Statistical analysis to evaluate the effects of temperature and pH on anammox activity

Anaerobic ammonium oxidation (anammox) is the most efficient process of nitrogen removal from wastewater. A central composite design was employed to study the individual and interactive effects of temperature and pH on the activity of anammox bacteria. A total of 13 experiments including 5 center point replicates with specific anammox activity as a response were carried out. The maximum specific anammox activity (0.0286 ± 0.0003 g N2-N g VSS−1 d−1) was observed at temperature and pH of 43 °C and 7.5, respectively. The results of estimated regression coefficients for specific anammox activity suggested that both temperature and pH positively affects the specific anammox activity of bacteria. The effect of pH was found to be more significant than temperature as evident from the P value (P < 0.05). Further, contour plot revealed that the interaction between pH and temperature is also significant. Regression model equation was developed to predict the specific anammox activity at particular pH and temperature. The regression model was validated by carrying out experiment at lower temperatures (25, 20 and 15 °C) with varying pH, which revealed that an appropriate anammox activity could be maintained at lower temperature by changing pH. The study suggested that higher pH should be maintained in wastewater treatment plant to achieve the high anammox activity at lower temperatures.