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Filtration for Rainwater-based Irrigation Systems

by Doug Pushard

Although rain is almost pure, it needs filtration for every use including irrigation. Rainwater in the sky is unusually clean, it is not by the time it reaches your irrigation system. Your roof, your conveyance system, your tanks all add contaminants to your rainwater that need to be removed prior to using it in your irrigation system.

One of the key challenges with rainwater is filter, filter, filter.

Your roof and conveyance system are the two areas that much of the debris can be introduced into your stored rainwater. Using proper screening and the proper maintenance of your gutters can keep much of the debris out. Additionally, first flush devices or further downstream filtering also assists in keeping the rainwater as clean as possible. Lastly, ensuring your storage system is properly sealed so critters or debris cannot enter this portion of your system further keeps your system clean.

All these steps will reduce the need to clean the storage system for most likely years, but also reduce the filtering needs prior to using your rainwater for irrigation.

  • Proper screening of gutters
  • Proper maintenance of gutters
  • First flush devices
  • Downstream filtering
  • Proper sealed storage system

All irrigation systems need to have some type of filter. Even city water-based irrigation systems using drinking water should have a screen mesh filter prior to the valves. These mesh filter remove sand and other small particulates before they can clog irrigation valves or irrigation heads. These mesh filters come in a range of mesh sizes (e.g. 120 - 150 mesh) with the higher the mesh number the finer the screen and the more it will filter out.

These filters have a working pressure specified as part of the product. It is critical the filter be installed per the specs for this working pressure range. Too high a pressure and debris will be forced through the screen or damage it although. If pressure is too low, not enough water will be forced through the filter to reach the irrigation heads.

Drip irrigation systems have a lower working pressure than spray irrigation system. For drip irrigation systems the filter is preceded with a Pressure Reducer valve. These devices come in fixed pressure settings (e.g. 30 PSI or 40 PSI) or variable pressure valves. The variable pressure devices can be adjusted via a bolt on the head of the device.

In systems with both spray and drip zones the pressure reducer for the drip zones would be installed after the irrigation valves. The spray zones would not require a pressure reducer. Otherwise drip heads would be outside their working recommended pressure and be blown off by too high a pressure.

Mesh FilterA typical irrigation filter has four pieces – a lower housing body that is attached to the irrigation line, an upper housing body that can be unscrewed to reveal the filter, a filter installed inside the housing body, and an end-cap or valve. The housing body is installed in the irrigation line. The filter body has a flow arrow on it. It is critical that the housing body be installed with the flow arrow pointing in the direction of the water flow; otherwise the filter will not work properly.

Generally, the irrigation filter body is installed with this upper body housing pointed down towards the ground. This allows the filter to be most effective. The upper housing body unscrews and allows the filter to be taken out to be cleaned. Having an extra spare mesh filter is a great way to easily maintain your system eliminating that last minute rush to the store.

I recommend visually inspecting the mesh filter once a year. These screen filters do tear over time. Put inspecting your filters on your spring cleaning calendar the same time every year. For example, before you turn on the irrigation system in the spring take out the used filter and install the spare filter. Inspect and clean the used filter and store it for next year.

The end cap, if your filter has one, can also be used to clean debris out of your filter. For example, mid-irrigation season, remove the cap and turn on your irrigation system for 5 - 10 seconds to spray clean the inside of the filter. Caution should be taken when doing this operation as water will spray out of the valve box onto you and anything else nearby. If the irrigation water is rainwater, it will contain harmful bacteria. DO NOT SPRAY YOURSELF, PETS, OR OTHERS, not fun!

With city water, well water, or rainwater as a water source, irrigation filters should have these simple screen filters. Plumbing code requires screen filters for rainwater. Rainwater for spray or above ground irrigation requires appropriate signage (non-potable), more filtration and purification are required! Keep your filters and system clean and you will enjoy the benefits of FREE rainwater for years to come.

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