ABSTRACT
Each of the systems previously considered in this section was developed to allow use of the biodegradable substrate in the wastewater as the electron donor for denitrification. In some cases, however, it may be necessary to denitrify the effluent from a nitrifying activated sludge system, which will be very low in organic matter. Such a stream can be readily denitrified in a separate stage suspended growth denitrification system13'66'68'84 if a supplemental electron donor, such as methanol, is added, as illustrated in Figure 11.7. The electron donor is added in proportion to the nitrate-N content of the influent as it enters the anoxic zone, as discussed in Section 6.4. An aerobic zone is required to strip the entrained nitrogen gas and to prepare the biomass to settle in the clarifier. It will also allow metabolism of any excess electron donor that was not removed in the anoxic zone, thereby maintaining a high effluent quality. The SRT for such systems is typically between 3 and 5 days. Because of the low yields typically observed with substrates like methanol, the bioreactor HRT can be in the 2 to 4 hr range while maintaining typical MLSS concentrations. The aerobic HRT will typically be 45 to 60 min.
