Research On Strategy Of Combined Sewer System Retrofits And Pollution Control Ability Of The Intercepting Facility

The combined sewer system overflow pollution control is an urgent and arduous task in the battle of water pollution prevention and control in China.The combined drainage system is mostly used in the old urban area,and it is prone to combined sewer overflows(CSOs).The experience of many cities shows that retaining the combined system and transforming it can effectively CSOs,and the intercepting facility is one of the key structures in the reconstruction of the combined system.Therefore,it is of great significance to study the retrofit strategy of the combined system and the intercepting capacity of the intercepting facilities for CSOs control.In this study,the current problems of the combined system and the corresponding solutions were analyzed,and the different CSOs pollution control objectives and water environment capacity determination methods were compared and analyzed.A CSOs pollution control transformation strategy was established.The full-scale engineering tests and simulated fluid mechanics simulations were conducted for researching the water flow of different intercepting facilities.The methods for determining the different intercepting capacity of the intercepting facilities were established.The reconstruction design analysis of the combined system area was based on the simulation of the drainage network model and combined with the strategy and the method.The main conclusions are as follows:(1)For the reconstruction of the combined system,the existing drainage facilities need to be repaired and dredged to restore the original flow space.At the same time,the water environment capacity and the CSOs pollution control target and its limit value should be determined.The gap between the current situation and the control target was analyzed.The improved design standards of drainage facilities were determined according to the gap.And flow-restricting devices were set up.Finally,the pollution control goals can be achieved by improving the rainy weather treatment capacity of the wastewater treatment plant or adding on-site treatment facilities.(2)The turning of the water flow and the reduction of the flow cross section occur in the intercepting facility.In addition,an easy deposition point appears on the side away from the intercepting pipe due to the water blocking effect in the weir intercepting facility.The the water flow is broken and a large amount of air is entrained in the trough intercepting facility due to the water drop,and increase energy loss.The weir-tough intercepting facilities are mainly used for the function of the tough when the ratio of the weir height to the tough depth is less than 1.2,and the function of the weir is mainly used when the ratio is greater than 1.4.The intercepting pipe can be divided into approximate wide crest weir flow,sluice free outflow,sluice flooded outflow and full pipe pressure flow according to the different water flow characteristic.(3)The methods for determining the flow intercepting capacity of different intercepting facilities have been established,which can be used to determine the intercepting volume of existing facilities,or to determine the weir height or tough depth of the proposed facility according to the designed intercepting volume.(4)The retrofit analysis of a combined sewer system is carried out.The drainage network model simulation results shows that the total annual overflow COD is 4.27 t,and the rainwater intercepting rate is 58.9%,and the overflow frequency is 29 times of the current pipeline network in the study area under the conditions of typical rainfall years.Studies have shown that the area needs to reduce 85% of pollutants(calculated as COD)from entering the river,and the total amount of interception and storage in the area needs to be increased to 4.86 mm/h.The finally retrofit plan was to remain the scale of the original intercepting facility,set up a flow limiting device,and add a 4 mm storage tank.The total annual overflow COD is 0.51 t,the pollutant reduction rate is 88.06%,the rainwater interception rate is 85.3% and the overflow frequency is 4 times after the renovation,which meets the pollution control requirements.