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Why Do It
Subsurface Sand Filters retain, filter, and slowly release stormwater runoff. Research at the University of New Hampshire Stormwater Center shows they are highly effective at protecting the environment. They have an excellent ability to reduce peak stormwater flows and remove many common stormwater pollutants, plus they bring stormwater temperature very close to groundwater temperature. One of their primary advantages is their ability to be installed under parking or access areas. Some drawbacks are the extra engineering and careful installation they require, and the fact that they may be more difficult to inspect, maintain, and repair because they are underground.
In South Portland, Subsurface Sand Filters are among the recommended approaches to meet Basic or Post-Construction Stormwater Management Plan permit requirements.
Subsurface Sand Filters are designed to temporarily retain stormwater flow in underground chambers that sit over a sand filter media, similar to some septic systems. It is critical to prevent sediments from clogging the drain, so all systems require a pretreatment component for this purpose. Hydrocarbon (oil) pretreatment may also be included for some locations. Water enters the system, then slowly filtrates through the sand filter, then exits the system either via infiltration or an underdrain system.
Subsurface Sand Filters generally are placed with the bottom of the system at least one foot above the seasonal high groundwater level. They can be adapted to soils with good infiltration abilities, or they may be underdrained if infiltration is not possible. In setting over or adjacent to groundwater aquifers, a liner may be necessary to prevent stormwater from polluting groundwater.
Design and Installation
Subsurface Sand Filters should be designed by an engineer, and incorporate manufactured components.
The system needs to be routinely inspected to ensure sedimentation is not clogging the sand filter, and that the filter is draining down within 24 – 36 hours after a storm. Pretreatment components need to be cleaned of sediment periodically. If there is a hydrocarbon pretreatment system, it may need to have filters changed regularly.