Effect of pH on Carbon and Phosphorus Transformations in Biological Phosphorus Removal with High Propionate and Acetate Wastewater

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Two parallel VIP/UCT configured BNR systems were operated identically except for acidification of anaerobic zone pH in Train 1 (pH=6.5) versus Train 2 (pH=7.2). Lowering anaerobic zone pH decreased anaerobic P release, and observed yields for biomass decreased from 0.38 mgVSS/mgCOD to 0.28 mgVSS/mgCOD. The net implications for EBPR were a higher Pup/Prel ratio, and a higher biomass P content. In addition it is possible that lower pH resulted in a higher net P removal but no definitive conclusion can be reached due to the interference of P release in the secondary clarifier of Train 2.

Carbon transformations were largely unaffected by the pH change other than direct consequences of the lower observed yield. One minor exception was that lowering pH seemed to favor specific 3HB biosynthesis over 3HV, although the specific total PHA biosynthesis was not affected much. In addition the use of carbon sources for PHA biosynthesis other than acetic acid or degraded glycogen (e.g. propionic acid and other unknown endogenous or exogenous carbon sources) may have been inhibited by lowering pH and this might conceivably have been linked
to the lower observed yields.

The reason for the lower observed yields in Train 1 may have been due to the inorganic acid H2SO4 (which was used to lower the anaerobic zone pH) acting as an uncoupler. However since the zones where growth occurred (anoxic and aerobic zones) only had pHs lower by 0.5 and 0.3 units, respectively, it seems unlikely that such a dramatic difference could be explained by the pH difference alone. Another remote possibility is that sulfide was produced from the
sulfate in the acid and acted in an inhibitory manner. It is uncertain whether or not the difference in observed yields was a) specific to this wastewater, b) an experimental artifact of some kind, or c) represents a phenomenon that might be observed in a significant fraction of low pH EBPR systems.

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