The Influence of pH on the Nitrification Rate

In the literature, the optimum pH value forthe nitrification process varies between 8 and 9. Figure 3.7 summarizes investigations of pH effects on the nitrification rate. Usually the nitrification rate decreases, as the pH decreases. By measuring the nitrification rates Meyerhof (1916) found the pH optimum for Nitrosomonas to be between 8,5 and 8,8, and for Nitrobacter to be 8,3 to 9,3.

Hofman et al. (1973) made similar investigations, and found for both organisms an optimum pH of 8.3, and that the nitrification rate fell almost to zero at pH 9,6. They also found that nitrification proceeded with considerable speed until the pH was as low as 6,5. Hofman et al. (1973) further reported that the optimum pH for nitrite oxidation by Nitrobacter was 7,7 and not 8,8 as found by Mayerhof (1915). Wild et al. (1964) suggested the optimum pH for nitrification to be 8,4 and that 90 per cent of the maximum nitrification rate occurs between pH 7,8 and 8,9. Less than 50 per cent of the optimum rate occurs outside the range of pH 7,0 to 9,8. Painter (1972) reported that the point at which the rate of nitrification decreased was between pH 6,3 and 6,7, and that between pH 5 and 5,5, nitrification ceased.

Anthonisen (1974) suggested the following mechanism by which pH affects the rate of nitrification. His hypothesis is based on the fact that the ammonia/ammonium and nitrite/nitrous acid equilibria depend on pH. Both "free ammonia" NH3 and "free nitrous acid" HN02 inhibit the nitrifying organisms. When the intracellular pH of a nitrifying organism is lower than the pH of the extracellular environment, free ammonia will penetrate the cell membrane, and inhibit the bacteria.

Nitrification Free Ammonia Inhibition
Figure 3.7 The influence of pH on the nitrification process.

Ionized ammonia NH4+, on the other hand, will remain in the extracellular environment. Similarly, when intracellular pH is higher than that of the extracellular environment, free nitrous acid penetrates the cell, not the nitrite ions. Anthonisen proposed, therefore, that the ability of ammonia and nitrous acid to penetrate the nitrifying organisms was one of the reasons why the nitrification process is less affected at pH values between 8 and 9.

Equation (3.3) shows that H+ is produced by the oxidation of ammonia and carbon dioxide. When the biomass synthesis is neglected, it can be calculated that 7,14 mg of alkalinity, as CaC03, is destroyed per mg of ammonia nitrogen oxidized. Experimentally determined ratios are presented in Table 3.13. A ratio of 7,1 mg alkalinity (as CaC03) destroyed per mg of ammonia nitrogen oxidized may be used theoretically in plant design.

As the nitrification process reduces the HC03" level and increases the H2C03 level, it is obvious that the pH would tend to be decreased. This effect is mediated by stripping of carbon dioxide from the liquid by aeration, and the pH is therefore often raised. If the carbon dioxide is not stripped from the liquid, the pH may be depressed to as low as 6,0. Haug etal. (1974) calculated that to maintain thepH greater than 6,0 the alkalinity of the waste water must be 10 times higher than the amount of ammonium nitrified.

It is important to distinguish between long-term and short-term pH effects on the environment where the nitrification process is to occur.

There is a great difference in the effects that can be observed in the nitrification process, if pH varies over short (hours, days) or long periods (months, years). Most investigations referred to in this text have been on a short-term basis. Investigations of long-term effects have not been described in the literature.

Table 3.13. Alkalinity destruction ratios in experimental studies

System

X mg alkalinity destroyed mg NH* -N oxidized

Reference

Suspended growth

6,4

Mulbager et al. (1971)

Suspended growth

6,1

Horstkotte et al.{1973)

Suspended growth

7,1

Newton etal. (1973)

Attached growth

6,5

Gasser et al. (1974)

Attched growth

6,3 to 7,4

Osborn etal. (1965)

Attached growth

7,3

Haug etal. (1972)

as CaC03, the theoretical value is 7.1 From EPA (1975).

The hydrogen ion concentration (pH) has been found to have a strong effect on the rate of nitrification. There is a wide range in reported pH optima; the almost universal finding is that, as the pH moves into the acid range, the rate of ammonia oxidation declines. This has been found to be true for both unacclimatized and acclimatized cultures, although acclimation tends to moderate pH effects.

Downing et al. (1966) showed that the effect of pH on nitrification for pH values less than 7,2 can be estimated from the following relationship:

This expression was developed for combined carbon oxidation-nitrification systems, but its application to separate stage nitrification systems would appear useful. For pH levels between 7,2 and 8,0 the rate is assumed constant.

Table 3.14 Effect of pH on the nitrification.

Degree of inhibition %

Circumttancet of obtervation

Reference

0 100 100

0 50

Pure culture of Nitrosomonas

Pure culture, testtube scale

Pure culture of Nitrosomonas

Pure culture of Nitrosomonas

Pure culture of Nitrobacter

Pure culture of Nitrosomonas

Batch Culture

Pure culture of Nitrobacter

Pure culture of Nitrosomonas isolated from activated sludge.

Submerged filter, mixed but predominantly nitrifying bacteria.

Mixed culture; lab. scale

Two-stage, activated sludge pilot plant.

Mayertiof (1917)

Barritt (1933)

Buswell etal. (1954) Lees (1954) Lees (1954)

Engel & Alexander (1958)

Engel & Alexander (1958) Boon & Landelout (1962)

Loveless & Painter (1968) Haug & McCarty (1972)

Praksam & Loehr (1972)

Rimer & Woodward (1972)

Table 3.14 (continued)

8,0-8.8 0 Batch activated Medcalf & Eddy (1973)

7,1 50 sludge; lab. study

8,0 0 Percolating filter Forster (1974)

5.9 50 lab. scale mixed population.

7,45 0 Marine nitrifying Sma & Baggaley (1975)

filter system; batch studies

7,8 0 Simultaneous nltri- Halling-Ssrensen &

fication and de- Hjuler (1992)

nitrification attached growth UFBR.

(*):Adaptation in 10 days, the rate of ammonia oxidation becomes the same as that at pH 7-8,5. (+): pH not controlled, nitrification occured at pH 4.9; no improvement between pH 5 and 11.

Because of the effect of pH on the nitrification rate (see Fig. 3.8), it is especially important that there be sufficient alkalinity in the waste water to balance the acid produced by nitrification. Addition of alkalinity to the waste water may be necessary.

Boon and Laudelout (1962) developed a kinetic expression for the effect of pH on the nitrite oxidation by Nitrobacter winogradskyi. They suggested that inhibition of high nitrite concentration results from non-competitive inhibition of nitrous acid, while at pH over 7 there is a competitive inhibition of the adsorption of nitrite on the enzyme sites by OH" -ions.

The rate equations for pH below 7 and pH above 7 are shown separately in equations (3.33) and (3.34) respectively.

where:

S = nitrite concentration.

Ka = equilibrium constant of nitrous acid and nitrite ion dissociation.

K| = dissociation constant of the enzyme-nitrous acid complex.

Kb = basic acid-base dissociation constant of the active enzyme site.

The total rate equation for pH effects was thus determined by combining equations (3.33) and (3.34) as in equation (3.35).

Results showed that Kb and K, were 0,004 and 8,2 |iM of N02", respectively.

Suzuki et at. (1974), using the Lineweaver-Burke plot, in the study of the pH effect on the oxidation of ammonia by Nitrosomonas europaea, found that the value of the Monod saturation ammonia constant decreased when pH increased. This means that having pH as the parameter, the plot shows competitive inhibition.

As Nitrosomonas and Nitrobacter are both sensitive to their own substrates of unionized ammonia and nitrite, and the unionized-ionized nitrogen equilibria depend on pH, it follows that the pH value is an important factor.

• Engel and Alexander (1959)

% of Maximum Oxidation Rate

% of Maximum Oxidation Rate

10.0

Figure 3.8 The influence of pH on the nitrification rate. A summary of different results found in the literature. Source: Shammas (1986).

10.0

Figure 3.8 The influence of pH on the nitrification rate. A summary of different results found in the literature. Source: Shammas (1986).

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