I

Stabilization Reactor _i_I_i_L

Fractional Volume in Contact Reactor

Figure 7.27 Effect of the relative volumes of the two reactors on the steady-state oxygen requirement in each reactor of the CSAS system described in Figure 7.20. The total system volume was constant at 250 m . SRT = 10 days. The solid curves represent the stabilization (first) reactor and the dashed curves the contact (second) reactor.

10 we will review some general rules that can be used by the engineer designing such a system.

7.5 MODIFIED LUDZACK-ETTINGER PROCESS 7.5.1 Description

All of the systems considered so far in this chapter have been totally aerobic. As a consequence, no significant denitrification occurs, and thus no nitrogen is removed; it is simply transformed from ammonia-N to nitrate-N. If one wished to add a de-nitrification reactor to any of the systems studied previously, it would be necessary to add an electron donor because by the time the SRT is sufficiently long to allow oxidation of the ammonia-N to nitrate-N, all of the soluble organic substrate has been degraded. As a consequence, the only source of organic matter to serve as the electron donor for denitrification is through hydrolysis of particulate substrate entrapped in the MLSS or released through biomass death and lysis. Hydrolysis is a slow reaction, however, and very large anoxic bioreactors would be needed to achieve even partial denitrification in this manner. Adding organic matter, such as methanol, to the bioreactor would overcome this problem, but adds to the system's operating costs.

Ludzack and Ettinger" reasoned that it should be possible to use the readily biodegradable substrate in the wastewater itself as an electron donor to achieve partial denitrification if there was a way to bring the nitrate formed in the reactor system back to a point where the substrate was available. They achieved this by separating the bioreactor into two compartments, with the first receiving no aeration, and by pumping a stream of mixed liquor from the actively nitrifying aerobic zone back to the anoxic zone to carry nitrate to it. This system is referred to as the modified

Schematic Oxidation Ditch

Figure 7.28 Schematic diagram of two CSTRs in series with all influent and all biomass recycle to the first reactor, in which the first reactor is anoxic and receives MLR flow from the second, which is aerobic. Although not shown, solids wastage is directly from both reactors. This configuration simulates the MLE process.

Figure 7.28 Schematic diagram of two CSTRs in series with all influent and all biomass recycle to the first reactor, in which the first reactor is anoxic and receives MLR flow from the second, which is aerobic. Although not shown, solids wastage is directly from both reactors. This configuration simulates the MLE process.

Ludzack-Ettinger system to differentiate it from some of their earlier work. Although many other systems have since been developed within which both nitrification and denitrification occur with greater efficiency,1"2' the MLE system represents one of the simplest within which both processes take place. Thus, it provides a convenient system with which to investigate through modeling the effects of various system variables on both processes.

The system chosen to simulate the MLE process is shown schematically in Figure 7.28. The bioreactors are each 125 m1, as in the last system, but all influent flow and biomass recycle enter the first one, which is anoxic with a dissolved oxygen concentration of zero. The second bioreactor is maintained at an oxygen concentration of 2.0 mg/L, as has been done in all previous simulations. To provide nitrate for anoxic growth of the heterotrophic bacteria in the first bioreactor, mixed liquor from bioreactor 2 is pumped to bioreactor 1 at a rate of 2000 m'/day, or twice the influent flow rate. To distinguish this stream from the biomass recycle stream, which has a higher MLSS concentration and is at a lower flow rate, it will be called the mixed liquor recirculation (MLR) stream.

7.5.2 Effect of SRT on Steady State Performance

The effects of SRT on the concentrations in the second bioreactor of the MLE system are shown by the solid curves in Figure 7.29. For comparison, the concentrations in a single CSTR with a volume of 250 m1 are shown as the dashed curves. As expected, the most obvious difference between the two systems is the nitrate-N concentration, which is much lower in the MLE system due to the denitrification in the first bioreactor. There are some other differences that should be recognized, however.

Examination of the ammonia-N, nitrate-N, and autotrophic biomass curves shows that a longer system SRT is required for the onset of nitrification in the MLE system. This is because nitrifiers can grow in only half of the system volume, the second half where dissolved oxygen is present. As discussed in Section 6.5.2, the aerobic SRT is recognized as the variable of importance in determining the growth of autotrophic bacteria in systems containing anoxic zones. Since the mass of biomass in the aerobic zone is half the total mass in this MLE system, the aerobic SRT

Ammonia Oxygen Nitrate Nitrifiers
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