Biological phosphorus removal processes - Effect of pH on anaerobic substrate metabolism

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 0.7-0.75 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.