3.9 Nitrification and Denitrification for Nitrogen Removal
Due to the stricter effluent standards, in particular for nutrients, many existing
wastewater treatment plants need to be upgraded. Upgrading of plants with conventional processes may require the extension of tank volumes and the construction of
new reactors. It is necessary to develop innovative processes, which satisfy the
following requirements: high removal efficiencies, upgrading with minimal interference with the existing facilities, low investments, and simple technologies with low
operating costs [39].
Conventional activated sludge treatments for nitrogen removal include several
phases with different oxygen concentrations or may employ only one reactor in
which alternating aerobic and anoxic phases are achieved in time or space. In such
processes, the oxygen concentration must be controlled in order to obtain the
simultaneous (in time and in space, i.e., with constant conditions and in the same
reactor) activity of denitrifying and nitrifying bacteria. It has been shown, in fact,
that denitrification reaction takes place also under aerobic conditions and that
nitrification is possible at low levels of dissolved oxygen. As far as denitrification
is concerned, “aerobic denitrification” and “microzone denitrification” can be
defined [40]. In the former, microorganisms utilize nitrate and oxygen simultaneously as terminal electron acceptors; and in the latter, denitrification occurs in
the anoxic microzones with a biological floc. This last phenomenon is supposed to
occur due to the particular aeration conditions at very low dissolved oxygen
concentration.
Low oxygen concentrations can be maintained by direct DO control or indirect
DO control, like oxidation-reduction potential (ORP). Since nitrification and denitrification have become major aims in wastewater treatment, attempts have been
made to use ORP as a control parameter, cheap but reliable control strategies, for
wastewater treatment plants [41]. Most ORP controllers based on curve interpretation also have implemented time-control strategies as an emergency control whenever characteristic points cannot be found by controller [42].
Other studies for the nitrification-denitrification process include nitrification
performance in an integrated fixed activated sludge process [43] and iterative design
of a nitrate controller using an external carbon source in an activated sludge process
[44]. In an activated sludge process, the nitrogen removal efficiency may be
improved by adding an external carbon source. Automatic control of the nitrate
level can be achieved by regulating external carbon flow [45–47].
3.10 Membrane Bioreactor
In recent years, new membranes specially developed for use in wastewater treatment
have made membrane bioreactor application become a promising alternative to the
well-known aerobic processes. Ultra- or micro-filtration membrane possesses
3 Biological Processes
109
Due to the stricter effluent standards, in particular for nutrients, many existing
wastewater treatment plants need to be upgraded. Upgrading of plants with conventional processes may require the extension of tank volumes and the construction of
new reactors. It is necessary to develop innovative processes, which satisfy the
following requirements: high removal efficiencies, upgrading with minimal interference with the existing facilities, low investments, and simple technologies with low
operating costs [39].
Conventional activated sludge treatments for nitrogen removal include several
phases with different oxygen concentrations or may employ only one reactor in
which alternating aerobic and anoxic phases are achieved in time or space. In such
processes, the oxygen concentration must be controlled in order to obtain the
simultaneous (in time and in space, i.e., with constant conditions and in the same
reactor) activity of denitrifying and nitrifying bacteria. It has been shown, in fact,
that denitrification reaction takes place also under aerobic conditions and that
nitrification is possible at low levels of dissolved oxygen. As far as denitrification
is concerned, “aerobic denitrification” and “microzone denitrification” can be
defined [40]. In the former, microorganisms utilize nitrate and oxygen simultaneously as terminal electron acceptors; and in the latter, denitrification occurs in
the anoxic microzones with a biological floc. This last phenomenon is supposed to
occur due to the particular aeration conditions at very low dissolved oxygen
concentration.
Low oxygen concentrations can be maintained by direct DO control or indirect
DO control, like oxidation-reduction potential (ORP). Since nitrification and denitrification have become major aims in wastewater treatment, attempts have been
made to use ORP as a control parameter, cheap but reliable control strategies, for
wastewater treatment plants [41]. Most ORP controllers based on curve interpretation also have implemented time-control strategies as an emergency control whenever characteristic points cannot be found by controller [42].
Other studies for the nitrification-denitrification process include nitrification
performance in an integrated fixed activated sludge process [43] and iterative design
of a nitrate controller using an external carbon source in an activated sludge process
[44]. In an activated sludge process, the nitrogen removal efficiency may be
improved by adding an external carbon source. Automatic control of the nitrate
level can be achieved by regulating external carbon flow [45–47].
3.10 Membrane Bioreactor
In recent years, new membranes specially developed for use in wastewater treatment
have made membrane bioreactor application become a promising alternative to the
well-known aerobic processes. Ultra- or micro-filtration membrane possesses
3 Biological Processes
109