Study On The Influence Of Sewage Flow Regime Change On The Carbon Source Recovery Of Activated Primary Sedimentation Tank

In the urban sewage treatment plant,the primary sedimentation tank can not only remove the particulate pollutants in the sewage,but also lead to a huge mass of carbon loss,which will affect the subsequent biological nitrogen and phosphorus removal effect.With the aid of mechanical agitation and sludge return system,the activated primary sedimentation tank can break up the returned sludge of large particles,release the dissolved organic matter on the surface and inside of the sludge,and enhance the carbon source recovery effect.However,the relatively stable flow regime of sewage in the settling zone of the activated primary sedimentation tank makes the particulate matter easy to settle here.Therefore,after entering the settling zone,the dissolved carbon source released in the mechanical agitation zone may be readsorbed by the particulate matter and settle together again,resulting in the loss of the carbon source of the effluent,which can not fully realize the role of carbon source recovery.To solve the above problems,this study changed the flow regime of sewage in the activated primary sedimentation tank by setting baffles in the settling zone,and studied the flow characteristics of sewage under different baffle setting conditions by using the method of numerical simulation.And then,according to the pilot experiment results,the operation effect of the activated primary sedimentation tank under different conditions was explored,the optimal operation condition,baffle setting and flow regime were determined,and the impact mechanism of the flow regime change on the carbon source recovery was analyzed.The results showed that the optimal operation condition of the activated primary sedimentation tank was to control the mechanical stirring rate to 60 rad / min and the sludge reflux ratio to 20%.Theoptimal baffle setting situation was to set a baffle at an angle of 90°with the sewage inlet direction at the front end of the settling zone.In this situation,the area of the backflow area was the largest,and the area of the effective settlement area was the smallest,and the flow regime was the most conducive way to enhance the effect of carbon source recovery.Combined with the the results of numerical simulation and pilot experiment,it was found that in this flow regime,three large backflow areas would be formed in the settling zone of the activated primary sedimentation tank,which reduced the effective settlement area of particulate matter.The increase of fluid turbulence in the tank would change the movement state of the particulate matter,increase the probability of collision friction between the particulate matter,release the organic matter adsorbed on the surface of the particles,and the carbon source deposited on the bottom would also be resuspended due to the flow regime of the reflux zone.Therefore,the TCOD loss rate of the effluent was reduced to 21.95 %,the content of highquality carbon sources SCOD and VFA in the effluent was increased by 36.36% and 27.43% compared with the influent,respectively.The structure of the effluent carbon source was optimized and the phenomenon of carbon loss was improved at the same time.This study changed the flow regime of sewage in the settling zone of the activated primary sedimentation tank by setting baffles,strengthened the effect of carbon source recovery,and provided new ideas and theoretical support for the improvement of nitrogen and phosphorus removal efficiency of the subsequent biological treatment unit.