Research On The Optimal Hydrologic Conditions Aimed At Reclaimed Water Deployment For Urban Rivers In Beijing

Reclaimed water has become the main water source of urban rivers and lakes in Beijing,contributing to a near 60%.While urban rivers are closely related to the living activities of nearby residents,algae bloom control,ecological system and landscape maintenance should be concerned during river management.In order to probe into the relationship between hydrology and water quality,an integrated method was applied in this study,combining with literatur e research,field monitoring and simulation tests.Firstly,based on existing research results about the influence of hydrologic conditions on water quality,the technology roadmap of following analysis was raised.Secondly,taking the upper reaches of Beijing's Qinghe River as a typical case,7 cross sect ions and 3 inflow points were set to collect data.By analysis and evaluation with the monitoring data from June to October in 2015 and July to October in 2016,major influence factors on water quality in Qinghe River were identified.Thirdly,with physical models,simulation experiments were designed to study the relationship between hydrology and phytoplankton growth,including an orthogonal pretest and three groups of gradient tests made in 3 preset water depth under 7 flow rates.Finally,taking the results of literature review,field monitoring and simulation tests into consideration synthetically,optimal hydrologic conditions were suggested for the management of urban rivers originated with reclaimed water in Beijing.The conclusions drawn from the research include the following points.(1)In Qinghe River,the conventional physical and biochemical indices had meet national standard of class IV,except Total Nitrogen(TN).Consequently,the reach was shown with mild-to-moderate eutrophication.Together with poor phytoplankton diversity,there was risk of algae bloom in the reach.(2)As suggested by monitoring data,the increase of velocity and decrease of water depth could lower the potential of algae bloom.However,it might also boost the resuspension of Total Organic Carbon(TOC),thus,the passing rate of turbidity had dropped by 37% in study area during 2016.In other words,the optimal hydrologic conditions should make both phytoplankton growth and sediment resuspension under control.(3)It was indicated in model tests that the growth rate of phytoplankton fluctuated with the change of flow velocity,and at the water depth of 0.70 m,the poorer phytoplankton diversity was revealed.In general,when the velocity was below 0.05 m/s,the higher potential of algae bloom was presented.Afterwards,the algae-bloom potential would touch the bottom around the flow rate of 0.10-0.15 m/s,and rise again within a narrow range at about 0.20 m/s.(4)In practice,the lower current velocity than 0.05 m/s is not recommended in spring and summer.To avoid algae bloom and sediment resuspension,the flow rate is not supposed to exceed 0.20 m/s either.For key reaches,the water depth of 0.5 m or lower is suggested.In addition,the local climate,the amount of accessible reclaimed water,the sediment condition and the management objective also need to be considered,so as to determine an optimal scheme for the management of rivers originated with reclaimed water.