Evaluation On The Performance Of CAST Process In Municipal Wastewater Treatment Plant For Nitrogen And Phosphorus Removal

CAST(Cyclic Activated Sludge Technology)process is a modified SBR(Sequencing Batch Reactor)process,in which influent,reaction,sedimentation and drainage are integrated in one tank,and is widely used in urban wastewater treatment plants.However,there are few reports on the nitrogen removal pathways and the contribution of each pathway to nitrogen removal in CAST process.In this study,we analyzed the nitrogen removal pathways of CAST process by measuring the changes of nitrogen in typical cycles under different operation modes of CAST process in Ankang Jiangbei wastewater treatment plant,and calculated the contribution of different pathways to the total nitrogen removal based on mathematical models.SND(Simultaneous Nitrification and Denitrification)rates at different DO(Dissolved Oxygen)concentrations and batch experiments of microbial storage of intracellular polymers under different Famine time conditions were carried out in a sequential batch reactor.The nitrogen and phosphorus removal performance of CAST process was analyzed and evaluated by integrating the water quality date for influent and effluent water,nitrifying activity,and microbial community structure,and the main conclusions were drawn as follows:(1)The CAST process in Ankang Jiangbei wastewater treatment plant has good performance in nitrogen removal and phosphorus removal,ammonia nitrogen removal rate can reach more than 95.1%,total nitrogen removal rate can reach more than 71.9%,total phosphorus removal rate can reach more than 97.7%.(2)The nitrogen removal pathways of CAST process include: simultaneous nitrification denitrification(SND)in the aeration stage,endogenous denitrification(ED)in the sedimentation stage and anoxic denitrification(AD)in the anoxic tank,of which the first two pathways can contribute more than 97% to the total nitrogen removal.(3)Feast-Famine batch experiments demonstrated that the sludge stored the most acetic acid at a Famine time of 36 h at 0.173 g Hac/g VSS,from which the optimal mixture reflux ratio of 45% was deduced.(4)The dissolved oxygen concentration had a great influence on the nitrogen removal efficiency of the CAST process,with the most satisfactory total nitrogen removal rate of 84.5% at an average sludge concentration of 4755 mg/L,DO concentration of 1.72 mg/L and an aeration time of 2 hours.(5)The high-throughput sequencing results showed that the phosphorus-polymerizing bacteria in the activated sludge of CAST process in Ankang Jiangbei wastewater treatment plant were dominated by Tetrasphaera with fermentation function,accounting for 6.96%,which may be the main reason for the good phosphorus removal performance of CAST process.