relatively high because it is driven by the readily biodegradable substrate contained in the influent wastewater. Excellent nitrification and a good degree of denitrification (down to effluent nitrate-N concentrations of 4 to 8 mg/L as N) can be obtained with this process. However, further reductions in the effluent nitrate-N concentration are limited by the requirement to recirculate nitrate to the upstream anoxic zone, as illustrated in Figures 7.29 and 7.31. Mixed liquor recirculation flow rates typically range between one and four times the influent flow rate. The anoxic SRT ranges from about 1 to 4 days, with longer values being required when the wastewater contains smaller quantities of readily biodegradable substrate and when lower effluent nitrate-N concentrations are required. Aerobic SRTs are controlled by the need to maintain stable nitrification and range from about 4 to 12 days, depending on temperature, as discussed in Sections 9.3.2 and 10.2.2. Like all activated sludge systems, the system volume, and hence the hydraulic retention time (HRT), depends on the chosen MLSS concentration because the mass of biomass in the system is fixed once the SRT has been fixed. Typically, the HRT in the anoxic zone is in the 1 to 4 hr range, whereas the HRT in the aerobic zone may range from 4 to 12 hr. As defined by Eq. 4.15, HRTs for all BNR systems are calculated on the basis of influent flow alone; they do not include recycle and recirculation flows. The anoxic zone may be staged to increase denitrification efficiency and organism selection by provision of a substrate gradient.
Further nitrogen removal can be achieved by the addition of a second anoxic zone and a small final aerobic zone as provided in the four-stage Bardenpho" process discussed in Section 7.6 and illustrated in Figure 11.5. The second anoxic zone is used to remove the nitrate-N remaining in the effluent from the first aerobic zone. Although the denitrification rate is low in the second anoxic zone, the concentration of nitrate-N entering it is low and nearly complete nitrate-N removal can be obtained at anoxic SRTs of 2 to 4 days. HRTs are typically 2 to 4 hrs. The final aerobic zone is used only to strip nitrogen gas and to oxygenate the mixed liquor prior to entering the clarifier. The size of this zone is small to minimize decay reactions, which would liberate soluble nitrogen that would then pass into the effluent. Consequently, the HRT is typically around 30 min. The first anoxic and aerobic zones are similar in size to those in the MLE process, as is the MLR rate.
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