which results from the effect of environmental parameters on enzyme reactions can be expressed by different models of enzyme inhibition.
An enzyme inhibitor is a compound which acts to reduce the rate of an enzymatically catalysed reaction by binding with either the free enzyme E and/or with the enzyme-substrate complex ES as shown in Table 3.16. Types of enzyme inhibition can be classified (following Grady and Lim 1980) into five groups for reversible inhibitors. Reversible inhibitors are inhibitors where the activity of the enzyme returns to normal, when the inhibitor is removed.
1. Competitive inhibition.
An inhibitor which is classed as competitive competes for the same active sites as the substrate.
An uncompetitive inhibitor binds with the enzyme-substrate complex to form an inactive enzyme substrate-inhibitor complex which cannot undergo further reaction to yield the product.
3. Non-competitive inhibition.
A non-competitive inhibitor can combine with both free enzyme and the enzyme substrate complex.
4. Substrate inhibition.
When their concentrations are very high, some substrates will bind with the enzyme substrate complex as well as with the free enzyme.
5. Product inhibition.
The product may bind with the enzyme substrate complex, forming an unreactive enzyme substrate product complex, ESP.
The mechanisms and inhibition-model of these different types are shown in Table 3.16 and Fig. 3.11. The figures show the inhibition models for competitive, uncompetitive and non-competitive inhibition.
Transforming the Michaelis-Menten expressions into one of the linear equations, i.e. Lineweaver-Burke, makes it easier to quantify the various parameters that are affected by the inhibitior. A specific nmax and Ksn can therefore easily be distinguished for each condition and type of inhibitor.
Krittiya (1984) used the Lineweaver-Burke plot to estimate the effect of sodium ion on the nitrite oxidizing bacteria, as shown in Fig. 3.12. Results showed that the sodium ion inhibiton on the nitrite oxidizing process was categorized as a noncompetitive type and the inhibition constant «¡nhib was 2,0 g/l as Na+.
Visut (1985) made similar experiments with sodium inhibition on ammonium oxidizing bacteria and proposed the following expression for the inhibitory effect of sodium ion on oxidizing bacteria:
H. = specific growth rate, d"1
Sn = ammonium concentration mg/l as N
I = inhibitor concentration g/l as Na+
(imax = maximum specific growth rate
Ks n = saturation constant
Kinhib. = inhibition constant
Visut (1985) found the following experimental values: ^ = 0,0313 h"1, Ks n = 11,6 /13,5 mg/l as N, Kinhib = 6,64 mg/l as Na+ and Kd = 3,1 • 10"3 h"1.
Hockenbury and Grady (1977); Beg et al. (1982) ; Akai et at. (1983) and Hassan et al. (1988) have all used the rate expression for enzyme inhibition in their studies of effects of inhibitors in the nitrification process.
d, e and f are Lineweaver - Burk plots
g. h and i are Hanes plots
g. h and i are Hanes plots
j, k and I are Hofstee plots
Fig 3.11 Typical plots for identifying the types of enzyme inhibition. The solid cu represent the uninhibited cases, the dashed curves the inhibited cases. (Ohgaki and Wanttawin 1990).
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