Enhanced biological phosphorus removal from wastewater by biomass with different phosphorus contents, part III: Anaerobic sources of reducing equivalents

Laboratory-scale sequencing batch reactors exhibiting enhanced biological phosphorus removal (EBPR) operated at different influent phosphorus/chemical oxygen demand (COD) ratios were analyzed to evaluate possible anaerobic sources of reducing equivalents. Assuming anaerobic glycogen degradation was the sole anaerobic reducing equivalent source, an anaerobic phase carbon balance showed that glycogen-accumulating metabolism (GAM)-dominated systems were nearly carbon-balanced, but that polyphosphate-accumulating metabolism (PAM)-dominated systems had end-anaerobic phase carbon deficits. An anaerobic-phase reducing equivalent balance showed a reducing equivalent excess for the GAM-dominated systems and a deficit for the PAM-dominated systems, suggesting that glycogen degradation was not the sole reducing equivalent source for PAM. Reducing equivalent balances showed that metabolic models including complete anaerobic tricarboxylic acid (TCA) cycle activity, partial TCA cycle activity, and the glyoxylate bypass could provide the reducing equivalents required in PAM. Metabolic precursors produced in glycolysis, the TCA cycle, or modified versions of the TCA cycle could allow anaerobic growth and account for the PAM carbon deficits. The importance of considering both PAM and GAM activity in evaluating EBPR metabolic models was illustrated.