Biological Nitrogen Removal Using Immobilized Bacteria
393
Although the nitrogen compounds are water contaminants, their presence in
the treatment plant effluent may be needed. Water intended for irrigation may
contain such a nutrient. Usually, this demand is seasonal, requiring removal of
nitrogen compounds during a certain period of time. Such a requirement makes
the advanced treatment more complicated.
2 Applicability of the Proposed Process for Wastewater
Recycling Projects
2.1 Process Development Background
Removal of nitrogen compounds is beneficial and commonly accomplished by
biological processes using nitrification-denitrification processes.
Both nitrification and denitrification are somewhat problematic. The weakness
of nitrification is the slow growth of the ammonia-oxidizing bacteria. Under
optimal conditions, the generation time of nitrifying bacteria is about 15 h.
Compared to heterotrophic microorganisms, which have generation times of 20 -
40 min., it is clear that nitrifying organisms tend to be washed out (Wijffels et al.
1993) when they occur in a mixture of activated sludge (AS) biomass. Hence, the
key requirement for nitrification in the AS wastewater treatment systems is that
the sludge age in the reactor should be more than that corresponding to the
specific growth rate of nitrifiers. In the case of lower solids retention time (SRT),
slow growing nitrifiers are washed out.
The problem of the high reactor volume is of particular importance in the case
of refitting the existing reactors designed for organics removal only. Tanaka et al.
(1993) reported that a common single-sludge AS reactor designed for wastewater
nitrogen and organics removal has a capacity approximately twice of that of a
conventional activated sludge process intended only for organic matter removal.
2.2 Conventional Wastewater Treatment Systems Including
Nitrogen Removal
The nitrifying and denitrifying phases are frequently run in separate reactors or in
different areas of the same tank. Conventional widely used systems for biological
nitrogen removal (illustrated in Fig.l) are suspended-growth processes based on
activated sludge treatment.
It should be noted that heterotrophic microorganisms degrading organics are
originally present in wastewater parallel with ammonia oxygenation by nitrifiers.
As a result of the large sludge retention time (SRT) needed for the nitrifying
reactor, its effluent does not contain a significant amount of biodegradable
393
Although the nitrogen compounds are water contaminants, their presence in
the treatment plant effluent may be needed. Water intended for irrigation may
contain such a nutrient. Usually, this demand is seasonal, requiring removal of
nitrogen compounds during a certain period of time. Such a requirement makes
the advanced treatment more complicated.
2 Applicability of the Proposed Process for Wastewater
Recycling Projects
2.1 Process Development Background
Removal of nitrogen compounds is beneficial and commonly accomplished by
biological processes using nitrification-denitrification processes.
Both nitrification and denitrification are somewhat problematic. The weakness
of nitrification is the slow growth of the ammonia-oxidizing bacteria. Under
optimal conditions, the generation time of nitrifying bacteria is about 15 h.
Compared to heterotrophic microorganisms, which have generation times of 20 -
40 min., it is clear that nitrifying organisms tend to be washed out (Wijffels et al.
1993) when they occur in a mixture of activated sludge (AS) biomass. Hence, the
key requirement for nitrification in the AS wastewater treatment systems is that
the sludge age in the reactor should be more than that corresponding to the
specific growth rate of nitrifiers. In the case of lower solids retention time (SRT),
slow growing nitrifiers are washed out.
The problem of the high reactor volume is of particular importance in the case
of refitting the existing reactors designed for organics removal only. Tanaka et al.
(1993) reported that a common single-sludge AS reactor designed for wastewater
nitrogen and organics removal has a capacity approximately twice of that of a
conventional activated sludge process intended only for organic matter removal.
2.2 Conventional Wastewater Treatment Systems Including
Nitrogen Removal
The nitrifying and denitrifying phases are frequently run in separate reactors or in
different areas of the same tank. Conventional widely used systems for biological
nitrogen removal (illustrated in Fig.l) are suspended-growth processes based on
activated sludge treatment.
It should be noted that heterotrophic microorganisms degrading organics are
originally present in wastewater parallel with ammonia oxygenation by nitrifiers.
As a result of the large sludge retention time (SRT) needed for the nitrifying
reactor, its effluent does not contain a significant amount of biodegradable