N2O production in a single stage nitritation/anammox MBBR process

aeration takes place through self drag from bottom to top of the filter (Henze et al.,
1997). Since the wastewater is sprinkled over and percolated through the filter media
there is little hydraulic control of the biofilm thickness resulting in uneven growth
throughout the filter. This causes local clogging that hinders free flow of water and air
through the filter resulting in decreased nutrient removal rate. Modern filling materials
in plastic have a specific filling area of about 100-250 m 2 /m 3 while the carrying material
in older treatment plants often is crushed stone or pumice that only has a specific area of
40-60 m 2 /m 3 , (Gillberg et al., 2003).
2.3.2 Biofilters
The carrier material in these filters can be either a granulated media or corrugated
sheets. In the granulated reactor wastewater passes through a stationary filter bed made
out of sand or plastic beads, the filter media is aerated from the bottom in the aerobic
version. The specific area of the filter media is high and ranges from 1000-1200 m 2 /m 3 ,
the effective area is however not this high since only 50% of the reactor volume is filled
with the carrier material (Ødegaard, 1993). This reactor clogs easily and has to be
backwashed. Clogging and substrate decrease from top to bottom in the reactor rules
out efficient nutrient removal throughout the whole reactor volume. The second type of
media is a stationary carrier material constituted of plastic sheets that are welded
together in cubes, the specific area of these filters ranges from 150-200 m 2 /m 3 . The
system is aerated from the bottom with blower systems and has to be backwashed at
intervals to prevent clogging (Ødegaard, 1993).
2.3.3 Fluidised bed
In a fluidised bed the biofilm grows on sand grains with a size of 0.4-0.5 mm. To keep the
sand grains in suspension at all times wastewater is pumped through the bottom of the
reactor at a high constant flow rate. The turbulence created from high volumetric flow
rates passing through the reactor implies great shear forces and very thin biofilms. With
bed depths ranging from 3 to 4 m a specific surface area of 1000 m 2 /m 3 can be achieved
(Metcalf & Eddy, 2004). High turbulence in combination with very high contact area
between microorganisms and wastewater assures efficient substrate transmission and
high conversion rates. The draw back with this process design in nitrification is that
oxygen transfer rates to the water phase are too slow to maintain sufficient oxygen
concentrations for microbial activity (Gillberg et al., 2003) Sine the biofilm is very thin in
this reactor configuration it does not allow development of a biofilm with layers of
different oxygen concentration. This reactor type can there for not combine microbial
communities with different dissolved oxygen demands. Recirculation is necessary to
maintain the high fluid velocity.
12