Study On Change Of Water Quality And Control Measures Of Cut-off Water Pipelines In DMAs

In order to explore the potential impact of cut-off pipelines and stagnant areas which are created in the construction of DMAs on water quality,the pilot platform was constructed with water pipelines of 9 years old to study the spatiotemporal changes of essential physical?chemical and biological parameters of water in cut-off pipelines.Utilize computational fluid dynamics simulation software to evaluate the specific impact level of the cut-off pipelines on main water supply pipelines and propose control measures.The main conclusions are as follows:(1)Spatiotemporal changes of water quality parameters of cut-off pipelines is obviousUnder stagnation conditions,the iron?turbidity and HPC get worse as stagnant time gets longer.After 12 hours,the water quality deterioration rate was significantly accelerated;the deeper the depth,the faster the water quality deterioration rate;the correlation analysis of the deterioration of water quality parameters shows that the deeper the depth,the less the water is disturbed by the main pipe,and the higher the correlation level of each parameter.(2)Deterioration of water quality in the cut-off pipe under special conditions will bring risks to the water quality of the main pipeThe results of indoor pilot platform test and CFD simulation show that the ion diffusion coefficient,the flow velocity of main pipes,the length of the cut-off pipelines and the pipe diameter all have an effect on the amount of iron diffusion.Under stable operating conditions,Excessive risk of the outlet water quality is low because the concentration of ferrous ion concentration in cut-off pipelines is quite low near the junction and the mixing time between the stagnant water and the outlet water is very short.Under special operating conditions,such as opening a cut-off valve after a stagnant time more than a certain time,the excessive risk of ferrous ion concentration of the outlet water will get higher.The installation of the valves should be avoided to near the junction of the main pipeline,ensuring the distance between the valves and the main pipes is at least 5 times the pipe diameter;Under the condition of intermittent water use,the ferrous ion in cut-off pipelines will get into main pipelines quickly,even if iron release doesn't exist in the main pipelines,and the excessive risk of concentration of ferrous ion of main pipelines will get higher after stagnation(3)Technical measures for water quality control of DMA cut-off pipeline are proposedThe pilot platform test shows that,in the pilot platform,aiming to ensure that the water in the cut-off pipe continually reaches the standard,the intermittent water discharge should be executed every 12 hours,and the amount of water discharged should be more than three times that of the stagnant water;When the bypass pipeline and low flow rate discharge arouse the cut-off pipelines internal flow velocity to reach 20%of the laminar critical velocity,the iron and dissolved oxygen can be controlled,and the water quality of the cut-off pipeline will be qualified continually.