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02468 10 02468 10 SRT, days SRT, days

Figure 7.39 Effect of SRT on the steady-state concentrations of various constituents in the aerobic (last) reactor of the Phoredox system depicted in Figure 7.38. For comparison, the dashed curves represent the performance of a single CSTR with a volume of 250 m'. Influent tlow = 1000 m'/day. Influent concentrations are given in Table 7.1. Biomass recycle How = 500 m /day: volume of the anaerobic (first) reactor = 50 m', volume of the aerobic (second) reactor - 200 m'. Parameters are the default parameters in ASIM.V The dissolved oxygen concentration is zero in the anaerobic reactor and 2.0 m^'L in the aerobic reactor.

differences in the wastewater characteristics and the kinetic and stoichiometric coefficients in the two activated sludge models. They are qualitatively similar, however.

The responses of the soluble phosphate and the PAOs are shown in panels a and d of Figure 7.39. No PAOs grow in the single CSTR because the proper environment is not provided, and the only phosphorus removal is that associated with its role as a macronutrient for biomass growth. Examination of Figure 7.39a reveals that the soluble phosphate concentration in the single CSTR increases as the SRT is increased. There are two reasons for this. First, the observed yield of biomass decreases as the SRT is increased, thereby decreasing the amount of nutrients required. Second, soluble phosphate is released from slowly biodegradable substrate as it is hydrolyzed. Hydrolysis is greater at longer SRTs, allowing more phosphate to be released. The net effect of these events is to make the amount of phosphate released exceed the amount incorporated into biomass. Thus, the concentration increases. Furthermore, at short SRTs, no phosphorus is removed in the Phoredox system because the SRT is below the minimum required for growth of PAOs. The minimum

SRT relates only to the aerobic SRT because growth of PAOs occurs only under aerobic conditions. The presence of the anaerobic zone is necessary for their growth, however, because they grow at the expense of stored PHAs, which are only formed under anaerobic conditions as acetate is taken up and stored at the expense of PolyP. Once the minimum SRT is exceeded, a significant population of PAOs is established in the Phoredox system and phosphate uptake by them in the aerobic bio-reactor is able to reduce the phosphate to very low levels.

Examination of Figure 7.39f reveals that there is a reduction in the amount of heterotrophic biomass relative to the single CSTR over the SRT range where good phosphorus removal is occurring. This is because of competition between the PAOs and the common heterotrophs for substrate. In the anaerobic bioreactor of the Phoredox system, the PAOs are able to store acetate. The common heterotrophs, on the other hand, cannot store or use acetate in the anaerobic zone; nor can they grow as long as oxygen or nitrate-N is unavailable as an electron acceptor. They can only produce acetate by fermentation of readily fermentable substrates. Consequently, the amount of substrate available to the common heterotrophs in the aerobic zone, where growth occurs, has been reduced by the activity of the PAOs in the anaerobic zone, thereby reducing the quantity of heterotrophs that can be formed.

Examination of Figure 7.39a reveals that excellent phosphorus removal occurs as long as the SRT is between 2.2 and 3.6 days, but once the SRT exceeds 3.6 days, phosphorus removal deteriorates sharply. This is because of nitrification and denitri-fication. Once the aerobic SRT is sufficiently long, autotrophic biomass can grow, converting ammonia-N to nitrate-N. The nitrate-N, in turn, is returned to the anaerobic zone via the biomass recycle, providing an electron acceptor for the heterotrophs in that zone. That allows them to compete with the PAOs for acetate and readily fermentable substrate in the anaerobic zone. It also reduces the amount of fermentation that occurs, thereby reducing the quantity of acetate produced. The impact of both of these mechanisms is to reduce the mass of PAOs that can be grown, as shown in Figure 7.39d, which reduces the amount of soluble phosphate that can be taken up in the system. As a consequence of these effects, Phoredox and A/O processes tend to be operated with short SRTs. Furthermore, because it is often desirable to achieve carbon oxidation, nitrification, denitrification, and phosphorus removal all in a single system, several process flow sheets have been devised to overcome these effects. They are discussed in Chapter 11.

In summary, the important point to draw from Figure 7.39 is that there is a limited range of SRTs over which a simple Phoredox (or A/O) system will work properly. If the SRT is too short, the PAOs will wash out. If it is too long, nitrifying bacteria will be able to grow, providing an inorganic electron acceptor to the anaerobic zone which allows the common heterotrophs to out-compete the PAOs for substrate, thereby reducing phosphate removal.

7.7.3 Effects of System Configuration

The role of the anaerobic bioreactor in the Phoredox process is two fold. First and foremost, it provides the selective advantage that allows PAOs to grow in the system. It is there that they take up acetate, forming the PHAs that will serve as their energy source for growth in the aerobic bioreactor. Second, it provides a regime wherein fermentation may occur, providing acetate in excess of that available in the influent.

As important as the anaerobic tank is, the entire system cannot be anaerobic because aerobic conditions are required for growth of both PAOs and heterotrophic bacteria. Thus, there is an optimal balance between the sizes of the two zones. In order to illustrate that balance, simulations were performed in which the total system volume was held constant while the relative sizes of the two zones were varied. The system SRT was held constant at 4 days. The results of the simulations are shown in Figures 7.40 and 7.41. The former shows the responses of the soluble constituents while the latter presents the particulate ones. The solid curves represent the anaerobic (first) bioreactor and the dashed curves the aerobic (second) bioreactor. The dashed curves, therefore, also represent the system effluent with respect to the soluble constituents.

Examination of Figure 7.40a reveals that the best phosphorus removal occurs when the volume of the anaerobic zone is between 25% and 48% of the total system volume. It should be emphasized that this range is unique to this SRT, wastewater characteristics, etc. It will be different for other situations. The important point is that there is indeed an optimal combination that maximizes the concentration of PAOs (Figure 7.41a), thereby allowing maximum phosphate-P removal. Panels a and d of Figure 7.40 also reveal that over that range, the concentration of phosphate-P reaches a maximum in the anaerobic zone while the concentration of acetate reaches

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