What is a sand trap?
A sand trap (also: sand interceptor or grit chamber) is a tank or chamber in which sand, grit and other heavy particles settle out of water before the water is discharged. On a building site it is often a box with baffles through which water pumped from an excavation flows before it goes to a ditch or the sewer. In wastewater treatment plants the grit chamber is a standard step in mechanical treatment, right after screening out coarse debris.
How does a sand trap work?
Fast-flowing water carries sand along. When the water slows down in a larger tank, the heavier particles sink to the bottom. Baffles force the water to take a longer route, which increases the settling time. The sand stays behind and the clearer water flows on at the top.
Application
- Site dewatering: water pumped from an excavation often carries sand and silt. A sand trap stops this material from silting up or blocking ditches and sewers.
- Wastewater treatment: the grit chamber removes sand, gravel and broken glass from sewage, protecting pumps and later treatment stages from wear.
- Drainage systems: gullies and chambers with a silt sump below the outlet work on the same principle.
Practical tips
- Empty regularly: a full sand trap no longer works. Check the level of settled sand and clear it in time.
- Allow enough settling time: if the water at the outlet is still cloudy, the flow is too high for the tank. Pump more slowly or add a second tank in series.
- Discharge rules: discharging site water is usually subject to rules from the local authority or the sewer operator. Check the requirements before you start.
- Disposal: dispose of the collected material properly, especially if the water may be contaminated.
Related terms
- Excavation pit
- Dewatering
- Sewer
- Gully
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