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Filtering Rainwater and Water Quality

Filtering and Water Quality

The degree to which rainwater needs to be filtered depends upon it's end use and how long the water needs to be stored. For simple rainbarrel systems which use gravity to supply a hose bib, an inlet screen that filters large leaf debris is all that is needed, provided the water is used within a few months. For more complex systems which supply drip irrigation, indoor plumbing fixtures, or for potable uses, the rainwater needs to be filtered to a higher degree.

 Pre-filtering: The importance of pre-filtering rainwater before it enters the storage container is often overlooked. Complaints of stored rainwater smelling or turning "rotten" can always be traced to improper pre-filtration. If rainwater is not pre-filtered, a large amount of organic matter in the form of leaves and dirt can enter the storage tank. Aerobic bacteria begin to consume the organic matter and use up all the dissolved oxygen. Anaerobic bacteria begin to predominate resulting in odor. Another benefit of pre-filtering rainwater is reduced sediment buildup at the bottom of the storage tank and less tank maintenance.

There are a variety of options when it comes to prefiltering rainwater. In general, look for filters which are self cleaning to reduce the amount of maintenance needed on the system. The following is a general summary of the most common methods:

Micro-mesh gutter screens: Gutter screens can be installed on the gutter to filter debris before it enters the gutter. One advantage of gutter screening is the large filtering surface area can reduce maintenance of the filter. Some higher quality gutter screening is nearly self cleaning and requires very little maintenance. The micro-mesh screen can filter debris in the 80-100 micron size which is beneficial for potable or indoor fixture systems which require very good filtration.

Downspout Filters: These filters are placed either at the top of the downspout where it meets the gutter, or somewhere along the length of the downspout. These filters generally only provide coarse filtration of 1/8"-1/16" size (3175-1587 microns) and should only be used for rainbarel systems. A few models can filter to 280 microns and make good pre-filters for irrigation systems and indoor non-potable uses.

In Ground Filters/In Tank Filters: These filters are usually buried in the ground or installed in the storage tank and connected to a drain pipe coming from all the downspouts from the building. They are often used for larger roof areas on residential and commercial sites. Filtering size can vary among different models with the best at around 280 microns.

Sand Filters: Sand filters are often used in potable water systems to remove debris from the rainwater before it enters the storage tanks. Sand filters can remove particles up to 30 micron size.

First Flush Diverters: Diverters keep the dirtiest water out of the storage tank and can help reduce sediment accumulation on the bottom of the storage tank. The water from the first few minutes of each rain event picks up dirt and dust which tends to accumulate on the roof. This dirty water is directed away from the storage tank in a "first flush diverter". Usually 1-2 gallon is diverted per 100 square feet of catchment area. Diverters keep the dirtiest water out the the storage tank and can help reduce sediment accumulation on the bottom of the storage tank. Diverters are essential for potable water systems. Particles of air pollution can settle on roofs and and potentially wash into storage tanks without first flush diverters. These particles can be very small and are not easily filtered by sediment or carbon filters.

Landscape Irrigation: Spray irrigation systems need to be filtered to roughly 500-1000 microns, while drip irrigation systems need filtering to 100 microns. If the rainwater has been adequately pre-filtered, a simple irrigation y-filter can achieve the necessary additional filtering for spray or drip irrigation.

Toilet Use: At minimum, a 50 micron sediment filter will prevent grit from interfering with valve mechanisms in the toilet. Often times a carbon filter is utilized to reduce any color or odor that may be present in the water. Many permitting agencies may require filtration to 5 microns and disinfection with chlorine, ozone, or UV light.

Laundry Use: At minimum, a sediment and 5 micron carbon filter should be used to filter rainwater for laundry use. Many permitting agencies may require disinfection with chlorine, ozone, or UV light.