| CAST reactor of Tangwang wastewater treatment plant in Yangzhou is decided to be object of study. Production tests have been done according to abnormal phenomena in real operation such as unsteadiness of float sludge, ammonical nitrogen. The results show that:1 The reasons of sludge floating were activated sludge aged, N removal (denitrification), excess aeration and the impact on the reactor of oily wastewater2 To find out the reasons for instability of NH3-N removal efficiency,production tests have been done in sludge age, dissolved oxygen ,pH ,sludge load and other aspects. The tests proved major causes of treatment effect unsteadiness .The sludge age are ignored when adjusting the parameters ,which leads to reduction of nitrifying bacteria and denitrifying bacteria in the sludge. Nitration can not progress because nitrogen removal is not met.3 There are contradiction and competition between the processes of nitrogen removal and dephosphorization. Usually the process of nitrogen removal is to create anaerobic, anoxic and aerobic surrounding in time spatial series in which nitration denitrification and aerobic excess absorption are accomplished so biological denitrification and dephosphorization are achieved . In order to strength the results of denitrification 1 and dephosphorization, production tests have been done combined with the character of CAST process. The tests proved that efficiency of COD and SS removal can be kept to80% above. Under the conditions: operation period 4h , simultaneous inflow and inverse sludge, reflux ratio about 20%, the concentration of dissolved oxygen below 0.5 mg·L-1 for 30 minutes before inflow/aeration, 2.0～3.0 mg·L-1 during pure aeration, and 1.0～3.0 mg·L-1 for 30 minutes after inflow/aeration. SND can be achieved 85%. Under the conditions: the concentration of dissolved oxygen below 0.5 mg·L-1 after inflow/aeration after inflow/aeration and 1.0～3.0 mg·L-1 during pure aeration, SND can be achieved ,but the result is unfavorable.
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