Influence of low pH on continuous anaerobic digestion of waste activated sludge

The influence of low pH on single stage continuous anaerobic digestion was evaluated, with the goal of increasing soluble phosphorus (P) concentration to mitigate in-reactor P precipitation. This was performed at pH 5.0, 5.5, 6.0, 6.5 and 7.0 using 1 L stirred-tank mesophilic reactors fed with sewage waste activated sludge. Low pH (5.5) caused a significant (p < 0.01) increase in soluble P concentration up to 79% of the total P, while methane yield was reduced by 50%. Total volatile fatty acids and soluble chemical oxygen demand concentrations increased from 40 to 504 mg L^-1 and 600 to 2017 mg L^-1 respectively, as the pH was reduced from 7.0 to 5.5. Higher concentrations of propionic acid (370-430 mg L^-1) were found at low pH (5.5). The reduction in methane yield was associated with a shift in microbial community and decreased destruction of particulate organics. Acidogens dominated at low pH (< 6.0), while methanogens decreased by 88% at pH 5.5 compared to neutral pH. Apart from the loss in methanogenic and hydrolytic capacity, chemical needs for acid dosing to maintain low pH conditions, and other negative impacts of chemical dosing were identified as key limitations.