Nitrification in Pure Oxygen Activated Sludge Systems

Mixed liquor pH and temperature are two parameters that affect the growth rate of nitrifying bacteria and therefore the minimum solids retention time required to achieve nitrification. The objective of this study was to determine the consequence of low mixed liquor pH, and to determine if pH depression could be alleviated by recovering alkalinity through denitrification in a pure oxygen activated sludge system. The study was conducted at the University of Manitoba using laboratory scale, pure oxygen activated sludge reactors, fed with primary effluent. The results indicated that when denitrification was not included in the process, the concentration of ${\mathrm{CO}}_2$ in the headspace of the pure oxygen reactors increased to as high as 15% due to carbon oxidation and endogenous respiration. The high ${\mathrm{CO}}_2$ concentration in the headspace combined with low alkalinity caused by nitrification resulted in bulk mixed liquor pHs below 5.5. In order to maintain complete nitrification at a temperature of 24°C and a mixed liquor pH of 5.5, a solids retention time (SRT) of 12 days was required. In comparison, when denitrification was included in the process the pH of the mixed liquor was increased to 6.4 allowing for full nitrification at an SRT of 5.6 days at a temperature of 24°C. The increase in pH in the denitrification trains was attributed to three factors: recovery of alkalinity through the denitrification process, the conversion of influent carbon to ${\mathrm{CO}}_2$ in the anoxic reactor allowing the ${\mathrm{CO}}_2$ to escape to the atmosphere, and the recycle of mixed liquor super saturated with ${\mathrm{CO}}_2$ from the pure oxygen reactor to the anoxic reactor allowing the ${\mathrm{CO}}_2$ to escape to the open atmosphere. It was determined that the nitrifier growth rate at 12°C was approximately 50% of the rate measured at 24°C. At mixed liquor pHs between 6.0 and 6.3 at a temperature of 12°C, the specific nitrifier growth rate was between 0.12 and $0.15d^{-1}\text{,}$ while at 24°C, the specific nitrifier growth rate was between 0.25 and $0.30d^{-1}$ at pHs ranging from 5.0 to 6.1