Volume 2-05, Chapter 3 - City of Wichita
Volume 2-05, Chapter 3 - City of Wichita Volume 2-05, Chapter 3 - City of Wichita
Section 3.2.8 - Surface Sand Filter 3.2.8.1 General Description Sand filters (also referred to as filtration basins) are structural stormwater controls that capture and temporarily store stormwater runoff and pass it through a filter bed of sand. Most sand filter systems consist of two-chamber structures. The first chamber is a sediment forebay or sedimentation chamber, which removes floatables and heavy sediments. The second is the filtration chamber, which removes additional pollutants by filtering the runoff through a sand bed. The filtered runoff is typically collected and returned to the conveyance system, though it can also be partially or fully infiltrated into the surrounding soil in areas with porous soils if not used for hotspot treatment, and if the infiltration face is at least 5 feet above the historically high groundwater elevation. Because they have few site constraints beside head requirements, sand filters can be used on development sites where the use of other structural controls may be precluded. However, sand filter systems can be relatively expensive to construct, install, and maintain. Surface Sand Filter: The surface sand filter is a ground-level open air structure that consists of a pretreatment sediment forebay and a filter bed chamber. This system can treat drainage areas up to 10 acres in size and is typically located off-line. Surface sand filters can be designed as an excavation with earthen embankments or as a concrete or block structure. Figure 3-28 Sand Filter Example 3.2.8.2 Stormwater Management Suitability Sand filter systems are designed primarily as off-line systems for stormwater quality (i.e., the removal of stormwater pollutants) and will typically need to be used in conjunction with another structural control to provide downstream channel protection, on-site flood control, and downstream flood control, if required. However, under certain circumstances, filters can provide limited runoff quantity control, particularly for smaller storm events. Page 3 - 98 Volume 2, Technical Guidance
Section 3.2.8 - Surface Sand Filter Water Quality In sand filter systems, stormwater pollutants are removed through a combination of gravitational settling, filtration, and adsorption. The filtration process effectively reduces suspended solids and particulates, biochemical oxygen demand (BOD), fecal coliform bacteria, and other pollutants. Surface sand filters with a grass cover have additional opportunities for vegetation uptake of pollutants, particularly nutrients. Channel Protection For smaller sites, a sand filter may be designed to capture the entire CPv in either an off- or on-line configuration. Given that a sand filter system is typically designed to completely drain over 40 hours, the requirement of extended detention of the 1-year, 24-hour storm runoff volume will be met. For larger sites or where only the WQv is diverted to the sand filter facility, another structural control must be used to provide CPv extended detention. On-Site Flood Control Another structural control must be used in conjunction with a sand filter system to reduce the post-development peak flow to pre-development levels (detention) if needed. Downstream Flood Control Sand filter facilities must provide flow diversion and/or be designed to safely pass design flood events and protect the filter bed and facility. The volume of runoff removed and treated by the sand filter may be included in the on-site flood control and downstream flood control calculations. (See Chapter 3.) 3.2.8.3 Pollutant Removal Capabilities The surface sand filter is presumed to be able to remove 80% of the total suspended solids load in typical urban post-development runoff when sized, designed, constructed and maintained in accordance with the recommended specifications. Undersized or poorly designed sand filters can reduce TSS removal performance. The following design pollutant removal rates are average pollutant reduction percentages for design purposes derived from sampling data, modeling, and professional judgment. In a situation where a removal rate is not deemed sufficient, additional controls may be put in place at the given site in a series or “treatment train” approach. • Total Suspended Solids – 80% • Total Phosphorus – 50% • Total Nitrogen – 30% • Heavy Metals – 50% Volume 2, Technical Guidance Page 3 - 99
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Section 3.2.8 - Surface Sand Filter<br />
Water Quality<br />
In sand filter systems, stormwater pollutants are removed through a combination <strong>of</strong><br />
gravitational settling, filtration, and adsorption. The filtration process effectively reduces<br />
suspended solids and particulates, biochemical oxygen demand (BOD), fecal coliform<br />
bacteria, and other pollutants. Surface sand filters with a grass cover have additional<br />
opportunities for vegetation uptake <strong>of</strong> pollutants, particularly nutrients.<br />
Channel Protection<br />
For smaller sites, a sand filter may be designed to capture the entire CPv in either an <strong>of</strong>f- or<br />
on-line configuration. Given that a sand filter system is typically designed to completely drain<br />
over 40 hours, the requirement <strong>of</strong> extended detention <strong>of</strong> the 1-year, 24-hour storm run<strong>of</strong>f<br />
volume will be met. For larger sites or where only the WQv is diverted to the sand filter<br />
facility, another structural control must be used to provide CPv extended detention.<br />
On-Site Flood Control<br />
Another structural control must be used in conjunction with a sand filter system to reduce the<br />
post-development peak flow to pre-development levels (detention) if needed.<br />
Downstream Flood Control<br />
Sand filter facilities must provide flow diversion and/or be designed to safely pass design flood<br />
events and protect the filter bed and facility.<br />
The volume <strong>of</strong> run<strong>of</strong>f removed and treated by the sand filter may be included in the on-site<br />
flood control and downstream flood control calculations. (See <strong>Chapter</strong> 3.)<br />
3.2.8.3 Pollutant Removal Capabilities<br />
The surface sand filter is presumed to be able to remove 80% <strong>of</strong> the total suspended solids<br />
load in typical urban post-development run<strong>of</strong>f when sized, designed, constructed and<br />
maintained in accordance with the recommended specifications. Undersized or poorly<br />
designed sand filters can reduce TSS removal performance.<br />
The following design pollutant removal rates are average pollutant reduction percentages for<br />
design purposes derived from sampling data, modeling, and pr<strong>of</strong>essional judgment. In a<br />
situation where a removal rate is not deemed sufficient, additional controls may be put in<br />
place at the given site in a series or “treatment train” approach.<br />
• Total Suspended Solids – 80%<br />
• Total Phosphorus – 50%<br />
• Total Nitrogen – 30%<br />
• Heavy Metals – 50%<br />
<strong>Volume</strong> 2, Technical Guidance Page 3 - 99
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