Classification of Nitrification Systems

Nitrification systems within activated sludge processes may be classified according to the degree of separation of cBOD and nBOD oxidation or removal. The removal of cBOD and nBOD may occur in the same aeration tank or in separate aeration tanks. If cBOD and nBOD removal occur in the same aeration tank, the treatment system is termed a "single-stage" system (Figure 20.1). If cBOD and nBOD removal occur in separate aeration tanks, the treatment system is termed a "separate-stage" or "two-stage" system (Figure 20.2). The activated sludge processes most commonly used for nitrification are complete mix, plug-flow, extended aeration, and the oxidation ditch.

Although nBOD removal can be accomplished in the same aeration tank as is used for cBOD removal (single-stage system), nitrification in a separate tank (two-stage system) allows greater process

Figure 20.1 Single-stage nitrification system. In a single-stage nitrification system, an aeration tank or a series of aeration tanks is used to oxidize cBOD and nBOD. In a single-stage system, oxidation of cBOD and oxidation of nBOD occur in the same aeration tank.

A

. \

V

Clarifier

V

Clarifier

Aeration Tank Oxidation of cBOD only

Aeration Tank Oxidation of nBOD only

Figure 20.2 Two-stage nitrification system. In a two-stage nitrification system, an aeration tank or series of aeration tanks is used to oxidize cBOD; another aeration tank or series of aeration tanks is used to oxidize nBOD. In a two-stage system, oxidation of cBOD and oxidation of nBOD occur in different aeration tanks.

Figure 20.2 Two-stage nitrification system. In a two-stage nitrification system, an aeration tank or series of aeration tanks is used to oxidize cBOD; another aeration tank or series of aeration tanks is used to oxidize nBOD. In a two-stage system, oxidation of cBOD and oxidation of nBOD occur in different aeration tanks.

control. In a two-stage system each aeration tank can be operated independently, and many wastes that are inhibitory or toxic to nitrifying bacteria may be removed or reduced in concentration in the first aeration tank or cBOD removal stage. It is commonly accepted that a cBOD of 40 to 50 mg/l fed to the second aeration tank or nitrification tank can be tolerated by nitrifying bacteria.

The system used for nitrification depends on a number of factors including (1) plant design, (2) whether seasonal or year-round ammonia discharges requirements are to be satisfied, (3) range of temperatures, (4) desired effluent concentration for ammonium ions, (5) other effluent, water quality requirements, and (6) costs. A comparison of advantages and disadvantages of single-stage and two-stage nitrification systems is presented in Table 20.1. Due to the difficulty in achieving complete nitrification year round in more northern geographic locations or temperate regions, it may become mandatory to accomplish nitrification in a two-stage system.

TABLE 20.1 Comparison of Single-Stage and Two-Stage Nitrification Systems

Factor

Single-Stage

Two-Stage

System

System

Capital Cost

Smaller

Larger

Difficulty in terminating nitrification

Greater

Smaller

Operational costs

Smaller

Greater

Operational problems

Smaller

Greater

Process control requirements

Smaller

Greater

Sludge production

Smaller

Greater

Susceptibility to changes in nBOD loading

Greater

Smaller

Susceptibility to changes in toxic loading

Greater

Smaller

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