Ar

RAS is recycled to the anoxic zone while a denitrified AR stream is directed from the end of the anoxic zone to the anaerobic zone. However, several differences exist between the VIP and UCT processes. In the VIP process, all zones are staged, i.e., they consist of at least two (and as many as six) completely mixed cells in series; the process is designed as a high-rate, i.e., a short SRT, process to maximize phosphorus removal; and the NR is mixed with the RAS for recycle to the anoxic zone. Staging of the various zones provides several benefits. Increased efficiency is obtained with the use of a tanks-in-series hydraulic regime, just as it is for activated sludge systems. A tanks-in-series configuration also allows the respective environmental conditions to be more completely established in each zone. Finally, staging may allow the selection of microorganisms with increased reaction rates, since each zone functions more like a selector. High-rate operation is accomplished by minimizing the SRT, and hence the HRT, in each reactor zone. The combined SRT of the anaerobic and anoxic zones is generally 1.5 to 3 days, while the anaerobic and anoxic HRTs are typically 60 to 90 minutes each. The aerobic zone is sized just large enough to achieve a sufficient degree of nitrification to meet process objectives. The AR flow rate is typically 50 to 100% of the influent flow rate, while the NR flow rate is typically equal to the influent flow rate. Mixing of the NR with the RAS allows deoxygenation of the NR by the oxygen deficient RAS before it is added to the anoxic zone. This improves sludge settling characteristics by reducing the oxygen loading on the anoxic zone and minimizing the growth of low DO filamentous bacteria.""

Many other biological nitrogen and phosphorus removal processes have been developed and have received some full-scale use. Some minimize nitrate-N recycle to the anaerobic zone by allowing the RAS to denitrify under endogenous conditions. Increasing the residence time of the RAS in the clarifier, as described in connection with the AVO process, is one example. Two others are the Johannesburg " and the R-D-N'" processes. Processes that use oxidation ditches include the VT2~"" and the Biodenitro" processes. The operation of SBRAS processes has also been modified to obtain both nitrogen and phosphorus removal."1"' Finally, full-scale facilities have been modified by simply turning off aerators and/or by other simple modifications to create the zones necessary to achieve nitrogen and phosphorus removal."'"' The potential to enhance the removal of nutrients by similar modifications of activated sludge systems appears to be limited only by the imagination and understanding of the process fundamentals by plant designers and operators.

Fermentation of primary sludge to generate an influent stream high in VFAs for use in systems that remove both nitrogen and phosphorus is a recent, and exciting, development." It offers significant potential for enhancing the performance and improving the reliability of BNR systems.'0""4 The impact of fermentation on the performance of BNR facilities is described in Section 11.2.3, while the basic principles of sludge fermentation and the design of fermenters are discussed in Chapter 13.

11.1.4 Comparison of Process Options

Table 11.2 summarizes the primary benefits and drawbacks of several BNR systems. The MLE process offers good nitrogen removal, moderate bioreactor volume requirements, alkalinity recovery, good sludge settleability, reduced oxygen requirements compared to traditional activated sludge systems, and simple control. However, a high level of nitrogen removal cannot generally be achieved, as discussed previously. Practical MLR flow rates limit nitrate-N removal to between 60 and 85%. As illustrated in Figure 7.36, this constraint does not exist for the four-stage Bar-denpho process, which includes a second anoxic zone. Performance data from full-scale wastewater treatment plants demonstrates this difference." Processes with one anoxic zone typically produce effluents with total nitrogen concentrations ranging between 5 and 10 mg/L as N, while processes with two anoxic zones typically produce effluents with total nitrogen concentrations ranging between 1.5 and 4 mg/ L as N.57 However, this improved performance is at the expense of a larger bioreactor volume. Another benefit of the MLE and four-stage Bardenpho processes is alkalinity production by denitrification in the initial anoxic zone, which off-sets some of the alkalinity consumed by nitrification in the aerobic zone. Denitrification also reduces the oxygen requirement in the aerobic zone because nitrate-N serves as the electron acceptor during oxidation of some of the biodegradable organic matter, thereby removing the need for oxygen to do so. These effects are discussed in Sections 6.3, 6.4, 7.5, and 7.6, and illustrated in Figure 7.30. The reduced power requirements for oxygen transfer in the aerobic zone off-set some or all of the energy required to mix the anoxic zone and to pump the MLR. Good sludge settleability can be obtained with both the MLE and four-stage Bardenpho processes because the initial anoxic zone acts as a selector to control the growth of filamentous bacteria, as discussed previously. The incorporation of an anoxic zone in the bioreactor can also minimize denitrification problems in the secondary clarifier by reducing nitrate-N concentrations, making it impossible to generate sufficient nitrogen gas to cause sludge flotation.

Systems that encourage denitrification in an aerobic bioreactor provide the benefits of alkalinity recovery and oxygen requirement reduction associated with the MLE and four-stage Bardenpho processes. In fact, the total energy requirements in such systems are smaller since mixing and MLR facilities are generally not required. Some existing activated sludge facilities can easily be retrofitted. However, relatively large bioreactor volumes may be required since the microbial environment is not optimized, control can be more complex to restrict oxygen input to allow the anoxic regions to develop, and poor sludge settleability may result due to the growth of Group IV filamentous bacteria.

Table 11.2 Biological Nutrient Removal Process Comparison

Process

Benefits

Drawbacks

Nitrogen Removal

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