Bacterial Population Dynamics Applied in the Nitrification Process

The kinetics of the growth of nitrifiers have been discussed in the previous sections. In all practical applications in waste water treatment, nitrifier growth takes place in waste treatment processes, where other types of biological growth occur. In no case are there opportunities for pure cultures to develop.

This fact has significant implications in process design for nitrification.

In combined carbon oxidation-nitrification systems as well as in separate stage nitrification systems, there is sufficient organic matter in the waste water to enable the growth of heterotrophic bacteria. In this situation, the yield of heterotrophic bacteria growth is greater than the yield of the autotrophic nitrifying bacteria. Because of this dominance of the culture, there is the danger that the growth rate of the heterotrophic organisms will be established at a value exceeding the maximum possible growth rate of the nitrifying organisms. When this occurs, the slower growing nitrifiers will gradually diminish in proportion to the total population, and be washed out of the system.

Because waste water is a mixed culture system, a knowledge of the mutual relationship between nitrifying and heterotrophic bacteria is very important in the construction of nitrifying waste water plants.

Painter (1977) showed that the maximum specific growth of nitrifying bacteria, determined in the treatment process, is significantly different from that observed in a pure culture.

The reasons for this difference may be explained as follows:

1) Domination of heterotrophic bacteria which suppress nitrifying growth, because growth conditions, i.e the COD/N ratio, in the treatment plant enable the growth of heterotrophic bacteria prior to nitrifying bacteria.

2) Because the half saturation constant Ks 0 for heterotrophs is generally lower than that for nitrifiers, heterotrophs will generally compete with the nitrifiers for the available dissolved oxygen.

3) The toxic constituents of waste water may inhibit nitrification.

4) Fluctuation or limitation of nutrients.

5) A genuine difference between isolated strains and those effecting nitrification in the treatment process.

Especially 1) is an important factor in the construction of nitrifying waste water systems. Stover etal. (1976) have presented experimental results showing the effects of different COD/N ratios on nitrification, in both the activated sludge process and in the UFBR, system in both cases applied using non-toxic synthetic media.

The competition for nitrogen by heterotrophs, or inhibition, interferes with the removal of ammonia and reduces the production of nitrate under the conditions of a high COD/N loading. Applying a high COD/N loading also favours the development of a heterotrophic bacteria population and producing a lower nitrifying population.

Christensen and Harremoes (1978) have explanied how it is to be expected that nitrification in the attached growth treatment process, under a high organic carbon loading will not occur in the upper part of the trickling filter, nor on the first disks of a rotating disk unit.

It may be assumed that in the upper layer, the nitrifying population will lose in the competition with the heterotrophic bacteria, and carbonaceous matter only will be removed. In the lower part of the trickling filter and at the last disk unit, the ammo-nium-N loading is now high, compared with the organic loading, and, therefore the heterotrophic bacteria will be suppressed by the nitrifying bacteria. Nitrification will consequently occur there.

A few models have been developed involving the competition between heterotrophic and nitrifying bacteria (Harremoes, 1982; Wanner and Gujer 1984). All of these models, developed recently, have predicted that the fraction of nitrifiers in relation to the heterotrophic population is greater in the inner layer (near the surface of the media) than in the outer layer of biofilm.

There are many types of competition between two or more microbial populations. Competition occurs when the component populations are restricted in either their growth rates or their final population sizes, as a result of a common dependence on an external factor.

Competition can occur in either a closed culture, where growth is ultimately limited by the availability of a particular growth resource, or in an open culture (as a waste water plant), where growth is continuously limited. In open culture systems, as in a waste water plant, it is inevitable that those populations which are the least competitive, are eliminated from the growth environment. In this case the saturation constant Ksn, usually becomes the most important factor determining the outcome of competitive growth.

Figure 3.9 shows different systems with competition between organisms A and B. Organism B is initially a minor population compared to A. The dilution rate of organisms, D is used to predict the washout of organisms from a system plant.

Theoretically, if the growth rate n > D, then ds/dt (the substrate removal per unit of time) is negative and the growth limiting substrate concentration decreases. The biomass concentration is increasing under this condition.

If the growth rate \i < D, then ds/dt is positive and the growth-limiting substrate concentration increases, and the biomass concentration decreases.

Finally, if n = D, then ds/dt = 0, and the growth limiting substrate concentration reaches a constant, steady-state value at the same time as the biomass concentration.

There are two basic cases to consider in assessing whether or not the growth of population B is more or less competitive than that of the established population A, where neither of the two organisms are limited by the substrate.

For the new population B to succeed in becomming greater than population A, dXB/dt from the Monod equation (3.11) has to be positive. This can be achieved, if m-b > D, and pertains if either n.maxB (the maximum growth rate for organism B) > hnax.A (Fi9- 3"9a) or Ks, ,b < (Fig- 3.9b). It must be noted, however, that it is the combined effect of these which is important, in determining whether or not organism B is more competitive than organism A. Fig. 3.9c illustrates the situation in which

but KSi g > Ks A. For this pair of organisms, at any growth-limiting concentration, organism B is the more competitive, sustaining a higher growth rate than organism A at all substrate concentrations.

Initially, the growth rate of organism B is determined by the steady-state conditions established by organism A ; that is at a dilution rate D, the growth limiting substrate concentration sA. Gradually, as the proportion of the two populations begins to change in favour of population B, s begins to decrease and tend towards sA (see

Fig. 3.9a and 3.9b) which is the growth-limiting substrate concentration, which supports a growth rate of nB = D. At this substrate concentration dSA/dt must be negative, and accordingly population A is unable to grow at the imposed dilution rate and must continue to be washed out of the culture vessel.

The opposite situation is that population B does not replace population A, if < D and so dXB/dt is negative, a situation which results if either |imax B < |imaxA (Fig. 3.9d) or Ks B > Ks A (Fig. 3.9e).

Table 3.15 Comparison of parameters of heterotrophs and autotrophs (nitrifier) determining bacterial population dynamics (Frühen et ai 1991).

Parameter Symbol Value

Heterotrophic bacteria maximum growth rate, d"1 Heterotrophic bacteria decay coefficient, d"1 Heterotrophic yield coefficient, g/g"1

Autotrophic bacteria maximum growth rate, d"1 Autotrophic bacteria decay coefficient, d"1 Autotrophic yield coefficient, g/g"1

VH 0,57

Yn 0,24

The parameters presented in Table 3.15 show that both n^^ and Ks for the heterotrophic population favour heterotrophic growth. Supplying a treatment plant with both heterotrophs and nitrifier (autotrophic bacteria), it is therefore important to stock the plant with a high nitrifying biomass Xn, so the nitrifying population initially dominates the plant. A combination of high nitrifier and a limitation of heterotrophic substrate may be necessary.

To establish condition for a consistent nitrification it is therefore important that the specific nitrifier growth nn is higher than the maximum heterotrophic growth assuming pH and DO do not limit the growth of the nitrifier. This can be expressed in the following terms:

KSA Ksb

Fig 3.9 The various possible Monod relationships between two organisms, A and B, used to predict the outcome of free competition between them under conditions of growth limited by the substrate. After Slater and Bull (1978).

where: = maximum growth rate of the nitrifying population.

(ih = growth rate of the herterotrophic population.

Reduced DO or pH can act to depress the growth rate of the peak nitrifier ^max,n and cause a wash out situation. A new growth rate nobs will then be the peak nitrifier growth rate. The Monod Equation for this special condition is presented in EPA 1975:

where: piobs = maximum possible nitrifier growth rate under environmental conditions of T, pH, DO and S» Ks.

To "correct" the calculations for the competition between the nitrifier and the heterotrophic bacteria in the application of biological treatment, Lawrence and McCarty (1968) introduced the concept of a safety factor (SF). A conservative safety factor is recommended to minimize process variation caused by pH extremes, low DO, fluctuation of substrate, and toxicants.

The growth rate can be expressed in reciprocal form in terms of a solid retention time.

where <(>c = solids retention time in days.

DOUBLINGTIME ln2

DOUBLINGTIME ln2

Equation (3.42) is useful from the standpoint of process design.

The safety factor was defined as the ratio of the minimum retention time for solids. The safety factor can also be related to the nitrifier growth rate.

where <j>obs = the minimum retention time for solids in days for nitrification at a given pH, T and DO.

EPA 1975 proposes that the safety factor should equal or exceed the ratio of peak load expected in the suspended growth nitrification system.

Today the safety factor approach is rarely used in the literature, but it is absolutely necessary to use some form of safety factor in designing biological nitrification plants, because the knowledge of the risk of introducing more species of bacteria into the same system is still very limited.

Today, therefore, too many treatment plants still show too many differences in their efficiency of nitrogen removal.

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