N2O production in a single stage nitritation/anammox MBBR process

electrical potential than the sensor port which can create an electric potential and
increased nitrate flux over the biochamber membrane. (There are two different
possibilities to ground the environment in the close range of the microsensors. The first
option is to use the ground channel connected to sensor port on the piccoameter, the
electric potential of the sensor and ground channel is the same. The other option is use
the ground port on the backside of the piccoameter, this port has another electric
potential than the sensor port).
Since the biosensor relies on denitrifying bacteria converting NO2-N to N2O, the sensor
was hard to work with. The bacteria in the biochamber are changing and adapting their
metabolism as their physical environment with available substrates changes (Larsen et
al., 1997). This means that their metabolism might be influenced by moving from the
environment in which they are kept in between measurements, via the calibration setup
into the MBBR where measurements are performed. At some occasions the biosensor
had to be recalibrated one to three times before giving a stable signal, which is very time
consuming. The sensor also has to be well nursed in between measurements in order to
keep the microorganisms viable.
To obtain the same salinity during calibration and measurement the biosensor was
calibrated in the synthetic wastewater feeding the MBBR. The microbial nitrogen
conversion in the MBBR is changing the ionic composition of the influent wastewater
with a difference in ionic strength of influent medium and effluent as result. Since the
biosensor is sensitive to ionic strength as well as salinity (Nielsen et al., 2004) better
results might have been obtained by calibration of the biosensor in the effluent water.
(Since the effluent water contains NO2-N this calibration method gives a background
signal of NO2-N which has to be corrected for).
The biosensor might be a good option if changes of NO2-N are going to be studied during
cyclic changes of a microbial process. However the sensitivity of the sensor and the fact
that it has to be well looked after in between measurements has to be taken into account
when considering the biosensor as an option to conventional methods of determining
the NO2-N concentrations. The biosensor and required equipment is also a significant
investment cost.
5.4 Diffusivity and stripping test of N2O
Testing the diffusivity of N2O through mechanical mixing with K1–heavy carriers
without biofilm showed that