Zoning

Desired or intentional denitrification is usually achieved in an activated sludge process by establishing an anoxic zone or an alternating series of anoxic zones (Figure 32.1). The anoxic zone is the first corn-

return activated sludge

Figure 32.1 Alternating series of anoxic zones. In an activated sludge process that has the capability to operate in plug-flow, it is a common practice to control denitrification in the secondary clarifier by using the first aeration tank as an anoxic zone. In this practice, return activated sludge and nitrate ions within the secondary clarifier are removed from the clarifier as rapidly as possible and discharged with the aeration tank influent ( primary clarifier effluent) to the first aeration tank. This tank is mixed but not aerated. In the first aeration tank the returned solids (bacteria), returned nitrate ions, and the influent soluble cBOD establish an anoxic condition. Depending on the flow through the first aeration tank, the anoxic zone may vary from one-half to several hours. In the plug-flow mode of operation addition anoxic zones may be placed downstream of aeration tanks that produce nitrate ions. The additional anoxic zones reduce the quantity of nitrate ions discharged to the secondary clarifier.

\ °

DO = 5.0 mg/1 o

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'O NOr = 10.0 mg/1

o

\ oyT

o °

N03"= 10.0 mg/1

o

o

o

o

o

DO = 0.5 mg/1

NO3- = 3.0 mg/1

o

O

o

N03" = 1.0 mg/1

Figure 32.2 Anoxic zone at the bottom of an aeration tank. By carefully regulating the amount of surface aeration of a tank, it is possible to produce an oxic zone at the top of the tank and an anoxic zone at the bottom of the tank. These zones can be established because oxygen is poorly soluble and motile in water, while nitrate ions are highly soluble and motile in water. With the lack of adequate dissolved oxygen at the bottom of the tank, cBOD is degraded with the use of nitrate ions. Therefore simultaneous nitrification and denitrification occur in the same tank by carefully regulating the amount of surface aeration of a tank.

partment of the treatment process that denitrifies. Nitrite ions and nitrate ions used for denitrification in the anoxic zone are produced in the aeration tank. The ions enter the anoxic zone through the RAS or aeration tank e¿uent.

Mixing is provided in the anoxic zone. Slow speed subsurface mixers are used to keep the biomass in suspension and to ensure rapid depletion of any dissolved oxygen.

The size of the anoxic zone provides a retention time of 0.5 to 1 hour. This retention time achieves about 30% to 40% removal of

primary clarifier

j

1

2 o

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secondary clarifier

Figure 32.3 Denitrification in plug-flow mode of operation. In plug-flow mode of operation the first aeration tank is used as a denitrification tank. It is the first tank in plug-flow mode of operation that receives the soluble cBOD required to drive denitrification in the presence of nitrate ions and denitrifying bacteria. The nitrate ions and denitrifying bacteria enter the first aeration tank through the solids returned to the tank from the secondary clarifier. The first aeration tank is not aerated when it is used as a denitrification tank.

Figure 32.3 Denitrification in plug-flow mode of operation. In plug-flow mode of operation the first aeration tank is used as a denitrification tank. It is the first tank in plug-flow mode of operation that receives the soluble cBOD required to drive denitrification in the presence of nitrate ions and denitrifying bacteria. The nitrate ions and denitrifying bacteria enter the first aeration tank through the solids returned to the tank from the secondary clarifier. The first aeration tank is not aerated when it is used as a denitrification tank.

nitrogen. Because the rate of denitrification declines with increasing retention time, larger tanks providing greater retention times are not economical to operate. However, anoxic retention times of 1 to 2 hours are needed to control the undesired growth of filamentous organisms.

Although the anoxic zone is most effective if it is in a separate tank, denitrification can occur in a tank that is required to perform an additional biological role, such as nitrification. Denitrification stops in the anoxic zone if the denitrifying bacteria are exposed to dissolved oxygen. Nitrification stops if the dissolved oxygen concentration in contact with the nitrifying bacteria is too low.

There are several means to achieve zoning. Surface aeration of a mixed liquor tank can be adjusted to provide an aerobic zone at the surface of the tank and an anoxic zone at the bottom of the tank (Figure 32.2). Denitrification also can be achieved in a plug-flow mode of operation (Figure 32.3). In this mode of operation the an-oxic zone is placed in the front of the system. Here the zone has the largest amounts of nitrite ions and nitrate ions and the largest amount of soluble cBOD.

Denitrification can be achieved in oxidation ditches (Figure 32.4). Oxidation ditches have a high degree of natural internal recirculation that makes denitrification affordable and inexpensive compared to the plug-flow mode of operation. An oxidation ditch can be regulated to establish discrete aerobic zones and anoxic zones along the channels.

Influent X Effluent

Figure 32.4 Denitrification in an oxidation ditch. The design and internal mixing of an oxidation ditch permit the establishment of aerated and non-aerated zones with the oxidation ditch for nitrification and denitrification. Aerated zones that produce nitrate ions are followed by non-aerated zones that use the nitrate ions for degradation of soluble cBOD.

Influent X Effluent

Figure 32.4 Denitrification in an oxidation ditch. The design and internal mixing of an oxidation ditch permit the establishment of aerated and non-aerated zones with the oxidation ditch for nitrification and denitrification. Aerated zones that produce nitrate ions are followed by non-aerated zones that use the nitrate ions for degradation of soluble cBOD.

Zoning also can occur in an aeration tank when the tank is poorly mixed. Under this condition anoxic pockets are produced where denitrification can occur. Zoning can be mimicked in sequential batch reactors (SBR) by extending the aeration time to achieve nitrification and extending the settling time to achieve denitrification.

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