Effect of pH on the anaerobic acetate metabolism of biological phosphorus (P) removal processes was determined using anaerobic-aerobic activated sludge cultured with mainly acetate and containing a 12% or an 8% of total biomass as P (Px). The acetate uptake coupling to phosphate (Pi) release was found to be kinetically and thermodynamically influenced by pH. In the pH range 5.0–6.5, the acetate uptake rate (AUR) increased linearly with pH from 0 to about 50 (mg C/g VSS/h), and the Pi release rate (PiRR) from 20 to about 50 (mg P/g VSS/h). In the pH range 6.5–8.0, AUR remained at a constant range but PiRR continuously increased. Above pH 8.0, both AUR and PiRR started to decrease. With regard to the molar ratio of Pi released per acetate taken up (Pi/Ac), it was about 1.0 or 0.70–0.75 in the pH range 5.5–6.5, and proportionally increased to 1.75 or 1.50 in the pH range 6.6–8.5 for sludge containing a 12% Px or an 8% Px, respectively. Apparently, acidic pH inactivated the acetate metabolism, and basic pH stimulated too much Pi release, resulting an increase in energy consumption for acetate uptake (i.e. the Pi/Ac ratio). As a compromise, an optimum pH 6.8±0.7 was proposed for anaerobic acetate metabolism, because that a relatively high AUR with less energy consumption can be maintained by the bacteria that respond for biological P removal.

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