Drainage System Sewer Overflow On-site Control Technology And The Treatment Unit Simulation Research Using CFD

With the accelerated urbanization of China, the Combined Sewer Overflows (CSOs) pollution had become increasingly serious. The CSOs contained a large number of pollutants that would seriously damage the environment and threaten to the human health if they were directly discharged into rivers without any effective treatment.According to the water quality of the wet sewer overflow from the Iranian wastewater transfer pipes in Shanghai, a set of device to treat the overflow was developed using the enhanced flocculation combined with the high-efficiency sedimentation. The design ideas, the mechanism and functions of the device were introduced. The pipeline static mixer were applied to replace the traditional mixing unit, the draft-tube was installed to rearrange the flow and enhance flocculation process . The inclined tubes and the suspended filters were added in the settling tank to improve the sedimentation efficiency. Moreover, the software FLUENT was used to simulate and optimize the flow status in the sedimentation unit.Pilot experiments were conducted to treat the wet sewer overflow on-site. The relevant influence factors were investigated. Different kinds of wet sewer overflows in various weather conditions were researched. The flow of dry weather, drizzly day, strong rainfall, or storm even that in Meiyu period and winter season with persistent rainfall were treated by the polit device. The results showed that the device could greatly reduce SS, TP, COD and other pollutants, effectively control the wet sewer overflow pollution. The removal rate of SS, turbidity, TP and COD of the storm overflow reached 82%,79%, 77% and 69% on average, respectively. The surface hydraulic load of the sedimentation tank could reach 32m~3/h.m~2 that is 10 times more than the general precipitation used in wastewater plant. The impacts of sludge recycling, the surface hydraulic load of the sedimentation and the pharmaceutical dosage positions on the water quality were investigated. It was to find out whether such measures as the underflow emissions, inclined tubes adding in sedimentation unit, the channel mixer application could improve outflow quality. Sludge recycling was not recommended in the overflow pollution control. The optimal pharmaceutical dosage point should set at the inlet pipe of the reaction tank. The concentration of effluent SS and other water quality index were increased with the hydraulic load of sedimentation unit. Underflow discharge could effectively control redundant sewage overflow.The main functions of the inclined tubes in the sedimentation unit are to rectifier the flow and improve the water-solid separation. The applying of pipeline static mixer could reduce the mixed pool and relevant mixing equipments and prevent the pharmaceutical dosage from mixing with the air, which was effectively reduced the scum in the reaction unit and the sedimentation unit and improved outflow quality.The respondent time of the device in different start-up modes and its impact factors were researched. The results showed that the respondent time was 25～30min in the dry mode start-up and the effluent quality was not so good. The NTU and SS reached to 120 degrees and 100mg/L, respectively. However, with the wet mode start-up, the respondent time was about 10min. The effluent quality was satisfactory. The NTU was about 60 degrees and the SS was less than 60mg/L during the start-up period. The wet mode start-up was recommended to apply in control with the wet sew overflow pollution.Suspended filters were placed upper the inclined tubes to intercept the floc in the upflow. The performance of inclined tube combined filters sediment tank was investigated. Results showed the new sedimentation tank could increase the surface hydraulic load and greatly reduce the flocs in the effluent. Good and stable effluent quality was achieved. The flow-field distribution in the sedimentation unit of the device was simulated by FLUENT using theκ-εtwo equations model. Serious back-flow and large hydraulic dead zones were found. Based on the analysis, the baffle plate was decided to set under the weir of the inlet zone, at the left of the tank bottom. The simulated result of the modified tank was showed that the flow-field distribution was much more reasonable than the old one.The high-efficiency sewer overflow treatment device had a lot of advantages, such as quick start-up, high treating capacity, hydraulic Shock-Resistant, stable effluent quality, simple and compact treatment units, small occupied area, and easy operation and so on. Furthermore, the sludge produced by the device could flow back into the sewage pipeline, so the sludge treatment facilities were not necessary. The device can be used to control the wet sewer overflow pollution effectively in those rainy cities.