Variations In Dissolved Oxygen Under Different Mixing Conditions And Its Effects On Controlling Iron And Manganese Pollution In A Stratified Reservoir

In recent years,the aesthetic problems and health risks associated with hypolimnetic anoxia,excess Fe and Mn in drinking water systems have received much attention worldwide.The naturally-or artificially-induced mixing can effectively increase the dissolved oxygen level of water and control the pollution status of iron and manganese.Based on the typical stratified reservoir(Jinpen Reservoir)in north China,by using multi-parameter analyzer,the microelectrode and diffusive gradient technology,this paper focusesed on ?the DO depletion in hypolimnion during the summer stratification and the kinetics of DO transfer at the sediment-water interface(SWI);?Characteristics of Fe and Mn release from sediment under different mixing condtions;?Controlling the Fe and Mn pollution in JinPen reservoir;?The connection of artificial and natural mixing periods.The main results are as follows:(1)During the stratification period,the DO consumption in hypolimnion and at the SWI were investigated.Before installation of the Water lifting and aerators(WLAs),the hypolimnetic oxygen comsumption rate is 955 kg/d,after years operation of the WLAs,the comsumption rates are 219 kg/d,208 kg/d and 403 kg/d in 2014,2016 and 2017,respectively.In 2015,the alage blooms largely increased the comsumption rates that the rates are 703 kg/d.The sediment oxygen demand(SOD)is made up of the sediment oxygen uptake(JO2)and the flux of the reduced substances(Fred).SOD accounts for 54.2?75%of HOD of Jinpen Reservoir.At the ealy stage of the stratification,the SOD is 0.534mg/m2/d,and the JO2 is the main component of sediment oxygen demand,accounting for 72.3%.Inversely,at the end of the stratification,the SOD is 0.386mg/m2/d,and the Frea is the main component of sediment oxygen demand,accounting for 56.5%.A model of SOD was expressed as a function(?)of the shear velocity(u*)and the bulk DO concentration(Cbuik).Theoretical predictions were validated by microelectrode measurements in laboratory experiments.(2)The effects of dissolved oxygen in different mixed states of water on the release of Fe and Mn from sediments were expounded.By in situ detection of the Fe and Mn pollutants in overlying water,pore water and sediment,the release of Fe and Mn at SWI is studied,the results show:The vertical concentration gradient of dissolved iron and manganese in pore water is the driving force for diffusion and release.JFe increases from 0.96 mg/m2/d to10.65 mg/m2/d,and JMn increases from 1.3 mg/m2/d to 12.35 mg/m2/d when the temperature increasing and DO decreasing in overlying water.The diffusion of dissolved Mn in pore water can lead more than 90%active manganese in sediment.(3)The in-situ control process of iron and manganese pollution by WLAs was revealed.The artificial forced mixed oxygenation process can maintain a 1?2 mm thick aerobic layer at the SWI.The existence of this aerobic layer can effectively prevent the release of dissolved iron and manganese pollutants from sediments to the overlying water.Dissolved Fe is easily removed by abiotic oxidization with a rate of 0.01 mg/L/d,but dissolved Mn is persistently accumulated in near-sediment regions during the WLAs operation with a rate of 0.002 mg/L/d.The JMn and were enhanced from 5.1 mg/m2/d to 14.3 mg/m2/d,while the JFe decreased to 0.3 mg/m2/d during WLA operations.(4)Succusesfully induced the continuously natural complete mixing and controlled the iron and manganese pollution.The operation of the water-lifting aeration system can induce the natural continuous complete mixing in advance about 78 days and extend the mixing period by 2.6 times.The condition is that the temperature of complete mixing should be higher than the daily average temperature of the basin.During the periods,the DO is more than 8 mg/L that the Fe and Mn concentrations was less than 0.05 mg/L,and the JMn was decreased from 4.8mg/m2/d to 3.79 mg/m2/d,and,JFe was to low to determined.