14.4 Design Criteria for All Sand Filtration BMPs

Comply with all criteria and standards in Modeling Your Best Management Practices, Design Criteria for All Stormwater Treatment and Flow Control BMPs, Constructing Your Best Management Practices and Accessing and Maintaining Your Best Management Practices as applicable to the project in addition to criteria within each BMP. Where criteria or standards conflict, utilize the criteria and standards contained within the specific BMP.

Sand filter design criteria are as follows:

Hydraulic head of 4 feet from inlet to outlet is required for BMP T808 - Basic Sand Filter, BMP T809 - Large Sand Filter, and BMP T810 - Sand Filter Vault.

Sand filters shall be sized to capture and treat the water quality design volume, which is 91% of the total runoff volume as predicted by an Ecology approved continuous simulation model, assuming a 15-minute timestep.

The design hydraulic conductivity shall be 1 in/hr.

Online sand filters must be placed downstream of a detention facility. This is to prevent exposure of the sand filter surface to high flow rates that could cause loss of media and previously removed pollutants.

Online sand filters placed downstream of a detention facility must be sized using an Ecology approved continuous simulation model, assuming a 15-minute timestep, to filter the on-line water quality design flowrate. This ensures the sand filter is treating 91% of the total runoff volume.

Offline sand filters placed upstream of a detention facility must have a flow splitter designed to send all flows at or below the 15-minute water quality flowrate, as predicted by an Ecology approved continuous simulation model, assuming a 15-minute timestep, to the sand filter. The sand filter must be sized to filter all the runoff sent to it (no overflows from the treatment facility should occur).

Offline sand filters placed downstream of a detention facility must have a flow splitter designed to send all flows at or below the 2-year return period flowrate as estimated by an Ecology approved continuous simulation model, assuming a 15-minute timestep, from the detention facility to the treatment facility. The treatment facility must be sized to filter all the runoff sent to it (no overflows from the treatment facility should occur).

For online sand filters include an overflow in the design. The overflow height shall be at the maximum hydraulic head of the water above the sand bed. Online sand filters shall provide overflow structures designed in accordance with BMP F110 - Detention Ponds.

Pretreat runoff to be treated by the sand filter (e.g., presettling basin, etc. depending on pollutants) to remove debris and other solids, and oil from high use sites.

Design inlet bypass and flow spreading structures (e.g., flow spreaders, weirs or multiple orifice openings) to capture the applicable design flowrate, minimize turbulence and to spread the flow uniformly across the surface of the sand filter. Install stone riprap or other energy dissipation devices to prevent gouging of the sand medium and to promote uniform flow. Include emergency spillway or overflow structures (see A400 - Outfall Systems and A300 - Flow Spreading Devices).

If the sand filter is curved or an irregular shape, provide a flow spreader for a minimum of 20% of the filter perimeter.

If the length-to-width ratio of the filter is 2:1 or greater, locate a flow spreader on the longer side of the filter and for a minimum length of 20% of the facility perimeter.

Provide erosion protection along the first foot of the sand bed adjacent to the flow spreader. Methods for this include geotextile weighted with sand bags at 15-foot intervals and quarry spalls. The geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Moderate Survivability, Class A.

Include underdrain piping in sand filter design. Types of underdrains include a central collector pipe with lateral feeder pipes; or a geotextile drain strip in an 8-inch gravel backfill or drain rock bed; or longitudinal pipes in an 8-inch gravel backfill or drain rock with a collector pipe at the outlet end.

Size the underdrain piping for the two-year return frequency flow indicated by an Ecology approved continuous simulation model, assuming a 15-minute timestep. Provide at least 1 foot of hydraulic head above the invert of the upstream end of the collector pipe.

Internal diameters of underdrain pipes shall be a minimum of six inches having two rows of ½-inch holes spaced 6 inches apart longitudinally (maximum), with rows 120 degrees apart (laid with holes downward). Maximum perpendicular distance between two feeder pipes or the edge of the filter and a feeder pipe, must be 15 feet. All piping is to be schedule 40 PVC or greater wall thickness.

Main collector underdrain pipe shall be at a slope of 0.5 percent minimum.

A geotextile fabric (specifications in Appendix B) must be used between the sand layer and drain rock or gravel and placed so that 1-inch of drain rock/gravel is above the fabric. The geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C. Drain rock shall be 0.75-1.5 inch rock or gravel backfill, washed free of clay and organic material.

Provide cleanout wyes with caps or junction boxes at both ends of the collector pipes. Extend cleanouts to the surface of the filter. Provide a valve box for access to the cleanouts. Provide access for cleaning all underdrain piping. This may consist of installing cleanout ports that tee into the underdrain system and surface above the top of the sand bed. To facilitate maintenance of the sand filter an inlet shutoff/bypass valve is recommended.

Sand specification: The sand in a filter shall be a minimum 18” in depth and must meet the size gradation (by weight) given in Table 4 - 20: Sand Specifications. The contractor must obtain a grain size analysis from the supplier to certify that the sieve requirements are met.

Impermeable Liners for Sand Bed Bottom: Impermeable liners are generally required for soluble pollutants such as metals and toxic organics and where the underflow could cause problems with structures. Impermeable liners may be made of clay, concrete or geomembrane. Clay liners shall have a minimum thickness of 12 inches and meet the specifications given in Table 4 - 21: Clay Liner Specifications.

If a geomembrane liner is used it shall have a minimum thickness of 30 mils and be ultraviolet light resistant. The geomembrane liner shall be protected from puncture, tearing, and abrasion by installing geotextile fabric on the top and bottom of the geomembrane. The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile Strength Properties for Impermeable Liner Protection.

Concrete liners may also be used for sedimentation chambers and for sedimentation and sand filtration basins less than 1,000 square feet in area. Concrete shall be 5 inches thick Class A or better and shall be reinforced by steel wire mesh. The steel wire mesh shall be 6 gauge wire or larger and 6-inch by 6-inch mesh or smaller. An "Ordinary Surface Finish" is required. When the underlying soil is clay or has an unconfined compressive strength of 0.25 ton per square foot or less, the concrete shall have a minimum 6-inch compacted aggregate base. This base must consist of coarse sand and river stone, crushed stone or equivalent with diameter of 0.75- to 1-inch.

If an impermeable liner is not required then a geotextile fabric liner shall be installed that retains the sand. The non-woven geotextile shall conform to A2000 - Geotextile Specifications, Geotextile for Underground Drainage - Low Survivability, Class C.

If an impermeable liner is not provided, then an analysis shall be made of possible adverse effects of seepage zones on groundwater, and near building foundations, basements, roads, parking lots and sloping sites. Sand filters without impermeable liners shall not be built on fill sites and shall be located at least 20-feet downslope and 100-feet upslope from building foundations.

Include an access ramp with a slope not to exceed 7H:1V, or equivalent, for maintenance purposes at the inlet and the outlet of a surface filter. Access ramp shall conform to A3000 - Access Ramps and Roads.

Side slopes for earthen/grass embankments shall not exceed 3H:1V to facilitate mowing.

There shall be at least 2 feet clearance between the seasonal high groundwater level and the bottom of the sand filter.