mixed liquor from the aerobic zone (referred to as nitrified mixed liquor recirculation, or NR) is also directed to the anoxic zone to increase the overall degree of nitrogen removal. A separate stream of denitrified mixed liquor from the end of the anoxic zone (referred to as anoxic mixed liquor recirculation, or AR) is recirculated to the anaerobic zone to provide the microorganisms needed there. Because mixed liquor, rather than RAS, is recycled to the anaerobic zone, the MLSS concentration in it is lower than the MLSS concentration in the remainder of the bioreactor. As a consequence, somewhat longer anaerobic zone HRTs (1 to 2 hr) are needed to achieve the desired SRTs. Anoxic and aerobic SRTs and HRTs are similar to those used in the MLE process. Both the anoxic and the nitrified mixed liquor recirculation rates are typically two times the process influent flow rate. One variation of the UCT process involves division of the anoxic zone into two compartments, one that receives and denitrifies the RAS and another that receives and denitrifies the NR. Referred to as the modified UCT (MUCT) " process, it is illustrated in Figure 11.12. The upstream anoxic zone, which receives RAS only, is oversized to ensure that complete denitri-fication occurs there and that essentially no nitrate-N is recirculated to the anaerobic zone. This makes control of the NR to the downstream anoxic zone less critical, and relatively high NR rates can be used to ensure that the full denitrification capability of this zone is used.
The term VIP stands for the Virginia Initiative Plant. As illustrated in Figure 11.13, it consists of anaerobic, anoxic, and aerobic zones and, like the UCT process.
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