A rainwater harvesting cistern is comprised of many components. Our experience is that some of these components are often overlooked, which can negatively impact water quality. Here is our list of the top five commonly missed rainwater cistern components and why they are important .
1. Calming Inlet
The calming inlet reduces the velocity of water entering the cistern to avoid the “waterfall effect” where the incoming water stirs up any accumulated sediment. This simple component allows full pipe flow to up-well into the bottom of the pipe – keeping the water cleaner.
2. Floating Outlet
When the pump is located inside the cistern, a floating outlet provides a basic level of screening before the water is pulled into the pump. The screen floats in the cleanest portion of the water column; below the surface and off the bottom. This way, water from the bottom of the vault where sediment may have collected, and water from the surface where dust or floatables may be present, is avoided. This basic element is a simple step to improve water quality.
3. Vent pipe
A small vent pipe allows air flow through the cistern, helping to keep the water oxygenated longer. The vent pipe should be inverted and have a screen at the opening above ground to ensure animals and insects do not enter the system. This may not be needed if the manhole covers provide sufficient venting
4. Locked frame and cover
A manhole opening, typically 24” – 36” in diameter, should be provided on all cisterns for maintenance access. The access cover should be a locking type to reduce possible contamination, or unwarranted entry into the cistern.
5. Overflow pipe
Typically, high flows are routed around the cistern and low flows travel to the cistern. The overflow pipe allows excess water to leave the cistern when a storm event occurs and the cistern is full. The overflow can be routed to an infiltration gallery, a stormwater treatment system like a biofiltration unit, or back into the stormwater conveyance pipe.