A sediment trap is a device for preventing debris and dirt from entering a rainwater storage tank.
Since rainwater is harvested from catchment surfaces and collectors, the initial part of any rainfall will always wash debris into the collection system, only later in the rainfall event will cleaner water flow into the system. So it becomes necessary to separate the dirt and debris, or “sediment”, from the clean rainwater.
Other than a sediment trap, another alternative is installing a first-flush or foul-flush device into the system; these divert the initial flush of dirtier rainwater away from the tank.
The design of the sediment trap itself will depend on how much rainfall water flow the collection system is handling. A good rule of thumb to go by to figure capacity is 1 mm rain, on 1 sq m surface, produces 1 liter of water.
So for the typical roof collection system, figure out your roof area in sq meters, multiply that by your local yearly rainfall figure in millimeters, and that will tell you how many liters of water you get on your roof per year.
A typical design for a residential rainwater sediment trap would be to channel the rainwater down the side of the house from the roof into PVC piping of large diameter (5 to 6 inches) and into a small holding tank for trapping debris. Construction can be of cement block, brick, or plastic tank, and the holding tank should be half filled with gravel of medium size, or sand.
The top of the tank can be open if covered with mesh wire to keep out insects, leafs and other falling debris. Water then is directed from another large diameter PVC pipe in the bottom of the sediment trap tank into the storage cistern.
For a potable water system, sterilization and filtering units are added to the system, but the above description would be suitable for gardening and clothes washing purposes.
Photo by sarahemcc, Creative Commons Attribution License