Operation Optimization Of CAST Process Sewage Treatment Works Based On Computational Simulation

With the rapid development of urbanization in China,the amount of water received by urban sewage treatment plants also increases rapidly.After the recent construction of some sewage treatment plants,the long-term growth rate of water quantity is much faster than the calculation of water quantity when the project is established.Even if the longterm facility construction or expansion is started in advance,the necessary approval procedures and construction cycle are also needed.Therefore,there are many problems in the process of rapid urbanization The urban sewage treatment plants in this area often face a long period of operation beyond the designed water volume.When the service area contains the old urban area,the mixed sewage transferred by the combined system pipe network in summer will make the sewage treatment plants which are close to or even more than the designed water volume in the dry season face huge pressure.Therefore,how to excavate the existing facilities and equipment for treatment Potential,it is a practical technical demand to ensure that the wastewater treatment plant can reduce the pollutant concentration to the maximum extent and ensure that the effluent meets the standard by optimizing regulation and adding a small amount of auxiliary means under such extreme conditions.Based on a circulating activated sludge process(CAST)wastewater treatment plant in Chongqing,this study puts forward a debugging plan through experimental detection and investigation analysis,and analyzes and predicts it by using fluent and West software,combining computational fluid dynamics simulation with activated sludge dynamics simulation,and finally puts forward the optimal operation and control scheme of the plant under the condition of over design water volume.The Volume of Fluid model,Standard model and Component Transport model are established in Fluent software,the dynamic model of CAST reaction tank was established to analyze the hourly variation of the disturbance of the influent to the sediment and outlet in the main reaction zone of CAST tank.The proportion of influent components,velocity distribution and velocity vector distribution were obtained by simulation.The proportion of influent increased from 0 to 0.1 when the influent was between 10 min and 15 min;when the influent was between 16 min and 20 min,the disturbance of outlet increases,and the proportion of inlet water increases from 0.1 to 0.5;when the influent was between 21 min and 25 min,the proportion of inlet water increases from 0.5 to 0.6.The flow velocity at the inlet is larger than that in the pool,which leads to the backflow area above the inflow,and the range of the backflow area increases with the increase of the inflow,and finally reaches stability.A CAST process wastewater treatment plant model based on ASM2 d is established in WEST software.Through the analysis of the influent components of the wastewater treatment plant,the measured data of the component parameters used to modify the model are obtained.Through on-site measurement and analysis of inlet and outlet water,the variation rules of inlet and outlet water quantity and water quality,as well as the main water quality indicators affecting the effluent discharge are obtained.After the sensitivity analysis of the model,the decay rate of kinetic parameters(),the rate constant of bacteria dissolution and decay(),the rate constant of PP storage()and the yield coefficient of phosphorus accumulating bacteria(),the yield coefficient of heterotrophic bacteria()and the parameters required for storage()have great influence on the effluent quality index.The parameters with higher sensitivity are adjusted manually,and the corrected values of parameters are obtained.The parameters are input into the software instead of the original ones to simulate dynamically again.The simulated values are in good agreement with the measured values.Therefore,it is considered that the model basically reflects the actual operation of the sewage treatment plant,and can be used in the subsequent parameter control analysis.Combined with the analysis results of flow field state of water decanting obtained by Fluent software simulation,the value group of water inflow time from 12 min to 22 min with a time interval of 2 min is selected as the value of water inflow time T1 in WEST software,and the water inflow time T1 in process cycle is the difference between the original water inflow time of 45 min and T1.After dynamic simulation,the results of effluent quality change under different influent time are obtained.COD,TN,TP,NH3-N and SS concentrations in the effluent increased with the increase of inlet time.The growth rate of COD,NH3-N and SS was faster than that of TN and TP.This is related to the higher removal concentration of COD,NH3-N and SS.When the influent time T1 was 33 min,the effluent quality standard rate at 31 min and 29 min was basically the same as that at 45 min;When T1 shortened to 27 min,the effluent standard rate decreased significantly;When T1 continues to shorten to 25 min and 23 min,the compliance rate still keeps a downward trend,and when T1=23 min,almost all effluent quality exceeds the standard.Because it is necessary to solve the problem of excessive treatment water and ensure the effluent quality up to the standard,the influent time T1=29min is selected after comprehensive analysis,as the best time to control the water treatment.The first class B standard compliance rates of COD,TN,TP,NH3-N and SS in the simulated effluent were100%,90%,50%,100% and 87% respectively.Under this condition,the time of each operation cycle can be reduced by 16 min.if the impact of rainstorm flow is estimated as three cycles,the inflow of these three cycles can be increased by 1/3 compared with the original operation mode,which effectively improves the treatment capacity of the sewage treatment plant.Aiming at the problem that it is difficult to achieve TP emission standard by biological phosphorus removal,the beaker experiment of chemical phosphorus removal in the plant was studied.Ferric chloride was selected as phosphorus removal agent,PAM(polyacrylamide)as coagulant aid,and dosing equipment was set up near the mixing well of CAST tank.If the TP concentration of CAST effluent exceeds 1.0mg/l,add it as30ml/m~3;If it was in the range of 0.5～1.0 mg/L,add it as 20ml/m~3.This measure can effectively reduce the effluent TP concentration and further control the effluent SS.Through the combined application of Fluent and WEST simulation,the optimal control mode of CAST process sewage treatment plant to deal with the inflow exceeding the design value is obtained.This mode can make full use of the existing facilities and equipment to improve the treatment capacity and reduce the overflow risk caused by the impact of mixed sewage in rainy season to a certain extent.The research results can provide a reference for the sequencing process sewage treatment plant to deal with the flow impact Emergency control technology reference.