N2O production in a single stage nitritation/anammox MBBR process

The microsensor is connected to a piccoameter that polarises the cathode surface where
the nitrous oxide that diffuses through the silicone membrane is reduced to N2 gas. As
nitrous oxide is reduced at the cathode surface two electrons from the silver anode is
used for each reduced N2O molecule. The electron transport gives rise to a current
proportional to the amount of reduced nitrous oxide. The current is registered and
converted to an out signal by the piccoameter. The guard cathode is also polarised to
deplete oxygen in the electrolyte which minimizes zero current (Unisense b, 2007).
The nitrous oxide sensor has a measuring range of about 0-1 atmosphere pN2O with a
response time less than 10 seconds. The stirring sensitivity is smaller than 2% and the
out signal is temperature dependent with a temperature coefficient of about 2-3% per
°C. Interference in the out signal might occur from electrical noise in the surrounding
environment (Unisense b, 2007).
2.6.2 Nitrite biosensor
The nitrite biosensor is a nitrous oxide sensor equipped with a replaceable biochamber
(Figure 9a), (Unisense e, 2009). A plastic tube containing a carbon source and a bacterial
culture constitutes the biochamber that is mounted in the front of the sensor tip (Figure
9b), (Unisense e, 2009). The biomass in the reaction chamber is positioned between the
carbon source required for their growth and an ion-permeable membrane separating
the microorganisms from the external environment (Unisense c, 2007).
The denitrifying bacterial culture used in the biochamber is deficient in NO3 − and N2O
reductase which means that it is only able to reduce NO2 − into N2O. As NO2 − diffuses into
the biochamber it is reduced to N2O by the biomass (Nielsen et al., 2004). Since
denitrifying bacteria are facultative aerobic they can use both oxygen and nitrite as
oxidation agent for their respiration (Larsen et al., 1997). Oxygen is used preferential to
nitrite as it results in a higher energy yield. This will create a NO2 − reducing gradient in
the biochamber with the bacteria closest to the membrane respiring with oxygen. The
NO2 − reducing capacity of the biosensor will depend on the length of the aerobic zone
resulting in higher maximum detectable concentrations of NO2 − in anaerobic
environments (Larsen et al., 1997).
Produced N2O diffuses through the silicone membrane and is reduced at the cathode in
the transducer part of the biosensor. A piccoameter measures the current arising from
the electron transport just as in the case with the nitrous oxide sensor. The output signal
is proportional to the amount of NO2 − that has been reduced after diffusion into the
biochamber (Unisense c, 2007).
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