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0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0 8 Volume of Anaerobic Reactor as a Fraction of Total System Volume

0.0 0.2 0.4 0.6 0.8 0.0 0.2 0.4 0.6 0 8 Volume of Anaerobic Reactor as a Fraction of Total System Volume

Figure 7.41 Effect of the relative volumes of the two reactors on the steady-state concentrations of various particulate constituents in each reactor of the Phoredox system described in Figure 7.38. The total system volume was constant at 250 m\ SRT = 4 days. The solid curves represent the anaerobic (first) reactor and the dashed curvcs the aerobic (second) reactor.

a minimum. This is because of the mechanism of phosphate removal. Recall that in the anaerobic zone the PAOs hydrolyze stored Poly-P to gain energy for the uptake and storage of acetate as PHAs, releasing orthophosphate in the process (see Section 2.4.6). Over the optimal volume split range, maximum utilization has been made of the available acetate in the anaerobic tank, leading to maximum release of soluble phosphate. That phosphate is then taken up again in the aerobic bioreactor as the PAOs grow by using the PHAs as substrate and store Poly-P.

The upper end of the optimal range is controlled by the minimum aerobic SRT for growth of the PAOs in the aerobic zone. Once the anaerobic zone becomes so large that the aerobic SRT is below the minimum required for growth of the PAOs, they wash out and the system fails from the perspective of phosphorus removal. This can be seen clearly in Figures 7.40a and 7.41a. The population of heterotrophic bacteria increases (Figure 7.41b) because they no longer have to compete with the PAOs for acetate. Ultimately, as the anaerobic zone gets even larger, the entire system begins to act like a totally anaerobic system. The model was not intended for simulating such systems, so anaerobic fractions greater than 0.8 are not shown.

The lower end of the optimal range is a little more complicated. If the anaerobic zone is very small, there is insufficient time for acetate production by fermentation and its uptake by the PAOs. Thus, the PAOs can't grow at all in the system (Figure 7.41a). In addition, when the anaerobic zone is very small, the aerobic SRT is sufficiently long to allow autotrophic nitrifying bacteria to grow, thereby providing nitrate-N as an electron acceptor in the anaerobic zone through the biomass recycle. As a consequence, the anaerobic zone actually behaves as an anoxic zone, such as in the MLE process. Moderately sized anaerobic zones allow for some growth of

PAOs, but their full potential cannot be reached because of the return of nitrate-N to the anaerobic zone, which reduces the amount of fermentation that can occur and allows the heterotrophic bacteria to out-compete the PAOs for acetate. However, as the anaerobic zone is made larger, there is more opportunity for fermentation, with a gain in PAOs at the expense of heterotrophs (Figure 7.41a and 7.41b). Furthermore, as the size of the anaerobic zone is increased, the aerobic SRT is decreased, thereby reducing the population of autotrophic bacteria (Figure 7.41c) and the concentration of nitrate-N being returned to the first bioreactor (Figure 7.40c). This allows better growth of the PAOs. Ultimately, the point is reached at which the aerobic SRT is smaller than the minimum SRT for the autotrophs and they wash out (Figure 7.41c), allowing the first tank to operate in a truly anaerobic mode, which maximizes phosphorus removal.

In summary, the important point to gain from these simulations is that there is an optimal split between the anaerobic and aerobic zones of the Phoredox (or A/O) system. Optimal phosphorus removal occurs when the aerobic SRT is small enough to exclude growth of autotrophic nitrifying bacteria, yet large enough to allow the PAOs to grow, and the time in the anaerobic zone is sufficiently large to allow efficient fermentation and uptake of the resulting acetate.

7.8 SEQUENCING BATCH REACTOR 7.8.1 Description

All of the process variations we have considered so far are continuous processes. As a consequence, the environments required to achieve a variety of objectives must be encountered spatially as the wastewater and biomass move from tank to tank within the system. Because each tank has a fixed volume, the relative amount of process time spent under each environmental condition is fixed for a given influent flow rate. Alteration of those times requires alteration of the sizes of the various tanks, something that may or may not be achieved easily. It is possible, however, to accomplish the same results in a batch reactor by altering the environment temporally. In this situation, if the relative times devoted to the particular environments are not attaining the desired result, they can be changed easily by reprogramming the controllers that turn the pumps and blowers on and off. This flexibility is the major advantage associated with batch bioreactors.

The term sequencing batch reactor (SBR) stems from the sequence of steps that the reactor goes through as it receives wastewater, treats it, and discharges it, since all steps are accomplished in a single tank. A typical sequence is illustrated in Figure 7.42. The cycle starts with the fill period in which the wastewater enters the bioreactor. The length of the fill period is chosen by the designer and depends upon a variety of factors, including the nature of the facility and the treatment objectives. The main effect of the fill period, however, is to determine the hydraulic characteristics of the bioreactor. If the fill period is short, the process will be characterized by a high instantaneous process loading factor, thereby making the system analogous to a continuous system with a tanks-in-series configuration. In that case, the biomass will be exposed initially to high concentrations of organic matter and other wastewater constituents, but the concentrations will drop over time. Conversely, if the fill period is long, the instantaneous process loading factor will be small and the system

Influent

Influent

Cycle

Fill

Activities

React

Mixing and/or aeration occur as necessary for biological reaction.

Mixing and/or aeration occur as necessary for biological reaction.

Settle

Mixing and aeration terminated. Biomass settles.

Effluent

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