4.3 Nutrient Removal/Recovery from Wastewater
105
4.3.1.1 Biological Methods
– Nitrification–Denitrification
Nitrification is a two-step process with different types of microorganisms. In the
first step, ammonia is converted to nitrite and then to nitrate, which is carried out by
ammonia-oxidizer microorganisms such as Nitrosomonas, Nitrosospira and Nitrosococcus. They take up ammonia and generate a very small amount of energy for their
growth. The nitrite is then oxidized to nitrate by nitrite-oxidizing bacteria including
Nitrospira, Nitrobacter, Nitrococcus and Nitrospinae. Oxidation of nitrite to nitrate
also produces a small amount of energy. The overall nitrification reactions are as
follows [20]:
NH 3 + O 2 + 2e
−
→ NH 2 OH + H 2 O
(4.1)
NH 2 OH + H 2 O → NO
−
2 + 5H
+
+ 4e
−
(4.2)
NO
−
2 + H 2 O → NO
−
3 + 2H
+
+ 2e
−
(4.3)
The nitrification microorganisms require an aerobic environment. They play an
important role in removing the harmful levels of ammonia from the receiving waters
[18].
Denitrification is a process in which the nitrate is converted to nitrogen and other
gaseous products (such as NO and N 2 O) and it is facilitated by a wide range of
microorganisms. Some of the denitrifying microorganisms include Bacillus, Paracoccus and Pseudomonas. The process requires an organic carbon source and anoxic
conditions, and involves a series of denitrification reactions as follows [18]:
NO
−
3 → NO
−
2 → NO → N 2 O → N 2
(4.4)
The overall reaction:
2NO
−
3 + 10e
−
+ 12H
+
→ N 2 + 6H 2 O
(4.5)
The nitrification–denitrification process requires the construction of aeration and
settling tanks, which significantly increases the cost of treatments.
– Modifications of Nitrification–Denitrification Method
One of the early denitrification configurations was the Ludzack–Ettinger process, as
illustrated in Fig. 4.1b, in which the denitrification tank (anoxic zone) is placed before
the aeration tank (zone) and the organic materials in the influent are used as the carbon
source for denitrification. After the wastewater is treated through nitrification in the
aeration tank, it is returned together with WAS as a return sludge to the denitrification
105
4.3.1.1 Biological Methods
– Nitrification–Denitrification
Nitrification is a two-step process with different types of microorganisms. In the
first step, ammonia is converted to nitrite and then to nitrate, which is carried out by
ammonia-oxidizer microorganisms such as Nitrosomonas, Nitrosospira and Nitrosococcus. They take up ammonia and generate a very small amount of energy for their
growth. The nitrite is then oxidized to nitrate by nitrite-oxidizing bacteria including
Nitrospira, Nitrobacter, Nitrococcus and Nitrospinae. Oxidation of nitrite to nitrate
also produces a small amount of energy. The overall nitrification reactions are as
follows [20]:
NH 3 + O 2 + 2e
−
→ NH 2 OH + H 2 O
(4.1)
NH 2 OH + H 2 O → NO
−
2 + 5H
+
+ 4e
−
(4.2)
NO
−
2 + H 2 O → NO
−
3 + 2H
+
+ 2e
−
(4.3)
The nitrification microorganisms require an aerobic environment. They play an
important role in removing the harmful levels of ammonia from the receiving waters
[18].
Denitrification is a process in which the nitrate is converted to nitrogen and other
gaseous products (such as NO and N 2 O) and it is facilitated by a wide range of
microorganisms. Some of the denitrifying microorganisms include Bacillus, Paracoccus and Pseudomonas. The process requires an organic carbon source and anoxic
conditions, and involves a series of denitrification reactions as follows [18]:
NO
−
3 → NO
−
2 → NO → N 2 O → N 2
(4.4)
The overall reaction:
2NO
−
3 + 10e
−
+ 12H
+
→ N 2 + 6H 2 O
(4.5)
The nitrification–denitrification process requires the construction of aeration and
settling tanks, which significantly increases the cost of treatments.
– Modifications of Nitrification–Denitrification Method
One of the early denitrification configurations was the Ludzack–Ettinger process, as
illustrated in Fig. 4.1b, in which the denitrification tank (anoxic zone) is placed before
the aeration tank (zone) and the organic materials in the influent are used as the carbon
source for denitrification. After the wastewater is treated through nitrification in the
aeration tank, it is returned together with WAS as a return sludge to the denitrification