| With the increasing attention to water environment protection in China in recent years,the stantards of effluent from sewage plants have become increasingly strict.In order to make the effluent concentrations of nitrogen、p Hosp Horus and other nutrients reach the corresponding standards,most sewage treatment processes need sludge recirculation.However,the CAST process in a Chongqing’s wastewater treatment plant has not carried out sludge reflux for a long time and the effluent quality still meet the corresponding sewage discharge standards,which is different from the traditional nitrogen and p Hosp Horus removal process.No sludge reflux simplifyied the production steps,saved energy consumption and brought considerable benefits to the sewage plant.However,the process control is completely based on experience.Neither the reason why the effluent can reach the standards without sludge reflux nor the risk of long-term maintenance without sludge reflux is clear.In this project,a series of productive experimental were carried out in this sewage treatment plant to find out the difference between the treatment effect with and without sludge ruflux in CAST process and the reason why the effluent quality could reach the standard without sludge reflux,and to provide a theoretical basis for simplifing the operation mode of other sewage plant and save the energy consumption.In this paper,the characteristics of the water quality and sludge of the sewage plant in 2019 were studied.The removal rules of nutrients were studied through the field experiments of different sludge return flows in summer and winter.In order to explain the influence of different return flow ratio on the treatment effect of the process in micro perspective,the material balance calculation was carried out to explore the distribution and utilization of substances.What’s more,the change of the concentration of ammonia and nitrate nitrogen and the sludge concentration in each reaction period of CAST process and the cause of sludge granulation in sewage treatment plant were also studied.Finally,the theory of the stable operation of the CAST process was also discussed.The conclusions are as follows:(1)Through the analysis of the concentration of the influent and effluent of the sewage plant and the sludge characteristic of CAST process in 2019,it was found that the concentration of pollutants in influent was lower than the national and Chongqing average levels.In addition,the plant needed to add additional carbon sources to remove nitrogen effectively.In addition,the sewage plant reached the first-level A standard of sewage discharge standard when there is no sludge recirculation.And the sludge concentration in CAST process was high throughout the year while the organic load of sludge was at a low level.Except for July,the SVI was long for the rest of the time.(2)The reflux ratio R = 0 and R = 50% were tested in summer and winter.And it was found that there was no significant difference in the removal of nutrients.The removal effect of nutrients in outlet water was better.In summer,under the working conditions of R = 0 and R = 50%,the reduction rates of COD were 63.70% and 67.7%,the removal rates of ammonia nitrogen were81.12% and 88.44%,the reduction rate of total nitrogen were 24.79% and 22.94%,and the reduction rates of total p Hosp Horus were are 76% and 73 %,respectively.In winter,the nutrient reduction rate under R = 50% condition was slightly higher than that under R = 0.Under the working conditions of R = 0 and R = 50%,the reduction rates of COD were 80.47% and 85.48%,the ammonia nitrogen reduction rates were 79.56% and 85.96%,the reduction rates were 40.23% and 42.63%,the reduction rates of total p Hosp Horus were 51.48% and 58.38%,respectively.At the same time,it was found that the buffering capacity of the selector and the anoxic tanks under the condition of R=50% was improved by the sludge recirculation in the influent stage.(3)Through the nutrient balance calculation,it was found that no matter in summer or winter,when R = 0 and R = 50%,there was no significant difference in the distribution of carbon,nitrogen and p Hosp Horus,and the proportion of remained nutrients in the system was relatively high.The calculation of the carbon balance in summer showed that 25%-30% of the carbon source in the two conditions of R = 0 and R = 50% were still in the CAST system when the reaction was completed.And the proportion of the oxidized carbon in the process was high and the carbon source was not effectively utilized.The calculation effect also showed that the effciency of denitrification rate was low,only about 12%was uesd by denitrification,and more than 70% of the TN in CAST process was not removed effectively.It was found that the concentration of total p Hosp Hous in influent was low and a large number of it was consumed mainly through microbial assimilation.But about 50% of TP was not effectively removed.At the same time,it was found that the concentration of nutrients up to the discharge standard in summer mainly depended on the low concentration of influent and the low sludge organic load.In winter,the result of balance calculation of carbon showed that about 1/3 of the carbon in the two working conditions of R = 0 and R = 50% was directly oxidized leading to the serious waste of carbon source.Compared with the effect in summer,the proportion of TN consumed by denitrification has nearly doubled.It meant that the denitrification in the CAST system was more robvious in winter.In terms of the calculation of p Hosp Horus balance,although the sludge age was relatively long,about1/3 of TP was absorbed by PAOs,and about 40%-50% of TP was not removed effectively.In addition,various nutrients reaching the discharge standard in winter depended on the better denitrification and the lower sluge load.(4)Through analyzing the changes of ammonia nitrogen concentration and sludge concentration under the condition of R = 0 and R = 50%,it was found that there was obviously sludge recirculation between the selection pond and anoxic pond and anoxic pond and aerobic pond.It was beneficial to suppress the expansion of filamentous bacteria,which may be one of the reasons for the stable operation of the sewage treatment plant.In addition,the sludge particle size tests showed that there was granular sludge in the aeration tank of the plant,which was conducive to the rapid settlement for the sludge and the inhibition of the filamentous expansion.And it was believed that the causes for sludge granulation might be due to the characteristics of CAST process itself,the PFS added at the front process of the sewage plant,and the operation mode without sludge recirculation. |
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