The MBR Book: Principles and Applications of Membrane

52 The MBR Book
(Equation (2.30)). Both these processes demand that specific micro-organisms prevail
(Section 2.2.3). The exact micro-organisms responsible for denitrification (nitrate
removal by biochemical reduction) are more varied – it is carried out by many different,
phylogenetically-unrelated heterotrophs (Stewart, 1998; Zumft, 1992).
The biological generation of nitrate from ammoniacal nitrogen (NH � 4) and aerobic
conditions (nitrification), takes place in two distinct stages:
Overall:
2NH � 4 � 3O 2 → 2NO 2 � � 2H � � 2H2O (ammonia → nitrite) (2.26)
2NO � 2 � O 2 → 2NO � 3 (nitrite → nitrate) (2.27)
NH � 4 � 2O 2 → NO � 3 � 2H � � H 2O (2.28)
Since the second step proceeds at a much faster rate than the first, nitrite does not
accumulate in most bioreactors. However, since these micro-organisms are autotrophic
and thus rather slow growing, they demand relatively long SRTs to accumulate and
provide close to complete nitrification (i.e. above 90% ammonia removal). This presents
another advantage of MBRs where long SRTs are readily attainable.
Denitrification takes place under anoxic conditions when oxidation of the organic
carbon takes place using the nitrate ion (NO 3 � ), generating molecular nitrogen (N2)
as the primary end product:
C 10H 19O 3N � 10NO � 3 → 5N 2 � 10CO 2 � 3H 2O � NH 3 � 10OH � (2.29)
where in this equation “C 10H 19O 3N” represents wastewater.
Nitrification relies on sufficient levels of carbon dioxide, ammonia and oxygen,
the carbon dioxide providing carbon for cell growth of the autotrophs. Since nitrifiers
are obligate aerobes, DO concentrations need to be 1.0–1.5 mg/L in suspended
growth systems for their survival. Denitrification takes place when facultative microorganisms,
which normally remove BOD under aerobic conditions, are able to convert
nitrates to nitrogen gas under anoxic conditions. Denitrification requires a
sufficient carbon source for the heterotrophic bacteria. This can be provided by the
raw wastewater, which is why the nitrate-rich waste from the aerobic zone is
recycled to mix with the raw wastewater. Complete nitrification is common in fullscale
MBR municipal installations, although, since it is temperature-sensitive,
ammonia removal generally decreases below 10°C. Most full-scale MBR sewage
treatment plants are also designed to achieve denitrification.
Most wastewaters treated by biological processes are carbon limited, and hence
phosphorus is not significantly removed. This applies as much to MBRs as to conventional
plants. It appears that membrane separation offers little or no advantage
regarding phosphorus removal (Yoon et al., 1999). Enhanced biological phosphate
removal can be achieved by the addition of an anaerobic zone at the front of an activated
sludge plant and returning nitrate-free sludge from the aerobic zone (Yeoman
et al., 1986). This has been applied to a some full-scale MBR plant where constraints