14.8 BMP T810 - Sand Filter Vault
A sand filter vault is similar to an open sand filter except that the sand layer and underdrains are installed below grade in a vault. It consists of presettling and sand filtration cells.
14.8.2 Design Criteria for Sand Filter Vaults
Comply with Section 14.4 Design Criteria for All Sand Filtration BMPs and Section 14.5 Construction Criteria for All Sand Filtration BMPs, in addition to the criteria below.
Sand filter vaults may be designed offline or online.
The sand filter bed shall consist of a sand top layer, a geotextile second layer and an underdrain.
Optimize sand inlet flow distribution with minimal sand bed disturbance. A maximum 8-inch distance between the top of the spreader and the top of the sand bed is required. Flows may enter the sand bed by spilling over the top of the wall into a flow spreader pad or alternatively a pipe and manifold system may be used. Any pipe and manifold system must retain the required dead storage volume in the first cell, minimize turbulence, and be readily maintainable.
If an inlet pipe and manifold system is used, the minimum pipe size shall be 8 inches. Multiple inlets are required to minimize turbulence and reduce local flow velocities.
Provide erosion protection along the first foot of the sand bed adjacent to the spreader. Geotextile fabric secured on the surface of the sand bed, or equivalent method, may be used. The non-woven geotextile shall conform to A2000 - Geotextile Specifications,, Geotextile for Underground Drainage - Moderate Survivability, Class A.
Design the presettling cell for sediment collection and removal. Use a V-shaped bottom, removable bottom panels, or equivalent sludge handling system. Provide one-foot of sediment storage in the presettling cell.
Seal the pre-settling chamber to trap oil and trash. This chamber is usually connected to the sand filtration chamber using a pipe with an inverted elbow to protect the filter surface from oil and trash.
If a retaining baffle is necessary for oil/floatables in the presettling cell, it must extend at least one foot above to one foot below the design flow water level. Provide provision for the passage of flows in the event of plugging. Provide access opening and ladder on both sides of the baffle.
To prevent anoxic conditions, provide a minimum of 24 square feet of ventilation grate for each 250 square feet of sand bed surface area. For sufficient distribution of airflow across the sand bed, grates may be located in one area if the sand filter is small, but placement at each end is preferred. Small grates may also be dispersed over the entire sand bed area.
Sand filter vaults must conform to the materials, structural suitability, and access criteria specified for wetvaults in BMP T1120 - Wetvaults.
Provide a sand filter inlet shutoff/bypass valve for maintenance
A geotextile fabric over the entire sand bed may be installed that is flexible, highly permeable, three-dimensional matrix, and adequately secured. This is useful in trapping trash and litter. A geotextile may be used though may not be the best product. A polyethylene or polypropylene geonet might be better. The geonet should have high UV resistance (90% or more strength retained after 500 hours in the weatherometer, ASTM D4355), and high permittivity (ASTM D4491, 0.8/sec or more) and percent open area (CWO-22125, 10% ore more). Tensile strength should be on the order of 200 lbs grab (ASTM D4632) or more.
Access shall be provided for inspection and maintenance.
Access openings shall be provided a maximum of 50 feet from any location within the vault.
Access openings shall be provided over the inlet to the presettling cell.
Access openings shall be provided over each cell and situated over the V in the vault floor.
Access openings (except the removable panel) may be round, solid lids or 3' square diamond plate covers.
A removable panel shall be placed over the entire sand bed area.
Vaults that are less than 10 feet wide shall have removable lids.
Vaults shall comply with OSHA confined space requirements including clearly marking entrances to confined areas.
Provide ventilation pipes in all corners of the presettling cell. Alternatively, provide a removable panel over the entire presettling cell. If access manholes are installed at 12 foot or less spacing, ventilation pipes are not needed.
Locate removable panels outside traveled lanes.
The maximum depth from finished grade to the vault invert is 20 feet.
The minimum internal height shall be 7 feet and the minimum width shall be 4 feet.
