Nutrient removal performance at low temperatures in the modified UCT step feed process

Optimization of the modified University of Cape Town (UCT) step feed process at low temperatures was investigated to improve the nitrogen and phosphorus removal capacities. Nearly complete nitrification, desired denitrification and phosphorus uptake could be obtained at temperatures ranging from 19.2°C and 27.3°C with the hydraulic retention time (HRT) of 8h. However, a drop in temperature to 18.5 °C and then to 16.5 °C led to the nitrification deterioration in spite of the increase of HRT to be 9h, but it gradually recovered to the normal level with HRT of 10h. Moreover, there were more nitrates accumulated in the effluents owing to lower denitrification rates, which decreased from 42.5 mgN/(gVSS·d) in phase 1 to 28.6 mgN/(gVSS·d) in phase 3. However, the PO₄^3--P removal capacities were not seriously influenced by the drop of temperatures, just with a little decrease of removal efficiency. Because the reduced nitrification temperatures led to change the anoxic zone into another additional anaerobic zone, which was beneficial to the enrichment of anaerobic microbial populations. In addition, good settling property with the mean SVI of 88.9 mL/g might be associated with the step feed strategy and the existence of anaerobic zone in the process configuration.