2021 Kitsap County Stormwater Design Manual
2021 Kitsap County
Stormwater Design Manual

5.4.19 Sand Filters

5.4.19.1 BMP Description

Sand filters are used to provide runoff treatment. The following three sand filter BMPs are described in this section:

  • Sand Filter Basins (BMP T8.10: Basic Sand Filter Basin and BMP T8.11: Large Sand Filter Basin in Volume V, Chapter 6 of the Ecology Manual) – Like an infiltration basin, the sand filter basin is an impoundment that temporarily stores stormwater runoff so that it can infiltrate, but instead of infiltrating through the underlying soil, stormwater passes through a constructed sand bed. Sand filters can be sized as either a basic or a large BMP to meet different water quality objectives. Sand filter basins are designed with underdrains to collect and route runoff following treatment to the downstream conveyance system.

  • Sand Filter Vaults (BMP T8.20 in Volume V, Chapter 6 of the Ecology Manual) – A sand filter vault is similar to a sand filter basin, except that the entire BMP is installed below grade in a vault. It typically consists of a pre-settling cell (if pretreatment is not already provided) and a sand filtration cell. Like a sand filter basin, a vault can be sized as either a basic or a large BMP to meet different water quality objectives.

  • Linear Sand Filters (BMP T8.30 in Volume V, Chapter 6 of the Ecology Manual) – Linear sand filters are similar to sand filter vaults, except the vault is configured as a long, shallow, linear system. The vault contains two cells or chambers, one for removing coarse sediment and the other containing sand overlying an underdrain. Runoff usually enters the settling chamber as unconcentrated flow from an adjacent area and overflows to a central weir into the sand portion of the vault.

For additional filtration treatment BMP types, see BMP T8.40: Media Filter Drain in Volume V, Chapter 6 of the Ecology Manual.

5.4.19.2 Performance Mechanism

Sand filters treat stormwater primarily via physical filtration. As stormwater passes through the sand media, pollutants are trapped in the small spaces between sand grains or adhere to the sand surface. Over time, soil bacteria may also grow in the sand bed and some biological removal may occur.

Sand filter media can also be amended with steel fiber, crushed calcitic limestone, and/or other approved amendments to increase dissolved metals removal.

5.4.19.3 Application and Limitations

Use a sand filter basin to capture and treat the Water Quality Design Storm volume; which is 91% of the total runoff volume as predicted by an approved continuous simulation model (see Vol I–4.2.6 Minimum Requirement #6: Runoff Treatment). Only 9% of the total runoff volume would bypass or overflow from the sand filter BMP.

The large sand filter is generally subject to the same applications and limitations as sand filter basin. The difference is that the large sand filter basin uses a higher Water Quality Design Storm volume: 95% of the runoff volume of the period modeled in an approved continuous simulation model. Only 5% of the total runoff volume as modeled would bypass or overflow from the sand filter BMP.

For sand filter vaults:

  • Use where space limitations preclude above ground BMPs.

  • Not suitable where high water table and heavy sediment loads are expected.

  • An elevation difference of 4 feet between inlet and outlet is needed.

For linear sand filters:

  • Applicable in long narrow spaces such as the perimeter of a paved surface.

  • As a part of a treatment train as downstream of a filter strip, upstream of an infiltration system, or upstream of a wetpond or a biofilter for oil control. Note, the linear sand filter is used in the Basic, Enhanced, and Phosphorus Treatment menus also. If used to satisfy one of those treatment requirements, the same BMP shall not also be used to satisfy the oil control requirement unless increased maintenance is ensured. This increase in maintenance is to prevent clogging of the filter by oil so that it will function for suspended solids, metals and phosphorus removal as well. Quarterly cleaning is required unless specified otherwise by the designer.

  • To treat small drainages (less than 2 acres of impervious area).

  • To treat runoff from high-use sites for TSS and oil/grease removal, if applicable.

Off-line sand filters shall be located upstream of detention BMPs whenever feasible. On-line sand filters shall be located downstream of detention BMPs to prevent exposure of the sand filter surface to high flow rates that could cause loss of media and previously removed pollutants. See Table II-5.22 for a summary of runoff treatment requirements that sand filter BMPs can be designed to meet.

Table II-5.22. Sand Filter BMPs Applicability.
BMP

MR #5: On-site Stormwater Management

MR #6: Runoff Treatment

MR #7: Flow Control

List

LID Performance Standard

Basic

Enhanced

Oil Control

Phosphorus

Basic Sand Filter

 

 

X

Xa

 

Xb

  

Large Sand Filter

 

 

X

X

 

X

  

Sand Filter Vault

 

 

X

Xa

 

Xb

  

Linear Sand Filter

 

 

X

Xa

Xc

Xb

  

Notes:

  1. Can be used to meet enhanced runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Section 1.2 of the Ecology Manual.

  2. Can be used to meet phosphorus runoff treatment requirement as part of a two-BMP treatment train; see Volume III, Section 1.2 of the Ecology Manual.

  3. Can be used to meet oil control requirement (see Volume III, Section 1.2 of the Ecology Manual) as part of a two-BMP treatment train, if used downstream of a filter strip (see Vol II–5.4.21 Vegetated Filter Strips), upstream of an infiltration system, or upstream of a wetpond (see Vol II–5.4.22 Wetponds). If used to meet basic, enhanced, or phosphorus treatment requirements, the same BMP cannot be used to meet oil control requirements unless provisions are made for additional maintenance.

5.4.19.4 Site Considerations

Consider the following site characteristics when siting a sand filtration system:

5.4.19.5 Design Information

See BMP T8.10: Basic Sand Filter Basin, BMP T8.11: Large Sand Filter Basin, BMP T8.20: Sand Filter Vault, and BMP T8.30: Linear Sand Filter in Volume V, Chapter 6 of the Ecology Manual for design requirements related to sand filters.

Pre-settling is required to prevent clogging and extend the service life of the filter media.

5.4.19.6 Minimum Construction Requirements

No runoff shall enter the sand filter prior to completion of construction and approval of site stabilization by the responsible inspector. Construction runoff may be routed to a pretreatment sedimentation BMP, but discharge from sedimentation BMPs shall bypass downstream sand filters. Careful level placement of the sand is necessary to avoid formation of voids within the sand that could lead to short-circuiting, (particularly around penetrations for underdrain cleanouts) and to prevent damage to the underlying geomembranes and underdrain system. Over-compaction shall be avoided to ensure adequate filtration capacity. Sand is best placed with a low ground pressure bulldozer (4 pounds per square inch gage [psig] or less). After the sand layer is placed water settling is recommended. Flood the sand with 10–15 gallons of water per cubic foot of sand.

5.4.19.7 Operations and Maintenance

Refer to BMP T8.20 in Volume V, Chapter 6 and Volume V, Appendix V‑A of the Ecology Manual for required maintenance practices for both above grade and below ground sand filter BMPs.