Biological Nitrogen Removal Using Immobilized Bacteria
395
Hence, the organics required are supplied either from external source (see
Fig.la), thus increasing the cost of treatment and excess sludge production, or by
large recycling of the mixed liquor (see Fig. Ib), thus decreasing the efficiency of
total nitrogen removal. Theoretically, if the recycling ratio is increased, the higher
total nitrogen removal efficiency may be attained. In practice, however, an
excessive recycling ratio leads to an increase in dissolved oxygen in the anoxic
tank, which hinders the denitrification reaction. Tanaka and Emori (1993) reported
that the maximum non limiting denitrification recycle ratio (r = QR/Q) is about 2,
and the maximum nitrogen removal efficiency attainable is about 70%.
The only type of nitrifying AS reactors with neither external organics addition
nor mixed liquor recycling is a combined carbon oxidation nitrificationdenitrification using an endogenous carbon source
(see Fig.lc). The
disadvantages of such a system are an extremely large reactor volume (HRT 19 -
25 h) and incomplete total nitrogen removal (efficiency of 87%) (Tanaka and
Emori 1993).
Thus, investigating advanced wastewater treatment processes, three main
problems of biological nitrogen removal should be considered:
1. Relatively large reactor volume.
2. Incomplete nitrogen removal.
3. Expenses for electron donor supply by means of external carbon source
addition and increased excess sludge quantity or large MLSS recycling.
2.3 Alternative Biological Nitrogen Removal Processes
2.3.1 Naturally Attached Cells
Use of immobilization on solid media increases the retention time of the biomass,
which is important for slow-growing bacteria such as nitrifiers (Shieh and Keenan
1986). Two types of fixed film processes have been used frequently for
nitrification: trickling filters and rotating biological contactors (RBC). The
biomass, which accumulates on the media surface, consists of both heterotrophic
and autotrophic bacteria. If the wastewater contains both biodegradable organic
matter and ammonia, most of the film will contain heterotrophic organisms,
because of their higher growth rate, and little or no nitrification will occur.
Therefore, the nitrification is reached after BOD removal below 15mgL· 1
(Eckenfelder and Argaman 1991). Both trickling filter and RBC have a limited
capacity because of the relatively low specific surface area of the support (100 -
500 m 2 m- 3 ).
High-rate biofilm reactors have a much higher specific surface area
(2000 m 2 m-\ usually accomplished using small solid particles (Tijhuis et al.
1992). Examples of compact systems are the packed-bed and fluidized-bed
reactors. A reactor incorporated with the fibrous packing materials effectively
removed (95-97 %) COD and nitrified up to 85% of the NJL-N available, when
tested with synthetic wastewater, having an average COD of958 mgL- 1 and
395
Hence, the organics required are supplied either from external source (see
Fig.la), thus increasing the cost of treatment and excess sludge production, or by
large recycling of the mixed liquor (see Fig. Ib), thus decreasing the efficiency of
total nitrogen removal. Theoretically, if the recycling ratio is increased, the higher
total nitrogen removal efficiency may be attained. In practice, however, an
excessive recycling ratio leads to an increase in dissolved oxygen in the anoxic
tank, which hinders the denitrification reaction. Tanaka and Emori (1993) reported
that the maximum non limiting denitrification recycle ratio (r = QR/Q) is about 2,
and the maximum nitrogen removal efficiency attainable is about 70%.
The only type of nitrifying AS reactors with neither external organics addition
nor mixed liquor recycling is a combined carbon oxidation nitrificationdenitrification using an endogenous carbon source
(see Fig.lc). The
disadvantages of such a system are an extremely large reactor volume (HRT 19 -
25 h) and incomplete total nitrogen removal (efficiency of 87%) (Tanaka and
Emori 1993).
Thus, investigating advanced wastewater treatment processes, three main
problems of biological nitrogen removal should be considered:
1. Relatively large reactor volume.
2. Incomplete nitrogen removal.
3. Expenses for electron donor supply by means of external carbon source
addition and increased excess sludge quantity or large MLSS recycling.
2.3 Alternative Biological Nitrogen Removal Processes
2.3.1 Naturally Attached Cells
Use of immobilization on solid media increases the retention time of the biomass,
which is important for slow-growing bacteria such as nitrifiers (Shieh and Keenan
1986). Two types of fixed film processes have been used frequently for
nitrification: trickling filters and rotating biological contactors (RBC). The
biomass, which accumulates on the media surface, consists of both heterotrophic
and autotrophic bacteria. If the wastewater contains both biodegradable organic
matter and ammonia, most of the film will contain heterotrophic organisms,
because of their higher growth rate, and little or no nitrification will occur.
Therefore, the nitrification is reached after BOD removal below 15mgL· 1
(Eckenfelder and Argaman 1991). Both trickling filter and RBC have a limited
capacity because of the relatively low specific surface area of the support (100 -
500 m 2 m- 3 ).
High-rate biofilm reactors have a much higher specific surface area
(2000 m 2 m-\ usually accomplished using small solid particles (Tijhuis et al.
1992). Examples of compact systems are the packed-bed and fluidized-bed
reactors. A reactor incorporated with the fibrous packing materials effectively
removed (95-97 %) COD and nitrified up to 85% of the NJL-N available, when
tested with synthetic wastewater, having an average COD of958 mgL- 1 and