Basic Biological Processes
3.4.2 Alkalinity
The nitrification process reduces the alkalinity in water. Expressions (3.26) and
(3.27) show that it is only the first step of the process which influences the alkalinity.
For every mole of NH!-N which is oxidized to N02-N, approx. 2 moles of HC03
are consumed, and this corresponds to 2 equivalents of alkalinity. This is essential
for the nitrification of relatively soft water where the pH in the water can be so low
that the nitrification process is limited or stops completely.
Example 3.6
Typical domestic wastewater contains about 50 g TN/m 3 . Part of this ends up in the sludge. The rest might be 30 g N/m 3 which can be nitrified.
The alkalinity consumption, a TAL, for this purpose is
30g Nlm 3
3
14 g N/mol N · 2 eqv/mol N = 4.29 eqv/m
If the wastewater therefore has an alkalinity which is lower than approx. 5 eqv/m 3 ,
which many wastewaters have, see Chapter 1, it may be necessary to increase the alkalinity (for example with lime or through denitrification - see Section 3.5).
3.4.3 Kinetics, nitrification
Kinetics can be described by a Monod expression as shown in Expression (3.14). The
saturation constants, Ks,NH4,Aand Ks,N02,A are small- see Table 3.10. For practical
calculations in treatment plants, we can consider the nitrification process as a
one-step process and use the reaction constants for the total process which are
shown in Table 3.10.
3.4.4 The influence of the environmental factors on
nitrification
A number' of environmental factors influencing the nitrification are discussed below. These are:
- substrate concentration
- temperature
-oxygen
-pH
- toxic substances
It is important to note that in practice it is the oxidation of ammonium which is the
rate limiting step in the overall process. This means that nitrite will only appear in
79
3.4.2 Alkalinity
The nitrification process reduces the alkalinity in water. Expressions (3.26) and
(3.27) show that it is only the first step of the process which influences the alkalinity.
For every mole of NH!-N which is oxidized to N02-N, approx. 2 moles of HC03
are consumed, and this corresponds to 2 equivalents of alkalinity. This is essential
for the nitrification of relatively soft water where the pH in the water can be so low
that the nitrification process is limited or stops completely.
Example 3.6
Typical domestic wastewater contains about 50 g TN/m 3 . Part of this ends up in the sludge. The rest might be 30 g N/m 3 which can be nitrified.
The alkalinity consumption, a TAL, for this purpose is
30g Nlm 3
3
14 g N/mol N · 2 eqv/mol N = 4.29 eqv/m
If the wastewater therefore has an alkalinity which is lower than approx. 5 eqv/m 3 ,
which many wastewaters have, see Chapter 1, it may be necessary to increase the alkalinity (for example with lime or through denitrification - see Section 3.5).
3.4.3 Kinetics, nitrification
Kinetics can be described by a Monod expression as shown in Expression (3.14). The
saturation constants, Ks,NH4,Aand Ks,N02,A are small- see Table 3.10. For practical
calculations in treatment plants, we can consider the nitrification process as a
one-step process and use the reaction constants for the total process which are
shown in Table 3.10.
3.4.4 The influence of the environmental factors on
nitrification
A number' of environmental factors influencing the nitrification are discussed below. These are:
- substrate concentration
- temperature
-oxygen
-pH
- toxic substances
It is important to note that in practice it is the oxidation of ammonium which is the
rate limiting step in the overall process. This means that nitrite will only appear in
79