The Influence of Dissolved Oxygen on the Nitrification Rate

In engineering calculations, an aeration requirement of 4,6 mg 02 per mg NH4+ -N is just sufficient to be used for the nitrification process.

In almost all treatment systems, oxygen is also required to oxidize other materials than ammonia present in the waste water. This, therefore, often raises the total oxygen demand in a nitrifying plant.

Results from a number of studies on the effect of dissolved oxygen concentrations on the nitrification efficiency are summarized in Table 3.12. Most studies were conducted on suspended-growth systems. In the case of attached growth systems, the oxygen availability to the nitrifying biofilm can be affected by many parameters.

The concentration of dissolved oxygen (DO) has a significant effect on the rates of nitrifier growth and nitrification in biological waste treatment systems. The Monod relationship has been used to model the effect of dissolved oxygen, considering oxygen to be a growth limiting substrate, as follows:

where: DO = dissolved oxygen, mg/l and

K 02,n = half-saturation constant for oxygen, mg/l, in the nitrification process.

While the general effect of DO on kinetics is firmly established, further study is needed to determine the factors affecting the value of K 02,n. All of the various estimates are from systems where combined carbon oxidation-nitrification is practiced, and no measurements have been made on separate stage nitrification systems. K 02,n values for separate stage nitrification systems may very well be different from those for combined carbon oxidation-nitrification systems. Most often the operating DO is 2.0 mg/l or less, in studies (see Table 3.12), therefore a value of K 02,n of approximately 1,3 mg/l, will give a nitrification (or nitrifier) growth rate (equation 3.30) of about 60 % of the peak rate, following Downing et al. (1978).

Table 3.12 The influence of dissolved oxygen on the nitrification process.

Dissolved oxygen concentration mg/l


Circumstance/Method ot observation


Below 2

Below 4

Below 3 0,08

0,5-0,7 Saturation 1

Limiting for Nitrosomonas growth (*)

Limiting for Nitrobacter growth

Degree of nitrate about 10% lower at 2 mg/l




Limiting for growth


Critical ("*) Limiting Limiting Limiting

No inhibition no increase in rate of ammonia oxidation

Dropping-mercury method used to measure oxygen uptake

10-1 batches; water from Thames; determination made from a model

Small-scale plant

Activated sludge

Pure culture of

Nitrosocystis oceanus

Activated sludge

Pure culture of

Nitrosocystis oceanus

Activated sludge

Batch tests with activated sludge

Pilot plant; activated sludge

Percolating filter receiving sea water marine nitrifiers

Submerged filter receiving pre-oxygenated waste water

Painter and Jones (1963)

Knowles, Downing & Barrett (1965)

British Ministry of Technology (1965)

Downing & Knowles (1966) Gunderson (1966)

Wuhrmann (1964)

Carlucci & McNally (1969)

Downing and Knowles (1966)

Kiff (1972)

Metcalf & Eddy (1973) Forster (1974)

Haug & McCarty (1972)

(*) Rate of nitrification is the concentration below this value.

(**) Minimum concentration necessary for nitrification to occur.

Dissolved Oxygen Nitrification
Figure 3.6 The influence of dissolved oxygen on the nitrification rate.

Most mathematical models for biological growth take into account only one substrate, such as the Monod model, since experimental studies are usually performed with all other nutrients in excess. But Stenstnam and Poduska (1980) used a double substrate-limiting kinetic expression to describe the combined effect of dissolved oxygen and ammonia-nitrogen on the growth rate, as shown in the following equation. The equation is a modified form of the Monod single substrate model.


Ix = Specific growth rate (d"1)

Hmax = Maximum specific growth rate (d"1)

SN = Ammonia concentration

DO = Dissolved oxygen concentration

Ksn = Half saturation constant for ammonia nitrogen

Kq2 = Half saturation constant for dissolved oxygen

Kd = decay or maintenance coefficient (d"1)

The double substrate-limiting kinetics is interesting, because substrate diffusion through biofilms will result in the limitation of either the electron donors or the electron acceptors in the biochemical reaction.

Typical values of the half saturation constant Kq 2 are shown in Table 3.7 It would appear, looking at Table 3.7 that the activity of Nitrobacter is suppressed under low dissolved oxygen concentrations more than that of Nitrosomonas. Painter (1977) noted that the presence of organic matter can directly inhibit nitrifiers by virtue of heterotrophs oxidizing the compounds and successfully competing for the available dissolved oxygen, if this is kept at a fairly low concentration, as the Ks 0 for heterotrophs is generally lower than that for nitrifiers.

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