Effect of the Internal Recycles on the Phosphorus Removal Efficiency of a WWTP

The aim of this work was to study the effects of the internal recycle flow rates on a University of Cape Town (UCT) process. The study was carried out by continuously feeding urban sewage to a UCT pilot-scale plant and by using batch tests. In the pilot-scale experiments, it was observed that the maximum nitrogen removal was obtained when both internal recycle flow rates ranged from 200 to 250% and that the maximum phosphorus removal was obtained when the aerobic-to-anoxic flow rate ranged from 100 to 150% and the anoxic-to-anaerobic recycle flow rate ranged from 75 to 100%. In order to obtain a deeper insight on the anaerobic and anoxic processes, batch tests were carried out and the results were modeled. From the results obtained in the batch test, it was concluded that the nitrate−nitrogen concentration in the anaerobic compartment should be always lower than 1 mg L-1. Taking into account these observations and the opposite effects of the internal flow rates on the nitrogen and phosphorus removals, equilibrium internal recycle flow rates were selected in the pilot-scale plant in order to obtain the best effluent characteristics. These values ranged from 100 to 150% for the aerobic-to-anoxic compartment recycle flow rate and from 75 to 100% for the anoxic-to-anaerobic compartment recycle flow rate.