AN EXPERIMENTAL STUDY OF THE OPTIMAL THICKNESS OF A SAND .

DC WRRC Report No. 178AN EXPERIMENTAL STUDY OF THE OPTIMAL THICKNESS OF A SANDLAYER IN A SAND FILTER WATER QUALITY STRUCTUREJuly 1994D.C. Water Resources Research CenterUniversity of the District of Columbia 4200Connecticut Ave, NW Building 50, MB5004 Washington, DC 20008

AN EXPERIMENTAL STUDY OF THE OPTIMAL THICKNESS OF A SAND LAYER INA SAND FILTER WATER QUALITY STRUCTURESubmitted by:Farshad AminiFred Chang andJutta Schneider

DC WRRC REPORT NO. 153AN EXPERIMENTAL STUDY OF THE OPTIMAL THICKNESS OF A SAND LAYERIN A SAND FILTER WATER QUALITY STRUCTUREJuly 1994D.C. Water Resources Research Center University of theDistrict of Columbia 4200 Connecticut Ave, NW Building50, MB 5004 Washington, DC 20008

TABLE OF CONTENTS1 INTRODUCTION . . 12 OBJECTIVES . . .23 SCALE MODEL AND TESTING PROCEDURE . 24 RESULTS . . 45 CONCLUSIONS . . 116 REFERENCES . . 11APPENDIX I. GRAIN SIZE DISTRIBUTION FOR THE THREE SANDS TESTEDAPPENDIX II. DETAIL OF LABORATORY TESTS

AN EXPERIMENTAL STUDY OF THE OPTIMAL THICKNESS OF A SAND LAYER IN ASAND FILTER WATER QUALITY STRUCTURE1.- INTRODUCTIONConventional infiltration devices are often used for water quality control of runoff in anurban environment. These types of best management practices (BMPs) adversely impactgroundwater quality (e.g. EPA, 1983, Nightingale, 1987). In addition, these conventionalBMPs may not be feasible in ultra.-urban environments because of the large land areasrequired for their installation.In an effort to address these limitations, in the District of Columbia, an alternativesolution, i.e. underground confined Sand Filter Water Quality (SFWQ) Structure has beenconsidered (Truong et al., 1993). A sand filter can be used to remove fine particles in water.While the water moves through the filter media, the fine particles in the water are trapped inthe voids of the filter media. This process continues until the size of the voids has beenreduced considerably. The water ceases to flow when all the porous spaces have beencompletely filled. When the flow rate reduces to a certain limit, the filter media needs to bereplaced or the deposited particles needs to be removed from the filter media. Sedimentparticles trapped in sand filters may be cleaned by back-washing with clean water. For a sandfilter in the field, back-washing is impractical because clean water is hardly accessible. As aresult, the filter media of a sand filter needs to be replaced when the flow rate reaches its lowerlimit.This device may be used in the following areas:a. Surface parking lots;b. Parking apron, taxiway and runway shoulders at airports;c. Underground parking lots or multi-level garages;d. Emergency stopping and parking lanes, and sidewalks;e. Vehicle maintenances area;f. On street parking aprons in residential area;g. Recreational vehicle camping area parking pads;h. Private roads, easement service roads, and fire lanes;i. Industrial storage yards and loading zones;j. Driveways for residential and light commercial use;k. Office complexes.This research was funded by a grant from the U.S. Geological Survey through the DCWRRC of the University of the District of Columbia. The research was performed in the SoilMechanics Laboratory of the Civil Engineering Department at the University of the District ofColumbia.

2. OBJECTIVESThe grain size of the filter needs to be chosen such that the sand filter is as effective as possible. Ifthe grain size is too small, incoming particles can be trapped easily, but the effective zone will belimited to the uppermost layer of the filter medium. If the grain size is too large, no incomingparticles will be trapped, but will instead flow through the filter medium. As a result, the grain sizeof the filter medium needs to be selected such that the whole thickness of the filter medium is usedeffectively. The first objective of this study was to investigate the effect of particle size on theeffectiveness and efficiency of sand filter, with the ultimate goal of establishing the optimal grainsize for the sand filters to be installed in the District of Columbia.In addition, though a thicker sand layer has more space to trap particles, the particles are not trappedevenly throughout the layer. In fact, the distribution of particles is highly skewed. The concentrationis very high at the upstream, or inflow, side of the sand layer, and drops rapidly. The secondobjective was to determine the relationship between the thickness of a sand filter and itseffectiveness in removing fine particles suspended in water. In this case, the ultimate goal is toestablish the optimal sand bed thickness of a sand filter to be installed in the District of Columbia.To accomplish these objective, laboratory tests were conducted using a scale model of a sandfilter. Three types of sands, and sand thicknesses ranging from 6 inches to 12 inches were utilized. Itmay be noted that the research plan does not consider the effect of the size of the incoming particle.A study by Sherard et al. (1984) indicated that uniform sand filters will catch particles with diameterof about 0.11D15 when the particles are carried in suspension in seeping water.3. SCALE MODEL AND TESTING PROCEDUREThe sand filter model, as shown in Figure 1, was built in the Soil Mechanics Laboratory at theUniversity of the District of Columbia. The model is built of plexiglass, and it consists of a sandfilter, a 2 inch thick gravel, and a filter fabric (geotextile) to separate sand from gravel. A 1 1/2 inchperforated pipe in a gravel bed is used to discharge and direct the flow to the outflow pipe.For this experiment, three types of sands, namely fine, medium, and coarse sand were used. Thegrain size distribution for these three sand types are shown in Appendix I. The coefficient ofpermeability for the fine, medium and coarse sand was found to be approximately 0.02, 0.10, and0.2 ft/min, respectively. To study the effect of sand thickness on the efficiency, three sandthicknesses, 6", 9", and 12" sand, were used for each sand type.Water-sediment mixtures were obtained from various storm drain inlet locations in the Districtof Columbia. After the sediment was added to the inflow tank, it was thoroughly mixed

OUTFLOWTO STORMINFLOW FROMFLOWSEPARATOR

with water. Every thirty minutes, two (2) samples, one 0.5 ft above the filter media to represent theinflow concentration, and the other from the outflow to indicate the concentration of the outflowwere taken. Each sample was about 100 ml. Two series of tests for each soil type and each sandthickness were conducted to obtain an average value. Each series were done for about 4 hours.To obtain weight of sediment for both inflow and outflow conditions, filters with a normal poresize of about 1.0 um were used. After the sample was filtered, it was placed in an oven at 110 C.The weight of the sediment was obtained by subtracting the total weight of the filter with sedimentafter it had been dried, from the weight of the filter.The sediment concentration by weight (ppm) for both inflow and outflow conditions can becomputed using the following equation.Sediment Concentration (ppm) weight of sediment x 106weight of the sampleSediment concentration (ppm) were obtained for each sand type and each sand thickness at 30minute time interval. After two series of tests were completed, average values of ppm could bedetermined. A detail of testing procedure is presented in Appendix II. The efficiency of the sandfilter was then determined for each test. The efficiency is one minus the ratio outflow/inflowsediment concentration.4. RESULTSPlots of sediment concentration (ppm) Vs. time for both inflow and outflow conditions for the 12inch sand filter and for each sand type are shown in Figures 2 through 4. The sedimentconcentration (ppm) for both inflow and outflow conditions decreased with time for all sand types.The sediment concentration (ppm) values corresponding to the outflow condition was much lessthan the ones for inflow conditions for all sand types, indicating the effectiveness of the scale modelsand filter.A comparison of the effect of sand type on the efficiency of the sand filter is shown in Figure 5.The efficiency of the sand filter system with the medium sand was the highest, indicating arelatively better performance for the medium sand than the other sand types. The efficiency for thesand filter with the fine sand was slightly higher than the one with the coarse sand.The effect of sand thickness on the efficiency for each sand type is shown in Figures 6 through8. Within the range of variables studied, as filter thickness increased from 6 inches to 12 inches, theefficiency also increased.

inflow & outflow (ppm X 1000)FINE SANDtime (hours)Figure 2. Inflow and Outflow Sediment Concentration As a Function of Time for Fine Sand-

Figure 4. Inflow and Outflow Sediment Concentration As a Function of Time for Coarse Sand.

Fine SandEfficiency100908706050Time (hours)Figure 6. Effect of sand thickness on the efficiency of sand filter for fine sand

5- CONCLUSIONSThe results of this preliminary study revealed that the use of the medium sand provided ascale model sand filter with the highest efficiency. The use of the coarse sand was found to bethe least effective. Within the range of variables studied, a thicker sand bed resulted in a moreeffective SFWQ. A more comprehensive study may need to be conducted to include the effectof a variety of factors including uniformity of sand particle size, sand particle shape, incomingsediment particle size, and longer durations. In addition, a comparison of the results betweenscale model laboratory testing and full scale testing in the field may be beneficial.6- REFERENCES1. Results of the Nationwide Urban Runoff Program, Final report, U.S. EnvironmentalProtection agency, Water Planning Division, 1983,2. Nightingale, H. T., "Water Quality Beneath Urban Runoff Water Management Basins,"Water Resources Research, 23(2), 1987, 197-208.3. Truong, H. V., Burrel, C. R., and Phua, M. S., "Application of Washington, DC Sand Filterfor Urban Runoff Control," D.C. Dept. of Consumer and Regulatory affairs, EnvironmentalRegulation Administration, 1993.4. Sherard, J. L., Lorn, P. D., and Talbot, J. R., "Basic Properties of Sand and Gravel Filters,"Journal of Geotechnical engineering, ASCE, 110(6), June 1984, 684-700.

APPENDIX IGRAIN SIZE DISTRIBUTION FOR THE THREE SAND TYPES

Fine SandPercent finer00.010.1110Grain Size, D (mm)Medium Sand100Percent finer8060402000.01LJ -LJ0.11Grain Size, D (mm)10

Coarse Sand100Percent finer8060402000.010.11Grain Size, D (mm)10

APPENDIX IIDETAIL OF LABORATORY TESTS