Pre-Filtration For Your Rainwater Harvesting System

We often get questions about how to filter the water that you capture in your rainwater harvesting system. Specifically, people want to know how to keep things such as leaves, shingle pebbles, dirt, and other debris from entering their rain barrels.

Today, we'd like to share a how-to tip with you on pre-filtration: how to design your system to filter the water before you start to collect it.

A good way to help reduce the debris that enters your rainwater tank once the water begins to flow is to add a first flush diverter (also known as a "roof washer") to your system. In addition to helping filter debris from the rainwater, a first flush diverter also helps to reduce the salinity of the harvested rainwater in coastal areas, where salt tends to get deposited on rooftops.

One of our favorite simple pre-filtration designs is called a stand pipe, which is one of the most cost-effective solutions. A stand pipe can be constructed from materials found at most retail home supply stores, and usually consists of 3-inch PVC pipe, two 90-degree elbows, one Tee fitting, a cleanout cap, and a downspout adapter.

Shown below is a SketchUp model we created of a rain barrel with a stand pipe.

Here's how the stand pipe works:

1) The water enters the pipe at the top where it connects to a downspout.

2) The first water that is captured in the gutters from the roof flows down the stand pipe.
*The dirt, leaves, pebbles, and other heavy debris contained in the rainwater collect at the bottom of the pipe behind the cleanout cap.

3) The water fills up the stand pipe until it reaches the Tee fitting, which acts as the pipe overflow, and then begins to flow into your rain barrel, cistern, tank, or other catchment.

4) The cleanout cap, located at the bottom of the stand pipe, should be connected loosely to allow a slight leak. Letting the pipe slowly leak ensures that the pipe empties between rainfall events, so the next time it rains and you begin the process again, there is plenty of room for the initial water with the most debris to collect before entering your rainwater harvesting system.

Below is a picture that shows our 3-barrel system with a stand pipe.

First Flush Diverter

This is just one DIY version of a stand pipe. There are many different types of first flush diverters that are commercially available in a variety of  shapes, sizes, and price ranges. We simply want to show you that adding a pre-filtration design to your rainwater harvesting system does not have to be costly or intimidating and can greatly improve the quality of the water before it even enters your tank.

Thanks for reading and happy harvesting!

~Sarasota Rain Barrels