Research On Optimal Operation Parameters Of CAST Process For Sewage Treatment

Cyclic activated sludge technology(CAST) is a variant of sequencing batchreactor(SBR),the CAST process has attracted a great deal of interest based on its performanceof low investment and operational cost,easy and flexible operation,the ability of prevention ofsludge bulking in biological selector and simultaneous nitrogen and phosphorus removal inmain reaction zone.In order to study the mechanism of CAST process, fed with domestic wastewater, abench-scale CAST reator made of glass was operated to investigate the influence ofparameters such as dissolved oxygen, aeration time and sludge return ratio on wastewatertreatment. Moreover, the profiles of dissolved oxygen(DO), pH and oxidation-reductionpotential(ORP) were evaluated. The study is expected to modify and optimize the processesof CAST and provide good reference for the engineering operation.Firstly, the effect of dissolved oxygen on the pollutants removal efficiency of CASTprocess was studied. Results showed that increasing DO concentration of main reaction zoneduring the aeration stage is conducive to the removal of COD and NH3-N. When the DOconcentration is2mg/L, the treatment effect of TN and TP better than that is0.5mg/L,1mg/Lor4mg/L. By considering the removal of various pollutants, ultimately determine the optimalDO concentration of main reaction zone during the aeration stage is2mg/L.The effect of aeration time on the pollutants removal efficiency of CAST process wasstudied. Results showed that the aeration time was increased from2h to3h, it can improve thepollutants removal efficiency.But,when the aeration time was4h, increasing aeration timehad a very small influence on the pollutants removal. Considering energy conservation, theoptimal aeration time was determined as3h.The effect of sludge return ratio on the pollutants removal efficiency of CAST processwas studied. Results showed that increasing sludge return ratio had a very small influence onthe removal of COD and NH3-N, but it can have some influence on TN and TP removal efficiency. when the sludge return ratio was increased from10%to30%, the TN and TPremoval efficiency was increased.When the sludge return ratio was increased to4h,the TNand TP removal efficiency was declined. The optimal sludge return ratio was determined as40%.Finally the relationships among the pH, ORP, DO and the concentration of pollutantswere studied.The pH and ORP curve had the inflection point in the late stage of influent inbiological selector, this showed that the denitrification reaction had been finished inbiological selector,the biological selector began to turn to anaerobic state. When NH3-N wasalmost exhausted in the late stage of aeration in main reaction zone, it would appear thephenomenon that the pH dropped to the lowest point and then increased, ORP from the slowrise to fast rise, the DO curve had a jump point at the same time, this showed that thenitrification reaction had been finished in main reaction zone.