Laboratory Research On The Law Of Suspended Solids Clogging During Urban Stormwater Groundwater Recharge

Urban stormwater groundwater recharge as an effective method in dealing waterresources storage issues is receiving increasing interests recent years. It has been occupied interms of relieving the pressure of water resources storage, optimizing the water resourcesallocation, minimizing the side effects of groundwater overexploitation and resolving theproblem of urban flood and drainage under the extreme hydrogeological conditions. However,this technique cannot be widely adopted because of uncertainty clogging behaviors which arecaused by suspended materials. Therefore, sound understandings of clogging mechanism bothin theory and practice are urgently needed. The research presented here are followingcomprehensive researching steps: background description of urban stormwater quality—mechanism experiments of suspended clogging—influences of water quality on suspendedclogging—prevention and government of clogging site. First, theories of flow and masstransportation in porous media are applied to describe the characteristics of urban stromwaterand suspended particle movements during various clogging types. Next, the mechanism ofsuspended clogging development was studied by combining laboratory experiments withnumerical models to simulate and predicate suspended clogging revolution, interpret theeffects of water quality on clogging and summarized methods of suspended cloggingprevention and control. The main conclusions of this study could be described as follow:Sampling sites were located around the Cultural Square and stormwater were collectedfrom different underlying surface for pollutants detecting. Chemical analysis results shownthat the nature rainfall contained only several kinds of contaminants with quite lowconcentrations; Water quality of the roof runoff was worse than the nature rainfall due to theretaining contacts with roof materials; Road surface stormwater runoff was affected by trafficand municipal refuses carried lots of pollutants that exceeded the Ⅴlevel of surface wateraccording to quality standard, which made the quality worst among those three runoffs.Besides, road surface stormwater quality outside the school was obvious worse than the insidebecause of frequently activities. Therefore, natural rain and later period roof runoff is the mostclean water in city which can be collected as the optimal water sources for groundwaterartificial recharge, and the road stormwater runoff must be pretreatment before using. The grain size characteristics, mineral and chemical content of suspended solid carried by thestormwater runoff erosion were analysed to demonstrate that the petrology of suspended solidwas mainly silt with a good particle grading. Through mineral and chemical composition tests,it can be concluded that quartz, plagioclase and alkaline feldspar took a large part in mineralcomposition and a small amount of clay was detected. Results revealed that Ca2+was themajor soluble salt in suspended, followed by Na+, Mg2+and K+for the cations; The anionicconditions are: concentration of HCO3-was the highest, SO42-, Cl-, NO3-and F-weredecreasing in turn. According to the occurrence mechanisms of different clogging types,experimental data, and urban stormwater quality characteristics, the clogging potential hadbeen evaluated. It displayed that suspended solids blocked the paths was the dominant reasonfor clogging and water quality were significant factors.According to the spatial distribution features of suspended clogging, the clogging can bedivided into4types: surface cake filtration, internal pore blockage, internal-surface cloggingand unclogging. For the purpose of accurately determining the criterion of different suspendedclogging, this study conducted a lot of laboratory experiments, and obtained the cloggingmodes of critical division Dp/d50(an important parameter of clogging mode) values. When theratio of Dp/d50was lower than1, surface cake filtration would take place. When the Dp/d50ratio exceeded1but was less than180, it would happen internal-surface clogging. When thevalue of Dp/d50was more than180, internal pore blockage happened. Hydraulic conductivity,TDS, the spatial distribution of the suspended particles and clogging rate during theseexperiments had been analysed. Conclusions on account of those works were:(1) There wasmere physical clogging during artificial recharge by comparing TDS concentration in inflowand outflow.(2) Suspended solids could only penetrate a few centimeters in surface cakefiltration process with the unevenly vertical distribution, which indicated that the bigger Dp/d50means deeper clogging effective depth.(3) During the same clogging process, obviousinfluences of medium and suspended particles on determine clogging rate were easilyobserved. More specifically, when the aggregates of medium were constant, faster cloggingrate might more frequently appear in smaller Dp/d50patterns. On the other hand, when theaggregates of suspended particles were constant, the medium with better penetration abilityhad the faster clogging rate. Membrane filtration model and equivalent permeability coefficient model were utilized toinvestigate clogging behavior during surface cake infiltration process. In membrane filtrationmodel, parameters are illustrated with standard error and minimum value. The parameters ofmedium sand were: α=0.45m2/kg, f 'R'=20000m/kg, and α=0.15m2/kg, f 'R'=20000m/kg of fine sand. The membrane filtering model obtained a good fitting result with theexperimental data and correlation index of the measured data and calculated value was0.98.On the other hand, the variation of permeability coefficient during clogging was obtainedbased on laboratory experimental data, which was then combined with equivalent permeabilitycoefficient formula to create the mathematical model to simulate the surface clogging process.This model was also able to reflect the real surface clogging behavior in details and can beused to as a prediction tool for surface clogging. As soon as the tow models were built, urbanstormwater infiltration experimental data was arranged to verify the applicability accuracy andboth obtained the ideal agreements. For internal pore blockage clogging simulation,modification of model expression and parameters determination through experimental resultswere carried based on the existing model (originally combined water flow movement andsolute transportation). It showed that this model had high practical value and could be taken asan efficient accurate tool to simulate the blocked development of internal process.Column experiments within different water injection (deionized water and tap-water)were applied to study the effects of chemical composition in suspended solids cloggingprocess. The results shown that the oxides (for example, Fe3+and Al3+) in deionized water andtap-water were the main reasons for permeability reduction of infiltration. In addition,suspended solids and tap-water were mixed evenly and then injected into quartz sand tocompare with the results of pure water suspended. Results shown that chemical precipitation(Fe (OH)3and Al (OH)3) in tap-water deposited mainly in the medium, which would increasethe surface area of the surface infiltration and lower down the head of injection process andconsequently aggravated the clogging development. This paper used infiltration model andequivalent permeability coefficient model to simulate surface clogging process during thetap-water and suspended solids mixture transported in quartz sand column, yet measured andcalculated value fitted poorly. Some modifications were operated on the model parameters andformula expressions, the outcomes have been obviously improved. The paper reviewed the existing clogging prevention and controlling methods through thedomestic and international clogging prevention and management literatures, which could bespecifically demonstrated in terms of recharging water quality, engineering structure ofinjection design and process controlling. Consequently, water quality turns out to be thedominant part. The available methods of restoring hydraulic conductivity includes mechanical(scraping and water flushing), gas injection, ultrasonic, etc. Water flashing, water disturbanceand gas injection were selected to control clogging in this paper. The results revealed that theeffect of water flashing was very small due to the weak strength of flashing water. On thecontrary, water injection disturbance can make the surface deposits refloating and obviousimproved infiltration medium to make recharging water moving smoothly. Gas injection canbe adopted to manage internal-surface clogging, but only achieve a better result during theinitial stage.