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COD for each mg of phosphorus that must be removed. Little COD is lost during fermentation of readily biodegradable organic matter to VFAs. so we would expect to get a mg of VFA COD for each mg of readily biodegradable COD fermented. To be conservative, assume that lf).7 mg of readily biodegradable COD will be needed to remove a mg of phosphorus after fermentation. Since the wastewater contains 115 mg/L of readily biodegradable COD, the maximum phosphorus removal capability is 115 + 10.7 = 10.7 mg/L. Furthermore, since the wastewater contains 7.5 mg/L of phosphorus, the system would be expected to remove essentially all of it in winter. Thus, we would expect the effluent soluble phosphorus concentration to be low.

Another way of addressing this question is with the BOD_/AP ratios given in Table 11.4. Since the A/O™ process is considered to be a high efficiency process when nitrification is not occurring, the BODJAP ratio will range from 15 to 20 mg BODJmg P. Again, to be conservative, assume that the worst case applies, i.e, a ratio of 20. Since the wastewater contains 155 mg/L of BOD, (Table E8.4), the maximum phosphorus removal capability is 155 -h 20 = 7.8 mg/L. This, too, suggests that excellent phosphorus removal should occur, giving a very low effluent soluble phosphate concentration.

e. What effluent soluble phosphorus concentration would be expected under summer operating conditions?

During the warmest part of the summer, it is likely that nitrification will occur in spite of the reduction in the SRT to three days. In Example 10.3.3.2, the concentration of nitrogen available to the nitrifiers at an SRT of 3 days was calculated to be 30.5 mg/L as N. Thus, we can expect the maximum amount of nitrate-N formed to be 29.9 mg/L as N (Eq. 11.5). The amount of nitrate-N recycled to the anaerobic zone depends on the RAS How rate. Typically, the RAS How rate is one-half the influent flow rate, or n is 0.5. Using that in Eq. 11.8 with 3 = 0 tells us that one-third of the nitrate-N formed will be returned, or an effective concentration of 10 mg/L as N. Since about 6 mg of COD will be required to denitrify each mg of nitrate-N, the recycle of the nitrate-N will reduce the readily biodegradable COD by 60 mg/L, to a value of 55 mg/L. If only 7.5 mg of readily biodegradable COD were required to remove a mg of phosphorus, the process would be capable of removing 55 -h 7.5 = 7.3 mg/L of P, which would be sufficient to remove almost all of the phosphorus. However, if 10.7 mg of COD were required, the system could only remove 55 + 10.7 = 5.1 mg/L of P. Although a small amount of phosphorus would be used in biomass synthesis, this suggests that a significant concentration of residual phosphorus would be left in the summer. The exact amount could only be determined with pilot studies. This analysis did not consider the slowly biodegradable substrate because relatively little of it will be available at the short SRTs involved. Again, onlv pilot studies or simulations will reveal whether any slowly biodegradable substrate would become available.

As in Part d, another way of addressing this question is with the BOD . AP ratio From Table 11.4 we see that the ratio is 20-25 mg BOD/mg P for an A/O™ process in which nitrification is occurring. A ratio of 20 would lead us to believe that almost all of the phosphorus could be removed, as we saw in Part d. If the ratio were 25, we would expect the removal to be 155 ^ 25 = 6.2 mg/L as P, which would leave a significant residual. Thus this analysis, like the previous, gives mixed results, with one assumption suggesting full phosphorus removal while the other suggests incomplete removal. Therefore, pilot studies should be performed to more accurately define the capability.

I. What is the phosphorus content of the MLSS in the winter?

Since all of the phosphorus will be removed via the waste solids and the solids are homogeneous throughout the system, the phosphorus content of the ML.SS can be determined by calculating the phosphorus content of the waste solids. The mass flow rate of phosphorus ill the influent to the process is (7.5 g/m' as P) (40,000 m'/day) ^ 1,000 g/kg = 300 kg/day of P. As discussed in Part d above, this phosphorus will be almost completely removed. Therefore, to be conservative assume that the effluent soluble PO,-P concentration is negligible. Therefore, the mass of phosphorus removed per day is 300 kg. From Table f-11.1, the solids wastage rate in winter is 5.330 kg/day. Therefore, the phosphorus content of the mixed liquor is 300 kg P day : 5,330 kg TSS/day = 0.056 mg P/mg TSS.

g. What will be the concentration of particulate phosphorus in the effluent if the effluent suspended solids concentration is 10 mg/'L as TSS? Since the phosphorus content of the solids is 0.056 mg P/mg TSS and the effluent suspended solids concentration is 10 mg/L as TSS. the effluent particulate phosphorus concentration is 0.56 mg/L as P. The total phosphorus concentration will be the sum of this value plus any soluble phosphorus that is present.

Sidestream Processes. Sidestream processes such as Phostrip® can be designed in a similar fashion. The primary differences are that the anaerobic zone is created in a sidestream stripper and that two phosphorus removal mechanisms arc provided, one through the waste solids like other BPR processes and the other in the stripper overflow. As discussed in Section 11.1.3, the low loading of biodegradable organic matter into the stripper in the Phostrip® process results in the need to retain the solids there for 8 to 12 hours to allow for the formation of sufficient quantities of VFAs by fermentation. The uptake of these VFAs results in substantial phosphate release, which is subsequently elutriated and removed in the stripper overflow stream. Mass balance procedures can be used to quantify the mass of phosphorus removed by stripping and elutriation. This is added to the phosphorus contained in the waste solids to give the total mass of phosphorus removed in the process. As discussed in Section 11.1.5, although the Phostrip® process is of historical importance, its use is relatively limited today, and this trend is expected to continue. Consequently, further discussion of the design procedures will not be given here. They are available elsewhere/ "

11.3.3 Processes that Remove both Nitrogen and Phosphorus

As discussed in Section 11.1.3, a large number of processes are available that remove both nitrogen and phosphorus. This would be of significant concern if unique design procedures were required for each. Fortunately this is not the case, and all processes that remove both nitrogen and phosphorus can be designed using the procedures already presented in Sections 11.3.1 and 11.3.2. Because those procedures are approximate, so is the procedure for designing systems to remove both nitrogen and phosphorus. The degree of interaction among the different components of the microbial community in a BNR system makes it impossible to develop exact analytical procedures. However, just as for systems that remove nitrogen or phosphorus, they are sufficiently exact to provide designs that can be verified through pilot studies and simulation. Just a few additional factors must be considered when designing a process that removes both nutrients.

Because complete nitrification must occur in the course of removing nitrogen, one consideration of particular importance is the minimization of nitrate-N recycle to the anaerobic zone. As discussed in Section 11.1.2, nitrate-N recycle adversely impacts phosphorus removal by allowing increased growth of non-PAO heterotrophs and by reducing fermentation in the anaerobic zone. Elimination of nitrate-N recycle is primarily a process selection issue, and several processes have been developed that minimize or eliminate it. The issues involved in process selection are discussed in Section 11.1.4 and the benefits and drawbacks of the alternative processes are presented in Table 11.2.

Another consideration in adapting the approaches of Sections 11.3.1 and 11.3.2 to the design of a process to remove both nutrients is the impact of the upstream anaerobic zone on denitrification in the downstream anoxic zone. It might be thought that the removal of readily biodegradable substrate in the anaerobic zone would result in reduced rates of denitrification in the anoxic zone because of the need to use slowly biodegradable substrate. Experience indicates that this is not the case.ls It is hypothesized that fermentation of slowly biodegradable organic matter in the anaerobic zone results in the formation of readily biodegradable substrate, which produces a rapid rate of denitrification in the anoxic zone. While additional research is needed to elucidate the mechanism, it is clear that similar anoxicxzone sizes can be used in processes that remove nitrogen alone and in processes that remove both nitrogen and phosphorus.

A final consideration is that some systems have different MLSS concentrations in the anaerobic zone than in the anoxic and aerobic zones. Examples are the UCT and VIP processes, shown in Figures 11.11 and 11.13, respectively, which add the RAS flow to the anoxic zone and provide biomass to the anaerobic zone by recirculating denitrified anoxic mixed liquor (AR) from the anoxic zone to the anaerobic zone. Although this is done to minimize nitrate-N recirculation to the anaerobic zone, it will result in lower MLSS concentrations in the anaerobic zone. The MLSS concentration in the anaerobic zone, XM , ANA, can be estimated from the MLSS concentration in the anoxic zone, XN1, ANX, by performing a mass balance on the anaerobic zone. Neglecting any change in the MLSS concentration that occurs in the zone, it can be shown that:

where 8 is the AR rate expressed as a fraction of the influent flow rate.

As with the other BNR processes, design begins with selection of the SRTs for the three environments. As discussed in Section 11.2.1, anaerobic SRTs can be reduced in processes that remove both nitrogen and phosphorus in comparison to the anaerobic SRT needed in a process that removes phosphorus alone. This is because readily biodegradable substrate is removed in both the anaerobic and anoxic zones of a process that removes both nutrients. Consequently, the anaerobic zone need not be relied upon to remove all of this material. However, reduction of the anaerobic SRT will result in reduced fermentation and reduced phosphorus removal capability for the process. Procedures are being developed to assess this trade-off, but at this time it must be done using judgment, experience, and simulation. Until more experience is gained, it is prudent to provide the flexibility to adjust the relative sizes of the anaerobic and anoxic zones, thereby adjusting their SRTs. Because it is the input of nitrate-N that distinguishes an anoxic zone from an anaerobic zone, one way to do this is to construct those zones as several completely mixed tanks in series, as in the VIP process (Figure 11.13), and to provide several possible discharge points for the RAS and NR. In this way, a tank can be changed from anaerobic to anoxic, and vice versa, simply by moving the RAS and NR input point. In addition, both mixing and oxygen transfer equipment can be installed in the initial sections of the aerobic zone to provide the flexibility to extend the anoxic SRT should the need exist. As discussed in Section 11.1.3, staging of the bioreactor provides several advantages including improved reaction rates and the selection of microorganisms with higher maximum specific growth rates. Thus, incorporation of staging not only provides operational flexibility, but also enhances overall process performance.

Just as with biological nitrogen removal processes, systems that remove both nitrogen and phosphorus can be designed for a widely varying degree of nitrogen removal. The principles presented in Section 11.3.1 can be directly applied here, depending on effluent total nitrogen objectives. The minimum degree of nitrogen removal required is that which will eliminate nitrate-N recycle to the anaerobic zone.

The design proceeds in exactly the same way that the other designs have proceeded. First, the SRTs of the three zones must be selected using the criteria presented above and in the preceding two sections, as well as in Section 11.2.1. This may require iteration, as discussed in Section 11.3.1. Using the total SRT, the mass of MLSS in the system is estimated with Eq. 9.11, giving the mass of MLSS in each zone by application of Eqs. 11.1-11.3. The RAS and NR flow rates are then selected to give the desired degree of nitrogen removal through the anoxic zone, using the principles articulated in Section 11.3.1 for an initial anoxic zone. The oxygen requirement in the absence of denitrification can be calculated with Eqs. 9.13 and 10.16, and corrected for denitrification to determine the net oxygen requirement as illustrated in Example 11.3.1.2. An MLSS concentration is then selected for the aerobic zone following consideration of the requirements for sedimentation, mixing, and oxygen transfer. This allows the total volume of the aerobic zone to be calculated. Since the MLSS concentration is the same in the anoxic zone as in the aerobic

zone, the chosen MLSS concentration can also be used to calculate the volume of the anoxic zone from the mass of MLSS present. A denitrified anoxic mixed liquor recirculation rate can then be selected, thereby setting the anaerobic MLSS concentration by Eq. 11.19. That, in turn is used to calculate the volume of the anaerobic zone from the known mass of MLSS present. Higher AR rates will result in higher anaerobic MLSS concentrations, thereby reducing the volume of the anaerobic zone. Thus, the opportunity exists for selecting a least-cost combination. Each zone can be subdivided into stages, if desired. If an effluent is desired with a lower nitrate-N concentration than can be accomplished with only an initial anoxic zone, then a second anoxic zone and a second aerobic zone can be added using the approach from Example 11.3.1.4.

### Example 11.3.3.1

A facility to remove both nitrogen and phosphorus is to be designed to treat the wastewater considered throughout Sections 10.3 and 11.3, which contains 7.5 mg/L of phosphorus. Process objectives include oxidizing ammonia-N, maximizing phosphorus removal, and obtaining a moderate degree of nitrogen removal to reduce the net oxygen requirement and alkalinity consumption. The VIP process was selected based on these objectives.

a. What SRTs are appropriate for this application?

As discussed in Part a of Example 11.3.1.1, the required aerobic SRT is 12.0 days. Although an anaerobic SRT as low as 0.5 days could be used, a value of 1.0 is selected to allow some fermentation to occur. An anoxic SRT of 1.5 days is selected to ensure complete removal of readily biodegradable organic matter, which will result in good sludge settling characteristics. Thus, the total SRT will be 1.0 + 1.5 + 12.0 or 14.5 days.

b. If the design MLSS concentration in the anoxic and aerobic zones is 3,000 mg/L as TSS and the AR rate is equal to the influent flow rate, what are the sizes of the bioreactor and its individual components, as well as the HRTs? First, calculate the mass of MLSS in the system at 15°C using Eq. 9.11 and an SRT of 14.5 days. Use of the procedure demonstrated in Example 10.3.3.3 gives a value of 60,000,000 g. Application of Eqs. 11.1-11.3 gives the mass in each zone:

Since the MLSS concentration in the aerobic and anoxic zones is 3,000 mg/L, the volumes of those zones are 16,600 and 2,100 m\ respectively.

The MLSS concentration in the anaerobic zone is obtained by application of Eq. 11.19. Since the AR flow rate is equal to the influent flow rate, 8 has a value of 1.0. Therefore:

Using this value gives a volume for the anaerobic basin of 2,700 m\

The HRTs of each zone can be calculated using the design flow of

40,000 m'/day. The volumes and resulting HRTs are summarized in Table El 1.2. Note that the size and HRTof the anaerobic zone are larger than those of the anoxic zone, even though its SRT is smaller. This is a result of the lower MLSS concentration in the anaerobic zone.

Appropriate values for the anaerobic and anoxic zone SRTs are only approximately known, and therefore staging of these zones is desirable, as discussed in Section 1 1.1.3 and above. Consequently, use a total volume of 4,800 m' for the anaerobic and anoxic zones and configure the volume as 5 stages, each with a volume of 960 m\ Provide flexibility in the NR and RAS discharge locations so that either two or three of the cells can function as the anaerobic zone, with the remainder functioning as the anoxic zone. Furthermore. because of the benefits of staging the aerobic zone with respect to nitrification, divide it into four cells, each with a volume of 4,150 m'.

c. What total nitrogen removal can be expected, and what NR flow rate is appropriate for this application?

As shown in Part b of Example 1 1.3.1.2, an anoxic selector with an SRT of 1.5 days is expected to remove 837 kg NO,-N/day based on the availability of readily biodegradable substrate. As discussed above, experience indicates that we should gel about the same amount of denilrification in this application in spite of the presence of the anaerobic zone preceding the anoxic zone. It was also shown in Pari b of Example I 1.3.2.2 that the sum of the RAS and MLR rates should be 163% of the influent How rate to provide sufficient nitrate-N to remove all of the readily biodegradable substrate. Thus, it is appropriate to provide an equivalent amount of RAS and NR flow in this case. However, because the goal is to fully denitrify in the anoxic zone to prevent recirculation of nitrale-N to the anaerobic zone, considerable flexibility should be provided in the possible pumping rates to ensure that this is accomplished.

d. How much phosphorus should this process be able to remove?

Because the PAOs are protected from nitrate by the process configuration, the system should be capable of removing equal quantities of phosphorus in winter and summer. Thus, the amount of phosphorus removal should be similar to that estimated for the A/O™ process in the winter in Part d of Example 11.3.2.1. Thus, essentially all of the influent phosphorus should be removed.

Table E11.2 Volumes and Hydraulic Residence Times for the Zones in the VIP Process of Example 11.3.3.1

Zone

Volume, m'

HRT, day

Anaerobic

2,700