7.6 Role of Ammonia-Oxidizing Bacteria (AOB)
in Wastewater Treatment
7.6.1 Removal of Micropollutants
Micropollutants (MPs) are toxic compounds that are present in nanogram to microgram per liter range in an engineered system. AOB are capable of removal of MP by
the action of ammonia monooxygenase enzyme via co-metabolism. Co-metabolism
involves simultaneous transformation of a nongrowth substrate as well as growth
substrate by the same enzyme, wherein the growth substrate further enters in the
metabolic pathway to produce energy. The removal of MP (nongrowth substrate) by
AOB is thus dependent on energy produced by the growth substrate (–NH 3 ). Low
substrate specificity and broad substrate spectrum allow co-metabolism of MPs by
AMO (Rasche et al. 1991). Simultaneous oxidation of both growth and nongrowth
substrate by AMO results in competitive inhibition of the primary growth substrate,
i.e., NH 3 . Biotransformation of MPs by AMO causes toxicity to cells, resulting in
loss of AMO activity. The removal of MPs therefore depends on the extent of
competitive inhibition to ammonia oxidation and the ability of cells to overcome
toxic effects of MPs (Yang et al. 1999).
The biotransformation of MPs follows either first-order or pseudo first-order
kinetics, and the transformation was found to increase with addition of nitrogen
load to nitrifying sludge (Fernandez-Fontaina et al. 2012; Dawas-Massalha et al.
2014; Xu et al. 2016; Zhou et al. 2019). However, extremely higher ammonia
concentrations competitively inhibit biotransformation until all ammonia gets
depleted (Dawas-Massalha et al. 2014). A particular ratio of NH 3 /MP is therefore
necessary to be maintained to bring about the removal of MP. Table 7.2 lists
micropollutants which have been biologically transformed or removed by the activity of AOB.
7.6.2 Removal of Aromatic Pollutants
Removal of aromatic pollutants through AOB has been achieved due to the broadspectrum specificity nature of AMO enzyme, although co-oxidation of aromatic
compounds and ammonia by AMO inhibits ammonia oxidation by varying extent.
Inhibition in ammonia oxidation arises possibly because of competition between
ammonia and other compounds for reactive oxygen species, availability of reducing
power, and other growth medium constituents. However, ammonia oxidation activity has been found to be restored to normal levels when pure culture of AOB such as
N. europaea cells were resuspended in fresh culture medium without aromatics
(Keener and Arp 1994). N. europaea has been extensively studied for its ability to
transform benzene and substituted benzenes such as ethylbenzene, styrene,
halobenzenes, pyridine, phenol, p-cresol, o-cresol, etc. (Keener and Arp 1994).
162
P. Chawley et al.
in Wastewater Treatment
7.6.1 Removal of Micropollutants
Micropollutants (MPs) are toxic compounds that are present in nanogram to microgram per liter range in an engineered system. AOB are capable of removal of MP by
the action of ammonia monooxygenase enzyme via co-metabolism. Co-metabolism
involves simultaneous transformation of a nongrowth substrate as well as growth
substrate by the same enzyme, wherein the growth substrate further enters in the
metabolic pathway to produce energy. The removal of MP (nongrowth substrate) by
AOB is thus dependent on energy produced by the growth substrate (–NH 3 ). Low
substrate specificity and broad substrate spectrum allow co-metabolism of MPs by
AMO (Rasche et al. 1991). Simultaneous oxidation of both growth and nongrowth
substrate by AMO results in competitive inhibition of the primary growth substrate,
i.e., NH 3 . Biotransformation of MPs by AMO causes toxicity to cells, resulting in
loss of AMO activity. The removal of MPs therefore depends on the extent of
competitive inhibition to ammonia oxidation and the ability of cells to overcome
toxic effects of MPs (Yang et al. 1999).
The biotransformation of MPs follows either first-order or pseudo first-order
kinetics, and the transformation was found to increase with addition of nitrogen
load to nitrifying sludge (Fernandez-Fontaina et al. 2012; Dawas-Massalha et al.
2014; Xu et al. 2016; Zhou et al. 2019). However, extremely higher ammonia
concentrations competitively inhibit biotransformation until all ammonia gets
depleted (Dawas-Massalha et al. 2014). A particular ratio of NH 3 /MP is therefore
necessary to be maintained to bring about the removal of MP. Table 7.2 lists
micropollutants which have been biologically transformed or removed by the activity of AOB.
7.6.2 Removal of Aromatic Pollutants
Removal of aromatic pollutants through AOB has been achieved due to the broadspectrum specificity nature of AMO enzyme, although co-oxidation of aromatic
compounds and ammonia by AMO inhibits ammonia oxidation by varying extent.
Inhibition in ammonia oxidation arises possibly because of competition between
ammonia and other compounds for reactive oxygen species, availability of reducing
power, and other growth medium constituents. However, ammonia oxidation activity has been found to be restored to normal levels when pure culture of AOB such as
N. europaea cells were resuspended in fresh culture medium without aromatics
(Keener and Arp 1994). N. europaea has been extensively studied for its ability to
transform benzene and substituted benzenes such as ethylbenzene, styrene,
halobenzenes, pyridine, phenol, p-cresol, o-cresol, etc. (Keener and Arp 1994).
162
P. Chawley et al.