N2O production in a single stage nitritation/anammox MBBR process

minimum concentration of 2 mg/l should be maintained (Gray, 2004). The nitrifying rate
increases up to DO levels of 3-4 mg/l, (Metcalf & Eddy). All figures given here yields for
DO concentrations in the water bulk phase of activated sludge processes, higher DO
concentrations are needed to satisfy the microbial oxygen demand in biofilm processes.
This since the oxygen concentration in the biofilm depends on diffusion of oxygen from
the water phase into the biofilm which is further explained in chapter 2.4.
Both denitrification and anammox processes are inhibited by oxygen. Denitrification has
been observed to be inhibited at DO concentrations above 0.2 mg/l (Metcalf & Eddy,
2003) and anammox organisms are reversibly inhibited by DO concentrations as low as
2 µmole/l or 0.032 mg/l, (Jetten et al., 1998).
2.2.2 Temperature
The temperature impacts the structure of the microbial community and is crucial for
growth and reaction rates in the system. Microbial reactions are often dependent on
enzyme-catalysed reactions that increase in velocity at higher temperatures. When the
time for a reaction to be catalysed is shortened the metabolism is more active and the
microorganism is allowed to grow faster (Prescott et al., 2005). Temperature does also
impact non viable factors like settling characteristics, gas solubility and transfer rates
(Gray, 2004).
Nitrification can be operated in a temperature interval of 0-40 °C with a temperature
optimum between 30-35 °C (Gray, 2004). Denitrifying bacteria are less sensitive to
temperature than nitrifiers and denitrification can take place in a temperature interval
from 2-75 °C with an optimum around 30 °C (Pierzynski et al., 2005)
Anammox bacteria are active in temperature range from 6-43 °C with an optimum at 30
̊C (Anammox online).
2.2.3 pH and alkalinity
pH, which is the measurement of a solutions acidity or alkalinity, is another important
environmental factor that impacts the growth rate of the microbial community. Since pH
is defined as the inverse logarithm of H + ions in solution a change of one pH unit
corresponds to a tenfold increase in the activity of H + ions. Each bacteria species have a
pH growth range and optimum.
Nitrification consumes alkalinity since two moles of OH − are used per mole ammonium
oxidised. Nitrification is favoured by mild alkaline conditions with pH optimum in the
range of pH 8.0-8.4 (Gray, 2004). The nitrification rate is significantly declined by low
pH values