methanotrophic bacteria, which uses CH 4 as their source of carbon and energy,
thereby oxidizing methane into methanol by methane monooxygenase (MMO)
enzyme (Jagadevan and Semrau 2013), and (ii) ammonia-oxidizing bacteria,
which can partially oxidize CH 4 to methanol by using ammonia as an energy source
(Taher and Chandran 2013). Besides these, some methane-oxidizing archaea are also
found to carry out anaerobic oxidation of methane (AOM) (Hanson and Hanson
1996; Boetius et al. 2000; Beal et al. 2009; Ge et al. 2014).
Ammonia monooxygenase (AMO) encoded by ammonia-oxidizing bacteria
(AOB) is another enzyme that can oxidize methane to methanol. Both MMO and
AMO enzyme are evolutionary related (Tavormina et al. 2011; Lawton et al. 2014).
AMO enzyme possesses a broad range specificity of substrates such as methane,
methanol, ethylene, methyl bromide, etc. (Taher and Chandran 2013). Hence, AOB
are capable to produce alternative liquid fuel (methanol) from methane oxidation,
while using NH 3 as energy source. The requirement of reducing power is essential
for this conversion. The pioneering work for methane as an alternative substrate for
AOB has been reported by Hyman and Wood (1983). They reported that CH 4
inhibits ammonia consumption by Nitrosomonas europaea with inhibition constant
of 2 mM. In addition, methane oxidation by pure culture of AOB such as
Nitrosomonas europaea and Nitrosomonas oceanus have been studied previously
(Jones and Morita 1983; Hyman and Wood 1983). N. europaea resists the inhibitory
effect of CH 4 up to 1 mM concentration, whereas N. oceanus was inhibited by less
than 0.1 mM CH 4 (Jones and Morita 1983).
N. europaea was investigated in the production of methanol in biofilm system
with maximum yield of 0.09 mg methanol/mg biomass (VS)/d (Thorn 2006). There
have been some evidences of oxidation of methane by nitrifying bacteria consisting
of Nitrosomonas spp. in a mixed consortium. For example, Nitrosomonas species
have been found in plastic waste biodegradation site where synthetic biogas has been
artificially pumped to promote methane oxidation (Muenmee et al. 2015).
Nitrosomonas and Nitrosospira species have been found in oxidation of dissolved
methane which is produced from anaerobic digestion of wastewater (Hatamoto et al.
2011). In a recent work, a maximum of 1.6 Æ 0.15 mg-COD CH 3 OH L
À1 was
reported to be produced in a continuous stirred tank reactor (CSTR) at hydraulic
retention time of 7 h by AOB (Su et al. 2019). However, the maximum yield of
methanol of 59.89 Æ 1.12 mg-COD CH 3 OH L
À1 was obtained from a mixed
nitrifying enrichment culture (Taher and Chandran 2013).
7.7 Inhibitors of AOB
Despite several functional capabilities of AOB, nitrification process in industrial
wastewater treatment plants is inhibited due to the presence of various toxic
chemicals. Heavy metals (e.g., zinc, copper, arsenic, etc.) are one of the major
contributors toward nitrification inhibition. Inhibitors of nitrification are classified
as chemical and biological inhibitors which are discussed in the following sections.
164
P. Chawley et al.
thereby oxidizing methane into methanol by methane monooxygenase (MMO)
enzyme (Jagadevan and Semrau 2013), and (ii) ammonia-oxidizing bacteria,
which can partially oxidize CH 4 to methanol by using ammonia as an energy source
(Taher and Chandran 2013). Besides these, some methane-oxidizing archaea are also
found to carry out anaerobic oxidation of methane (AOM) (Hanson and Hanson
1996; Boetius et al. 2000; Beal et al. 2009; Ge et al. 2014).
Ammonia monooxygenase (AMO) encoded by ammonia-oxidizing bacteria
(AOB) is another enzyme that can oxidize methane to methanol. Both MMO and
AMO enzyme are evolutionary related (Tavormina et al. 2011; Lawton et al. 2014).
AMO enzyme possesses a broad range specificity of substrates such as methane,
methanol, ethylene, methyl bromide, etc. (Taher and Chandran 2013). Hence, AOB
are capable to produce alternative liquid fuel (methanol) from methane oxidation,
while using NH 3 as energy source. The requirement of reducing power is essential
for this conversion. The pioneering work for methane as an alternative substrate for
AOB has been reported by Hyman and Wood (1983). They reported that CH 4
inhibits ammonia consumption by Nitrosomonas europaea with inhibition constant
of 2 mM. In addition, methane oxidation by pure culture of AOB such as
Nitrosomonas europaea and Nitrosomonas oceanus have been studied previously
(Jones and Morita 1983; Hyman and Wood 1983). N. europaea resists the inhibitory
effect of CH 4 up to 1 mM concentration, whereas N. oceanus was inhibited by less
than 0.1 mM CH 4 (Jones and Morita 1983).
N. europaea was investigated in the production of methanol in biofilm system
with maximum yield of 0.09 mg methanol/mg biomass (VS)/d (Thorn 2006). There
have been some evidences of oxidation of methane by nitrifying bacteria consisting
of Nitrosomonas spp. in a mixed consortium. For example, Nitrosomonas species
have been found in plastic waste biodegradation site where synthetic biogas has been
artificially pumped to promote methane oxidation (Muenmee et al. 2015).
Nitrosomonas and Nitrosospira species have been found in oxidation of dissolved
methane which is produced from anaerobic digestion of wastewater (Hatamoto et al.
2011). In a recent work, a maximum of 1.6 Æ 0.15 mg-COD CH 3 OH L
À1 was
reported to be produced in a continuous stirred tank reactor (CSTR) at hydraulic
retention time of 7 h by AOB (Su et al. 2019). However, the maximum yield of
methanol of 59.89 Æ 1.12 mg-COD CH 3 OH L
À1 was obtained from a mixed
nitrifying enrichment culture (Taher and Chandran 2013).
7.7 Inhibitors of AOB
Despite several functional capabilities of AOB, nitrification process in industrial
wastewater treatment plants is inhibited due to the presence of various toxic
chemicals. Heavy metals (e.g., zinc, copper, arsenic, etc.) are one of the major
contributors toward nitrification inhibition. Inhibitors of nitrification are classified
as chemical and biological inhibitors which are discussed in the following sections.
164
P. Chawley et al.