Figure 11.14 Example relationship between the intluent BODs/P ratio and the effluent soluble phosphorus concentration. (From M. J. Tetrault et al., Biological phosphorus removal: a technology evaluation. Journal, Water Pollution Control Federation 58:823-827, 1986. Copyright © Water Environment Federation; reprinted with permission.)
tration determined by the relative concentrations of phosphorus and organic matter in the influent. A phosphorus limited wastewater is one in which more than sufficient organic matter is available to remove the phosphorus. Consequently, the effluent phosphorus concentration will generally be low when it is treated in a BPR process. Thus, a phosphorus limited wastewater is desirable when a good quality effluent must be produced.
Recognition of the concept of carbon limited wastewaters has resulted in the development of benchmark ratios expressing the amount of organic matter required to remove a unit of phosphorus by various BPR processes. Such ratios have been determined from pilot- and full-scale BPR systems operating under carbon limited conditions and have been used to characterize the capabilities of those systems." ' A commonly used ratio is the BOD, to phosphorus removal ratio (BOD,/'AP), which is calculated as:
BOD, in biological process influent TP in biological process influent—SP in biological process effluent
Phosphorus removal is quantified as the total phosphorus in the biological process influent minus the soluble phosphorus in the biological process effluent. Influent total phosphorus is used because both soluble and particulate phosphorus are acted upon. Soluble phosphorus is in the form of, or rapidly converted to, inorganic phosphate, which is the form taken up and stored by the PAOs. Particulate phosphorus is either hydrolyzed and released as soluble phosphorus or it is entrapped in the MLSS and removed in the waste solids. In either case, the particulate phosphorus affects the overall phosphorus removal by the process. Effluent soluble phosphorus is used because effluent particulate phosphorus is generally associated with suspended solids that have escaped the clarifier and are a function of the efficiency of the clarifier, not the biological process.
Table 11.4 provides typical ranges for BOD,/AP for a variety of BPR processes. For consistency in this text, values are also presented for COD/AP which were calculated from the BOD, values using Eq. 8.33. Note that a low value for the ratio
Table 11.4 BOD, and COD to Phosphorus Removal Ratios for Various BPR Processes
BOD,/AP ratio COD/AP ratio
without nitrification, VIP, UCT)
Moderate efficiency (e.g., 20-25 34-43
indicates an efficient process since little organic matter is required to remove a unit of phosphorus. Highly efficient BPR processes, such as the A/O™ process operating under nonnitrifying conditions or the VIP process, require only 15-20 mg BOD, (26-34 mg COD) to remove a mg of phosphorus. In these processes, essentially no nitrate-N is recycled to the anaerobic zone, either because it is not generated (for the nonnitrifying A/O™ process) or it is removed (for the VIP process). They are also both high-rate processes, which maximizes phosphorus uptake and waste solids production. A moderately efficient process, such as a nitrifying A/O™ or A:/0™ process, will require 20-25 mg of BOD, (34-43 mg COD) to remove one mg of phosphorus. More organic matter is required for these systems because some will be consumed by non-PAO heterotrophs in the anaerobic zone due to the nitrate-N recycled there in the RAS. The ratio will be even higher for a low efficiency process, such as a live-stage Bardenpho process operating at long SRT, which require more than 25 mg of BOD, (43 mg COD) to remove a mg of phosphorus. Thus, it can be seen that the effects of many of the factors discussed in previous sections of this chapter are quantified in the organic matter to phosphorus removal ratio.
As discussed above, a BPR process will achieve good performance if it operates under phosphorus limited conditions. This occurs when the organic matter to phosphorus ratio of the influent wastewater is greater than the BOD,/AP value for the BPR process being used, i.e., when more organic matter is available per unit of phosphorus than is required by the process to remove the phosphorus. Therefore, appropriate BPR processes can be identified for a particular application by comparing the organic matter to phosphorus ratio for the wastewater to the BOD,/AP values for candidate BPR processes and selecting those processes with appropriate removal ratios. Thus, ratios such as those summarized in Table 11.4 can be quite useful in the initial stages of process evaluation and screening.
It must be emphasized that the organic matter to nutrient ratios discussed above are for the influent to the biological treatment system, not the ratios for the influent to the entire wastewater treatment plant. This is because the ratio can be significantly altered by treatment upstream of the biological process and by recycle streams from the solids handling system.
The potential for nutrient inputs to biological processes from solids handling systems is so great that it deserves emphasis. Some solids handling unit operations result in significant ammonification of organic nitrogen contained in the waste solids applied to them. Examples include anaerobic digesters, aerobic digesters, and heat treatment systems. A liquid stream with a high ammonia-N concentration is produced when the outflows from them are dewatered. If such streams are recycled to the liquid treatment process train, they can significantly increase the ammonia-N load without increasing the organic load. Moreover, such discharges are often periodic in nature, and the resulting ammonia-N shock load can overload the bioreactor system. Such solids handling systems can also result in solubilization of removed phosphorus and its recycle back to the liquid treatment process train, particularly if the solids are held under anaerobic conditions, which cause PAOs to release phosphorus. This includes, for example, wet wells and gravity sludge thickeners, as well as anaerobic digesters. Interestingly, full-scale experience indicates that of some of the phosphorus released during anaerobic digestion of BPR waste solids can precipitate and be retained with the solids/1" "" Precipitates include struvite (MgNH,PO:). brushite
(CaHPOj• 2H^O), and vivianite [Fe:(P04), ■ H_,0]. The process designer must be aware of these potential impacts and balance the requirements of the liquid and solids processing trains to obtain an optimum treatment system.
The composition of the organic matter present in a wastewater, particularly its bio-degradability, also affects the performance of BNR processes. In the anaerobic zone. PAOs transport short chain VFAs into the cell and store them as PHAs. There are two sources of VFAs for the PAOs; they are either present in the influent wastewater or they are produced by fermentation of other readily biodegradable substrate by facultative heterotrophs. The uptake of preformed VFAs is a rapid process, while fermentation is a slower process." 7 '' Ideally a wastewater that is to be treated in a BPR system will contain a high proportion of VFAs; this will result in their rapid uptake by the PAOs and a relatively small anaerobic SRT can be used. At a minimum, a sufficiently high concentration of fermentable organic matter must be present to generate VFAs for uptake by the PAOs. It has been estimated that a concentration of at least 25 mg/L as COD of readily biodegradable substrate must be available in the anaerobic zone to generate sufficient VFAs to allow adequate biological phosphorus removal.''" " Thus, the readily biodegradable substrate concentration in the influent wastewater, particularly the VFA concentration, will significantly affect the performance of a biological phosphorus removal system.
The readily biodegradable substrate concentration of the influent wastewater will also affect the denitrification rate in an initial anoxic zone. Denitrification is rapid when readily biodegradable substrate is available, but is much slower when only slowly biodegradable substrate is present. This is because the use of slowly biodegradable substrate is controlled by the rate of hydrolysis, which is relatively slow under anoxic conditions. Consequently, if the amount of readily biodegradable substrate entering an initial anoxic zone is insufficient to remove the nitrale-N added, the anoxic zone must be large enough to provide time for the hydrolysis of slowly biodegradable substrate. Hydrolysis and fermentation of slowly biodegradable substrate in an upstream anaerobic zone can produce readily biodegradable organic matter that can pass into an anoxic zone and produce a high rate of denitrification there.IS
Significant fermentation will occur in some wastewater collection systems, resulting in a wastewater that contains sufficient quantities of readily biodegradable substrate (particularly VFAs) to allow efficient biological phosphorus removal and denitrification. Warm temperatures, low velocities, which minimize reaeration. and force main systems, which maintain the wastewater under anaerobic conditions and in contact with the fermentative bacteria that grow as slimes on the walls of the collection system, provide ideal conditions for fermentation. When this does not occur in the wastewater collection system, the influent wastewater can be treated to convert slowly biodegradable organic matter into a more readily biodegradable form. As discussed in Section 11.1.2, fermentation is a developing technology that can be used to accomplish this conversion. Either the raw wastewater itself can be fermented, or primary solids can be separated and fermented. Solids fermentation is discusscd in Chapter 13.
The quality of the effluent from a biological wastewater treatment system is determined by the concentrations of soluble and particulate matter in it. Although this is the case for all wastewater treatment systems, it is particularly significant with BPR systems because of the elevated phosphorus content of the MLSS and their effect on the particulate phosphorus in the effluent. As discussed above, the phosphorus content of the MLSS in a BPR system will typically average about 6%, with values as high as 8 to 12% achievable in some cases. In contrast, conventional activated sludge will typically range from 1.5 to 2% on a P/VSS basis. Figure 11.15 illustrates the effect that increasing the phosphorus content of the MLSS can have on the particulate phosphorus concentration in the effluent from a BPR system. It indicates that significant quantities of phosphate can be contributed if effluent TSS concentrations exceed about 10 mg/L. Biological nutrient removal process mixed liquor also contains organic-N, with typical values in the 10 to 12% range on an N/VSS basis. This can amount to 1 to 2 mg/L of nitrogen for effluent TSS concentrations in the 10 to 30 mg/L range. These concentrations are significant in applications where an effluent low in total nitrogen must be produced.
Fortunately, BNR systems generally produce a well flocculated sludge that settles well in the clarifier and produces a clear effluent that is relatively low in suspended solids.4" Nevertheless, the impact of effluent suspended solids on effluent nitrogen and phosphorus concentrations must be considered carefully when BNR systems are used.
A number of environmental factors affect the performance of BNR systems. These factors also affect activated sludge systems, but their impacts can be more significant for BNR systems. The primary impact of temperature is on the kinetics of the various biochemical conversions, and its effect can be predicted quite well using the temperature correction factors described in Section 3.9. In general, temperature has the greatest impact on the nitrifying bacteria, as illustrated in Figure 9.4, but PAOs are also significantly affected.1' Decreasing temperature will also reduce denitrification rates, resulting in the need for larger anoxic zones and/or in reduced nitrogen removal. Decreasing temperature in the collection system can reduce the rate of fermentation of organic matter and alter the composition of the wastewater entering the biological treatment system/4'^ This impact is difficult to predict, but provides one of the major reasons that wastewater characterization or pilot plant studies should be conducted over an extended period of time.
Available data indicate that the activity of nitrifying bacteria is significantly reduced as the pH drops below 7.0, as illustrated in Figure 3.4, and that the impact of decreasing pH is much greater for them than for the PAOs or the denitrifying bacteria. Much of those data were collected in batch reactors using unacclimated cultures, but full-scale experience and some laboratory studies using acclimated cultures suggest that nitrifiers can acclimate to lower pHs, in the 6.5 to 7.0 range, with little decrease in activity/4"" Many nitrifying activated sludge systems operate quite successfully at pH values in this range, with efficiency dropping off only as the pH drops below 6.5. This potential should be considered in the design and operation of
0 10 20 30
Figure 11.15 Effect of the effluent TSS concentration and the mixed liquor P/VSS ratio on the effluent particulate phosphorus concentration.
0 10 20 30
Effluent TSS Cone., mg/L
Figure 11.15 Effect of the effluent TSS concentration and the mixed liquor P/VSS ratio on the effluent particulate phosphorus concentration.
BNR systems, and site-specific data should be collected using acclimated cultures if this becomes a significant cost or operational issue.
Dissolved oxygen concentrations affect the rates of nitrification, denitrification, and phosphorus removal in a variety of ways. Enough DO must be present in the aerobic zone to allow growth of nitrifiers and PAOs at adequate rates. In general, the DO requirements for nitrifiers are controlling due to their high half-saturation coefficient for oxygen. Although a DO concentration of 2 mg/L is often specified to obtain efficient nitrification, effluent quality goals can be obtained at lower DO concentrations if aerobic SRTs are sufficiently long. This effect is illustrated in Figure 3.3 where it can be seen that many combinations of DO and ammonia-N concentrations can exist for a specified nitrifier specific growth rate, i.e., for a given system aerobic SRT. At a fixed aerobic SRT, a reduction in the DO concentration will result in an increase in the ammonia-N concentration, but the increase may be small if the aerobic SRT is sufficiently long. Consequently, DO concentrations must be evaluated on a relative basis and adjusted in accordance with system performance requirements. Operation at aerobic zone DO concentrations below 2 mg/L may result in adequate nitrification and phosphorus uptake, while also encouraging additional denitrification. Low DO operation, on the other hand, may encourage the growth of Group IV filamentous bacteria, as discussed previously. The addition of DO to anoxic and anaerobic zones should be minimized because it is used preferentially as a terminal electron acceptor, thereby reducing the amount of readily biodegradable substrate available for denitrification or for uptake by the PAOs. If the quantity of readily biodegradable organic matter in the influent wastewater is high, however, larger oxygen inputs can be tolerated while still producing acceptable process performance because sufficient quantities will still be available for the other needs. Care should be taken not to introduce too much oxygen, however, because DO concentrations will be low in those situations and significant oxygen inputs can lead to excessive growths of low DO filamentous bacteria. In spite of the effects of oxygen entry into anoxic and anaerobic zones, both are often uncovered and simply rely on low surface turbulence to minimize oxygen transfer rates. As a consequence, care must be exercised in the selection and placement of mixers to minimize surface turbulence. In situations where interfacial oxygen transfer must be kept as low as possible, covers of various construction can be used. Since they need not exclude all air, but merely need to reduce surface transfer, the opportunity exists for the design engineer to be innovative in solving the problem.
The mixing energy provided to anoxic and anaerobic zones must be sufficient to keep the MLSS in suspension, but not so great as to cause significant surface turbulence, as discussed above. This means that the mixers in such zones should be selected with care. It is more difficult to generalize about the volumetric power input required to keep solids in suspension in anoxic and anaerobic zones than in aerobic systems because it depends very heavily on the type and placement of the mixer used. Consequently, mixer selection should be done in close cooperation with a qualified vendor. Nevertheless, as a rough approximation, vertical mixers often require around 12-16 kW/'lOOO m' whereas horizontal mixers require about 8 kW/1000 m.1
Bioreactor and aerator configurations also influence the environment produced in a BNR system. Both reaction kinetics and organism selection are improved by staging the reactor zones. This can be done cither by building separate tanks or by using curtain walls in a single tank. Aerator configuration will influence localized DO concentrations, as discussed previously, which can influence organisms selection and process reaction rates in the aerobic zone.
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