A modified cyclic activated sludge technology (CAST) treating domestic wastewater was employed to investigate the effects of different operating modes, such as C/P ratio, reflux ratio, and temperature on phosphorus removal. The results illustrated that at room temperature the phosphorus removal of the system improved significantly when the influent C/P ratio increased from 50 to 100, with the removal efficiency increasing rapidly from 15% to 95.6%. Decreasing the C/P ratio to 75, the phosphorus removal performance declined due to influent carbon source deficiency, and the mean phosphorous removal efficiency decreased to 51.4%. At the same time, the long-term addition of an easily degradable carbon source resulted in sludge bulking and a large amount of sludge loss. With a low C/P ratio, the phosphorus removal performance increased by 2.3 times when the sludge reflux ratio decreased from 25% to 12.5%. However, the phosphorus removal performance declined when reducing it to 0. The temperature experiment results showed that the phosphorous removal efficiency of the system remained stable, above 90%, in the low temperature system (14℃±1℃). However, the phosphorous removal efficiency of a shortcut nitrification system at high temperature (27℃±1℃) was only 14.1%, suggesting that a low temperature was beneficial for removing phosphorous. It was observed from the batch tests that the sludge at room temperature could utilize dissolved oxygen, nitrate, and nitrite as electron acceptors to take up phosphate. The sludge in the low temperature system could use both dissolved oxygen and nitrate as the electron acceptors. However, the sludge in the high temperature system could only use dissolved oxygen as the electron acceptors to take up phosphate. In addition, it was also found that sludge under starvation conditions resulting from short-term idling of the system favored phosphorus removal.
Keywords: COD/TP ratio; modified CAST; sludge reflux ratio; temperature.