1、Center for Watershed ProtectionCopyright 2000,CWPCopyright 2000,CWPFiltering SystemsSix Design Variations Surface sand filter Perimeter sand filter Organic sand filter Underground sand filter Pocket sand filter BioretentionCopyright 2000,CWPFilter System Design Components Flow Regulation Diversion o
2、f only water quality volume to facility Pretreatment Trapping of coarse sediments to extend design life Filter Bed and Filter Media Primary treatment component of facility Outflow/Overflow Safe conveyance of all storms through facilityCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPFilter Medi
3、aComparison of Different Media Properties Sand Silt Loam CompostPeatPermeability(cm/hr)3.30.1-0.4 -0.25-140Water holding capacity(cm/cm)0.14.07-0.1 -.01-0.2Bulk density(g/cm)2.651.25 1-20.1-0.3pH -5.7 7.83.6-6.0Organic matter(%)120 30-7080-98Cation exchange capacity1-312-18 66183-265Total phosphorus
4、(%)00.09 0.10.1Total nitrogen(%)00.15 1.02.5Filtration efficiency after18 in.(%)9394 1647Copyright 2000,CWPSand Filter Selection GuideMost Appropriate Option by Land UseFilter Ultra-Parking Roads Residential Pervious Rooftopurban lots SurfaceYesIdealMaybeMaybe NoYesUndergroundIdealYesMaybeMaybeNoYes
5、PerimeterYesIdealMaybeMaybeNoYesPocketYesYesMaybeYesNoYesOrganicMaybeYesNoMaybeMaybeYesBioretentionMaybeIdealYesYesYesYesIdeal:the best alternativeYes:greatly suitableMaybe:may be suitable under certain conditionsNo:seldom or never suitableCopyright 2000,CWPSand Filter Selection GuideKey Feasibility
6、 FactorsFilter Space Minimum Maintenance Costconsumed head burdenSurface2-3%5 feetannualmoderateUndergroundnone4 feetsemi-annualhighPerimeter2-3%3 feetannualmoderatePocket2-3%3 feet annualmoderateOrganic1-2%5 feetannualhighBioretention5%4 feetsemi-annuallowCopyright 2000,CWPSurface Sand FilterDesign
7、 Features Aboveground facility First developed in Austin,Texas Wet or dry pretreatment(3 ft.min.)18 inch sand filter bed Exfiltration or underdrain system Concrete or earth construction Designed to treat larger drainage areasCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCop
8、yright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPPerimeter Sand FilterDesign Features Located at the perimeter of parking lots Developed originally in Delaware Two parallel trench chambers Two foot wet pool pretreatment 18 inch sand filter bed Underdrain system Ideal for small,highly impervious ar
9、eas Ideal for flat areas with relatively low available head.Copyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPOrganic FilterDesign Features Aboveground filter system Organic medium replaces or augments sand Peat&leaf compost,two most common media 24 inch peat/
10、sand filter bed 18 inch compost filter bed(proprietary system:CSF Treatment Systems,Inc.)Exfiltration or underdrain system Cover crop desirable for peat/sand systemCopyright 2000,CWPPeat Sand Filters Peat Qualities:High cation exchange capacity High C:N:P ratio(microbial)High organic matter content(
11、80-98%)Moderately decomposed,fibric or hemic(reed-sedge)Stays in placeCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPUnderground Sand FilterDesign Features Below-ground facility Developed in District of Columbia Three foot wet pool pretreatment 24 inch sand
12、 filter bed Underdrain system Confined space considerationsCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPPocket Sand FilterSystem Components Simplified low cost alternative Primarily for very small sites Level spreader,grass filter,plunge pool pretreatment 18 inch sand filt
13、er bed Exfiltration or underdrain system Cover crop with pea gravel windowCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPSand Filter Flow Regulation Locate off-line to handle only WQv storm Requires flow diversion structure to bypass larger storms Diversion structure can either be located at
14、facility(preferred)or upstreamCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPSand Filter PretreatmentAlternative Techniques for Different Filter Options Surface sand and organic filters dry detention for 24 hours,or wet pool with dry detention above Underground sand filter wet pool at least 3
15、 feet deep&dry detention above Perimeter sand filter wet pool with 2 foot depth&dry detention above Pocket sand filter concrete level spreader,filter strip&plunge poolCopyright 2000,CWPSand Filter PretreatmentSizing Criteria Area based on WQV Camp-Hazen equation:As=-(Qo/W)*Ln(1-E)As=0.066(WQV)ft2 fo
16、r I 75%Vmin=3/4(WQV)Copyright 2000,CWPSand Filter bedSizing Criteria Darcys Law Af=WQV*(df)/k*(hf+df)(tf)where:Af=surface area of filter(ft2)WQV=treatment volume(ft3)df=filter bed depth(ft)-can vary depending on the site conditions but should not be more 24(18 is the standard)k=coefficient of permea
17、bility(ft/day)hf=average head above filter bed(ft)-varies depending on the site conditions,but should not exceed 6 feet tf=time to filter through bed(days)-A value of 40 hours is recommended Copyright 2000,CWPSand Filter MediaCoefficient of Permeability ValuesFilter MediaSandPeat/sandCompostCoeffici
18、ent of Permeability(k,ft/day)3.52.758.7Copyright 2000,CWPSand Filter MediaDesign Components 18-24 inch filter bed(sand or organic)Cover crop for some applications grass-peat/sand,surface,pocket pea gravel window-pocket gravel and geotextile-underground Observation wells/cleanout pipesCopyright 2000,
19、CWPSand Filter OverflowSystem Components Flow distribution vault or weir 6-11 inch gravel underdrain system 4-6 inch perforated collection pipe Overflow or bypass weir or pipe Gate valve for dewatering Outlet chamberCopyright 2000,CWPSand FilterConstruction SpecificationsParameter Specification Size
20、SandASTM C-33 concrete,medium agg.02-.04in.PeatAsh content:12 in.,limit vegetation heightRepair/replace damaged componentsRepair/replace clogged/failing elementsCopyright 2000,CWPBioretention Areas Economical for small sites(1 acre or less)Easy to construct Compatible with commercial landscaping nee
21、ds Utilizes existing open space Limited performance data suggests pollutant removal comparable to or better than other filtering practicesCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWPCopyright 2000,CWP
22、Copyright 2000,CWPBioretentionSystem Components Off-line design Pea gravel filter diaphragm Grass buffer strip Ponding area Pea gravel overflow drain Organic layer(mulch)Planting soil Plant materials(trees/shrubs)Gravel/pipe underdrain systemCopyright 2000,CWPBioretention Flow RegulationDiversion fo
23、r Off-line Design Runoff capture of WQv Two flow splitter design options within drainage system within filtering practice itself Simple three step design compute WQv and WQ peak discharge size low flow hydraulic structure to practice size larger storm overflow structureCopyright 2000,CWPBioretention
24、 PretreatmentFilter Strip Sizing CriteriaParam eterImp ervio us P arkin g Lo tsR esid ential Law nsN otesMaxim uminflow approachlength(feet)357575150Filter stripslope2%2%2%2%2%2%2%2%M axim umslope=6%Filter stripm inim um1015202510121518Copyright 2000,CWPCopyright 2000,CWPBioretention PretreatmentGra
25、ss Channel Sizing GuidanceParameter 33%ImperviousBetween34%&66%Impervious 67%ImperviousNotesSlope2%2%2%2%2%2%Maximumslope=4%Grassedchannel min.254030453550Assumes a2 wideCopyright 2000,CWPCopyright 2000,CWPBioretention Filter MediaDesign ComponentsAf=WQV(df)/k(h+df)(tf)where:Af=Surface area of the b
26、ioretention planting bed(ft2)WQv=Water quality treatment volume(ft3)df=Planting soil bed depth(ft)4 ft recommendedk=Coefficient of permeability for planting soil bed(ft/day)-k=0.5 ft/day:Median value of a silt loam h=Average height of water above the bioretention bed(ft);havg=*hmax-h is equal to 3,a
27、ssuming a maximum ponding depth of 6 above the planting soil bedtf=Time required for the Water Quality Treatment Volume(WQV)to filter through the planting soil bed-A value of 72 hours is recommended Copyright 2000,CWPBioretention Filter MediaDesign ComponentsAf=D.A.x 5.0%x Rvwhere,Af=the required su
28、rface area of the bioretention facility,D.A.=the drainage area,and Rv=the volumetric runoff coefficientCopyright 2000,CWPBioretention AreasSpecifications Minimum width=15 to 25 feet Minimum length=30 to 50 feet Length to width ratio of 2:1 for widths 15 feet Maximum ponding depth=6 inches Maximum pl
29、anting soil depth=4 feet Drainage area=0.25 to 1.0 acres Maximum slope=20%Maximum entry velocity=3 feet/secondCopyright 2000,CWPLandscaping a Bioretention Area Minimum 3 species of trees and shrubs(each)Trees planted 12 feet on center(1000 stems/acre)Native trees and shrubs selected for tolerance for:pollution ponding dry soil Mulch layer typically shredded hardwood mulch Locate plant material near perimeter but not at inflow Care and replacement warranty(80%-one year)Normal landscaping maintenance
