Description
A sand filter, or “filtration basin”, is a stormwater quality treatment system that works by using a two-component clarification system: the first is a sediment forebay for settling large particles and the second is a horizontal layer of coarse grained soil that acts as a screen.
Why use Sand Filters?
Sand filters are great for treating the quality of stormwater runoff from large impervious surfaces such as parking lots. They can treat the water from a drainage area of about 10 acres, making them practical for installation on commercial facilities. Sand filters can either collect the stormwater from a large drainage area for treatment at one centrally located filter, or surround the perimeter of an impervious area. These “surface sand filters” are usually constructed at grade. However, if there are serious land constraints, underground filters can be constructed
How do they manage stormwater?
The main purpose of a sand filter is to collect stormwater, rid it of pollutants, and then discharge the water into the storm sewer system. First, water is routed from an impervious surface to the first treatment cell, the sedimentation chamber. This chamber is either a large tank or just a leak proof depression in the ground where water can be contained. Here, water velocity is reduced, and it is retained long enough to allow suspended sediments to sink to the bottom of the chamber
Next, the water moves to the filter. The filter is simply a few layers of coarse sediment (usually sand and gravel) constructed at grade, and surround with earthen banks. At first, the water pools on top of the filter material, held in place by the banks. Gradually, gravity pulls the water beneath the surface, and infiltrates through the filter. Because the gaps between sand particles are very small, larger sediments are screened out and prevented from permeating any deeper. BOD and coliform bacteria concentrations are also reduced. After the water has propagated through the filter material, it is usually collected in a perforated PVC pipe underdrain, and then transported to the storm sewer system.
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