Rainwater Harvesting 101: Tank Overflow

Continuing our series of posts on Rainwater Harvesting 101, today we will look at using an overflow in a rainwater storage tank. This is the final, crucial step to consider when setting up a rainwater harvesting system. Although overflowing a rainwater tank may initially seem counter intuitive, it is actually a beneficial process for rainwater harvesting as it helps to move water which creates oxygenation and helps prevent stagnation. Using an overflow accomplishes two primary goals: 1.) Creates an exit path for excess incoming water, and 2.) helps to remove floating particulate from the surface of the water. 

Creating an exit path for incoming water is important so that the tank does not overflow through the manway, back-up through your pre-tank filter, or create excess pressure on the storage vessel. This also allows you to control where excess water goes and often excess water is routed to a garden or other areas for supplemental irrigation. Removing floating particulate from the surface of the water is also beneficial as it is part of the low-maintenance approach that we take to improve the quality of water in your tank. 

Overflow can be achieved either through standard PVC piping or by way of the WISY Multifunction Overflow devices. We recommend the WISY overflow devices for installations that have 4" or greater piping entering the tank as they siphon particulate off the surface off the water, use a stainless-steel device to prevent small animals from entering the tank, include a float that prevents back-up from a storm system from entering the tank, and are an easy installation. The picture below shows how the WISY Multifunction Overflow Device works.


As mentioned in previous posts, the small bits of sediment that make it into the tank after pre-filtration will typically either sink to the bottom of the tank to form the bio film or float to the top of the water level. As shown above, using an overflow device can help to remove those floating particles from the surface of the water, while also allowing excess water an exit path.

A few things to consider when installing an overflow:

  1. Match the overflow pipe size to the inlet pipe size. This makes sure the overflow can keep up with the incoming water if your tank is full of water. Example: If you have a 4" inlet to your tank, you'll want your overflow to be 4". 
  2. Ensure that the overflow pipe is installed at a lower elevation than the inlet pipe. In most instances, you want to make sure that the overflow is a minimum of half the pipe diameter below the inlet pipe. This allows for overflow before backing up into the pre-tank filter/inlet pipe.
  3. Make sure the overflow has a lip/bevel. It is recommended to have some sort of angle on the pipe that the water exits through. This creates a siphon effect and helps to draw water into the pipe (think of a pond skimmer). This also helps to siphon the floating particles through the overflow. If PVC piping is used, an angle of 11 or 22 degrees cut across the pipe is recommended. The WISY Multifunction Overflow Devices contain this angle already. See "ITEM A" below for an example.
  4. Prevent backflow. If your tank is tied into a storm drain, you will want to make sure that you either use a WISY Overflow Device (which contains a backflow prevention device) or a backwater valve. Both of these devices prevent water from entering the tank in the event that there is a sewer/storm line back up. This protects the quality of water in the tank. These both will also function as a method to prevent small animals such as mice from crawling into the tank. See "ITEM B" below for an example.
  5. Prevent mosquitoes/other bugs. Creating an air-tight connection on the tank helps to prevent mosquitoes from entering the rainwater storage tank. Using the example of ITEM A below, once enough water is in the tank to overflow it once, there will be water that sits in the bottom of the P-Trap and fills up enough to prevent any air gap to allow entrance into the tank. This isn't guaranteed to prevent all bugs from entering the tank, but it is a very effective method nonetheless.
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That covers the basics of rainwater tank overflow. If you have any questions or comments, please leave them below and we'll be happy to help!

In the final post of the series, we will cover post-tank filtration.

Final post: Post-tank Filtration