N2O production in a single stage nitritation/anammox MBBR process

2.3.4 Moving bed reactor
Another way to design a compact biofilm process is to use a suspended inert carrier that
moves freely within the reactor. The first carrier materials were small polyethylene
(density 0.95 g/cm 3 ) cylinders with a cross in side providing the microorganisms with a
protected surface to grow on. The carriers are kept in motion by aeration or mechanical
stirring, a sieve in the outlet keeps the moving carriers in the reactor. The reactor does
not clog and there is no need for backwashing or biomass recycling. (Ødegaard et al.,
1994). Different shapes and sizes of the carrier material provides an effective specific
area ranging from 220 -1200 m 2 /m 3 (AnoxKaldnes, 2009). Biofilm thickness depends on
carrier design and hydraulic conditions in the reactor. If a stagnant laminar layer is
formed around the carrier material this will increase the diffusional resistance that is
limiting in biofilm processes. The moving bed technology can also be combined with the
activated sludge process resulting in higher removal rates and more compact systems.
2.3.5 Rotating disc
Rotating filters are constituted of flat discs often made out of plastic, 2-3 meters in
diameter mounted in rows on a horizontal shaft. The filter medium that is semisubmerged
is alternately rotated through the water phase at right angles to the flow.
The filters are 10-20 mm thick, spaced about 20 mm apart and have an active surface
area of about 150-200 m 2 /m 3 (Gray N, 2004). Rotation of the filter discs keeps the
biofilm oxygenated and the motion creates an efficient contact between the water phase
and biofilm. Revolution speed of the filter is controlling the biofilm thickness (Henze et
al., 1997).
2.4 Biofilm kinetics
The kinetics of substrate conversion in a biofilm reactor is dependent of the reactor
configuration that decides the biofilm structure and of available nutrients in the
wastewater. Substrates in the water bulk phase are converted to biomass, energy and
end products through cellular metabolism in the bacteria. The mass balance for an
infinitely small section of the biofilm is described by:
(2.4.1)
(2.4.2)
where: Q is the volumetric flow, (dimension L -3 ∙T -1 ), C is the concentration, (dimension
M∙L -3 ), r describes the biological growth rate, (dimension M∙L -3 ∙ T -1 ) and V is the reactor
volume, (dimension L -3 ), (Warfvinge, 2008).
The substrate conversion rate in a biofilm reactor is dependent on three main
mechanisms, the diffusion resistance of substrates from the well mixed water bulk phase
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