N2O production in a single stage nitritation/anammox MBBR process

are intermediates in the chemical reaction. This reaction is the catabolic and energy
supplying part in anammox metabolism, it has to be carried out 15 times to fix one
molecule of carbon dioxide with nitrite as electron donor in the cellular synthesis or
anabolism (2.1.7) which produces nitrate (van Niftrik et al., 2004).
NH NO N 2H O (2.1.6)
CO 2NO
H O CH O 2NO (2.1.7)
Broda, (1977), predicted this microbial process through thermodynamic calculations for
over thirty years ago. The anammox process was discovered in the early nineties in a
rotating-disk plant treating landfill leachate at Mechernich, Germany (Rosenwinkel &
Cornelius, 2005). Mulder et al. also identified the anammox process in a denitrifying
fluidised bed reactor in Deltft, the Netherlands, at about the same time (Mulder et al.,
1995). Total stoichiometry of the anammox process has been estimated by Strous et al.,
(1998):
1NH
1.32NO 0.066HCO
0.13H
1.02 N 0.26NO
0.066CH2O . N . 2.03 H O. (2.1.7)
The bacterium performing the anammox reaction has been identified as a new member
of the order Planctomycete (Strous et al., 1999). Until now totally five anammox genera
have been identified, four from enriched wastewater sludge: Kuenenia, Brocadia,
Anammoxoglobus and Jettenia, the fifth genera of anammox bacteria Scalindua is often
found in marine environments (Jetten et al., 2009).
Planctomycetes are gram-negative bacteria with phenotypic properties such as absence
of peptidoglycan in the cell wall, budding reproduction and internal cell
compartmentalisation due to two membranes on the inside of the cell wall (Prescott et
al., 2005). Anammox bacteria are characterized by their deep red colour and ability to
form bio-films (Abma et al., 2006). Anammox cell structure is divided into three
compartments. The outer region closest to the cell wall called the paryphoplasm
encloses the second compartment which is the riboplasm. The riboplasm contains the
nucleoid and the anammoxosome, the third compartment where anammox catabolism
takes place, see Figure 2. All compartments are separated by bilayer membranes
constituted of impermeable and high density ladderane lipids (van Niftrik et al., 2004).
The higher membrane density is of importance to the anammox bacteria of two reasons,
one that it creates an electro potential force driving the ATP synthesis, and two it keeps
the toxic intermediates from the anammox process hydroxylamine and hydrazine inside
the anammoxosome (van Niftrik et al., 2004).
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