The first level A standard of the sewage discharge for phosphorus removal could not reach when only biological method is employed

such as the traditional CAST(cyclic activated sludge technology)process that has been widely used in municipal wastewater treatment plants.A modification for the traditional CAST process was made by introducing an independent stirring phase in the main reactor zone.The modified process has been demonstrated in a full-scale wastewater treatment plant.To optimize the metabolic environment for polyphosphate bacteria and to increase its competition ability

gradually the aeration time was shortened and the anoxic stirring time extended by two stage experiment.For the modified CAST process the efficiency of phosphorus removal is 89.1%

22% higher than non-modified CAST process

and the first level A standard of the sewage discharge could be steadily reached for the effluent TP.The results of static tests showed that the modification of process can increase the proportion of denitrifying polyphosphate accumulating organisms

and that the removal of 1 mg·L-1 PO43--P need consume of 1.67 mg·L-1 NO3--N.So

the denitrifying performance was also improved

and the concentration of NH4+-N and TN in effluent both could meet the first level A standard with 10.4% energy saved.