Biological phosphorus removal is a complex process that is dependent on the growth of specialized phosphate accumulating organisms (PAOs), which store phosphorus as polyphosphate (poly-P), as discussed in Section 2.4.6. Because biological phosphorus removal is still the subject of active experimental investigation, there is little consensus concerning all of the rate expressions describing it. Furthermore, the microbial events involved are subject to complex control through the concentrations of several constituents. As a consequence, a discussion beyond the scope of this chapter would be needed to fully define the kinetics and stoichiometry of biological phosphorus removal. There are, however, two events that should be considered briefly because they are distinctly different from the events discussed above. These are the uptake and release of phosphorus by the PAOs.
It will be recalled from Section 2.4.6 that two conceptual models exist for biological phosphorus removal, the Comeau/Wentzel model and the Mino model, illustrated in Figures 2.5 and 2.6, respectively. The major difference between them is that the Mino model incorporates glycogen formation and utilization, whereas the Comeau/Wentzel model does not. In developing a mathematical model for biological phosphorus removal, one group decided that the introduction of a detailed mechanistically based mathematical model for the process was premature."" Rather, they preferred to recommend the simplest mathematical model that allows adequate prediction of biological phosphorus removal. Consequently, they basically followed the Comeau/Wentzel model. Furthermore, they assumed that PAOs cannot use nitrate as is.ii is.ii
a terminal electron acceptor and that they can only grow on the PHB stored in the cell. Although these assumptions are severe restrictions, we have chosen to use activated sludge model (ASM) No. 2 as the basis for the rate expressions presented herein because it represents a consensus among several investigators in the field. It should be recognized, however, that more complete expressions are likely to be developed as more research is done.
Under anaerobic conditions, PAOs do not grow, but store acetic acid as PHB through the cleavage of Poly-P with the associated release of soluble phosphate. (In the following equations, all phosphate concentrations are expressed as phosphorus, P, and all organic materials are expressed as COD). The rate of removal of acetic acid, rSA, can be modeled with an interactive, dual limiting nutrient expression:
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