396
Y. Argaman et al.
Nl4-N of 94 mgL- 1 with 4-14 hr of HRT. Takizawa et al. (1996) obtained 73%
effective nitrogen removal using hollow polypropylene carrier, compared to 65%
of conventional AS treatment.
Thus, from the data given above, it can be concluded that fixed biomass allows
a significant increase in the rate of the nitrification process, and thus reduces the
required reactor volume needed for this step. However, this method cannot
provide effective denitrification, since BOD (electron donor) is removed along
with nitrification.
It should also be noted that both suspended and attached growth systems do
not provide an appropriate configuration for seasonal requirements (for
agricultural needs) of nitrogen removal. If nitrification is terminated, it may be
difficult to start it again within a short period of time due to the low growth rate of
the nitrifiers.
2.3.2 Nitrification USing Biomass Entrapped into a Gel
Entrapment is the most extensively investigated immobilization method used. The
enclosure of a cell with a gel structure is unlikely to change the function of the
cell. This is one of the least destructive immobilization methods (Tyagi and
Vembu 1991). Matsumura et al. (1997) reported that nitrifying bacteria entrapped
in gel provided an effective treatment and they were less sensitive to toxicants and
conditions than free ones.
An immobilized-cell reactor employing cells entrapped in gel beads forms an
alternative to attached-cell systems (Wijffels and Tramper 1995). Such a method
of artificial immobilization has several advantages in comparison with attachment
of cells and suspended growth:
• The microorganisms immobilized in this manner can be protected from others
and are not appreciably inhibited by toxic substances (Ozaki et al. 1991).
Selective immobilization of certain types of bacteria can selectively increase
the efficiency ofthe process, which is catalyzed by these microorganisms.
• Entrapped microorganisms are less sensitive to changes of temperature and pH
(Leenen et al. 1997).
Table 1 summarizes literature data on the different nitrification methods.
Based on the data presented in the Table, it can be concluded that entrapment of
nitrifying bacteria allows conducted nitrification to be at a high rate. In contrast
with AS biomass, the entrapped system shows much higher nitrifying activity.
High efficiency of the nitrification by entrapped bacteria during full-scale
wastewater treatment was confirmed by Japanese investigators (Tanaka et al.
1993). The immobilized cell reactor was installed in the aerobic stage of
conventional single-sludge AS treatment. Thus, in this aerobic zone, enhanced
nitrification by entrapped biomass occurs in parallel with BOD removal by AS
biomass. This process (named the PEGASUS process) allows removal of both
BOD and nitrogen within 8 h of the treatment.
Y. Argaman et al.
Nl4-N of 94 mgL- 1 with 4-14 hr of HRT. Takizawa et al. (1996) obtained 73%
effective nitrogen removal using hollow polypropylene carrier, compared to 65%
of conventional AS treatment.
Thus, from the data given above, it can be concluded that fixed biomass allows
a significant increase in the rate of the nitrification process, and thus reduces the
required reactor volume needed for this step. However, this method cannot
provide effective denitrification, since BOD (electron donor) is removed along
with nitrification.
It should also be noted that both suspended and attached growth systems do
not provide an appropriate configuration for seasonal requirements (for
agricultural needs) of nitrogen removal. If nitrification is terminated, it may be
difficult to start it again within a short period of time due to the low growth rate of
the nitrifiers.
2.3.2 Nitrification USing Biomass Entrapped into a Gel
Entrapment is the most extensively investigated immobilization method used. The
enclosure of a cell with a gel structure is unlikely to change the function of the
cell. This is one of the least destructive immobilization methods (Tyagi and
Vembu 1991). Matsumura et al. (1997) reported that nitrifying bacteria entrapped
in gel provided an effective treatment and they were less sensitive to toxicants and
conditions than free ones.
An immobilized-cell reactor employing cells entrapped in gel beads forms an
alternative to attached-cell systems (Wijffels and Tramper 1995). Such a method
of artificial immobilization has several advantages in comparison with attachment
of cells and suspended growth:
• The microorganisms immobilized in this manner can be protected from others
and are not appreciably inhibited by toxic substances (Ozaki et al. 1991).
Selective immobilization of certain types of bacteria can selectively increase
the efficiency ofthe process, which is catalyzed by these microorganisms.
• Entrapped microorganisms are less sensitive to changes of temperature and pH
(Leenen et al. 1997).
Table 1 summarizes literature data on the different nitrification methods.
Based on the data presented in the Table, it can be concluded that entrapment of
nitrifying bacteria allows conducted nitrification to be at a high rate. In contrast
with AS biomass, the entrapped system shows much higher nitrifying activity.
High efficiency of the nitrification by entrapped bacteria during full-scale
wastewater treatment was confirmed by Japanese investigators (Tanaka et al.
1993). The immobilized cell reactor was installed in the aerobic stage of
conventional single-sludge AS treatment. Thus, in this aerobic zone, enhanced
nitrification by entrapped biomass occurs in parallel with BOD removal by AS
biomass. This process (named the PEGASUS process) allows removal of both
BOD and nitrogen within 8 h of the treatment.