Rainwater From Rainwater Tanks: Filter Types According To Use

Rainwater is inherently pure before it reaches a collection surface or the ground but, as the article below discusses, rainwater from your rain water tanks must be appropriately filtered according to the intended use of the rainwater.  It is also important to store the rainwater in water tanks that are certified for storing potable water is it is to be used for drinking.
rainwater filterings
Rainwater and snowfall are the ultimate sources of all drinking water on the planet. Rainwater and melted snow runs off the land and collects in lakes and rivers. They also seep through the ground and recharge the aquifers that supply drinking water wells. Regardless of where you currently obtain your drinking water, it originally fell from the sky.

Rainwater’s Natural Qualities

The water in a raindrop is one of the cleanest sources of water available. Rainwater can absorb gases such as carbon dioxide, oxygen, nitrogen dioxide, and sulfur dioxide from the atmosphere. It can also capture soot and other microscopic particulates as it falls through the sky. Nevertheless, rainwater is almost 100% pure water before it reaches the ground.

Rainwater is soft water and leaves no limescale; washing clothes and hair in soft water requires less detergent and so reduces water pollution from these compounds. Plants LOVE rainwater. It doesn’t contain chlorine, which is carcinogenic.

Water is made “hard” by dissolved calcium or magnesium ions, neither of which is present in rain water (at least, not until it runs over calcium or magnesium containing compounds on the Earth’s surface). Pure water is considered the universal solvent; it can absorb or dissolve contaminants from almost anything it comes into contact with. That is why it is especially important to design and operate your system so that the rainwater picks up as few contaminants as possible before you consume it.

Although rainwater can be contaminated by absorbing airborne chemicals, most of the chemicals present in harvested rainwater are introduced during collection, treatment, and distribution. By properly designing and operating your rainwater harvesting system, you can minimize your exposure to a variety of chemical contaminants that include organic chemicals, such as volatile and synthetic organics, and inorganic chemicals, such as minerals and metals.


The degree to which rainwater needs to be filtered depends upon its end use, and how long the water will be stored. For simple rain barrel systems that 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 several weeks. For more complex systems which supply drip irrigation, indoor plumbing fixtures, or water for potable uses, the rainwater will need to be filtered to a higher degree.


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 water 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. Other benefits of pre-filtering rainwater are reduced sediment buildup at the bottom of the storage tank and less water tank maintenance.

There are a variety of options when it comes to pre-filtering rainwater. In general, look for filters that 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 surface. 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 superior 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 rain barrel 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 rainwater 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 water tanks. Sand filters can remove particles up to 30 micron size.

First Flush Diverters

Diverters keep the dirtiest water out of the rain water 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 that tend to accumulate on the roof. This dirty water is directed away from the storage tank in a “first flush diverter.” Usually 1-2 gallons are diverted per 100 square feet of catchment area. Diverters are essential for potable water systems. Particles of air pollution can settle on roofs 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.

Filter According To Use

Drip and Spray Irrigation

Spray irrigation systems need to be filtered to 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.

Potable Use

Rainwater is an inherently pure source of water. That said, rainwater can be exposed to considerable contaminants on the way to the tap. As rainwater falls through the air it can pick up soot and other airborne microscopic contaminants. Once on the roof, the rainwater can be exposed to herbicide or pesticide sprays which may have adhered to leaves or dirt particles. Bird, raccoon, possum or other animal droppings may be on the roof as well. Once in the piping and collection system, the rainwater can be exposed to synthetic compounds used in plastics, glue, and other manufactured products. Heavy metals may be present in some of the brass, copper, and iron pipe fittings used to construct a rainwater system.

When choosing products for a potable rainwater system, it is important to use products that have been certified for potable water use through ANSI or NSF. This will minimize the contaminants introduced into the water by the collection and distribution system. To reduce biological contaminants, filtering and sterilization is required.

The two main methods of sterilization are UV light and free chlorine. UV light’s advantage is that it leaves no residue in the water but it does need high doses to kill certain viral pathogens. Free chlorine is very effective against viruses but is virtually ineffective against Cryptosporidium cysts. In addition it leaves residual chlorine in the water which can be beneficial or detrimental depending upon your concerns.

To reduce VOC and SOC (Volatile/synthetic organic compounds) contaminants in the rainwater an activated carbon filter specifically made for these contaminants should be used.

Cross Connection Control

Rainwater systems supplying irrigation or indoor uses will require a backup water supply if the rainwater tanks empty. Backup water from the domestic supply or from a well can be added directly to the tank to ensure there is always water available to the rainwater pump. Water should be added to the tank through an air gap which will prevent untreated rainwater from coming into contact with the domestic or well supply. Consult your local building codes for specifics about air gaps.

Another method of providing a backup supply is to have a direct pipe connection between the rainwater system piping and the domestic water supply, with a reduced pressure principle device separating the two. When the rainwater tanks empty, the user can manually or automatically switch over to the domestic supply. This enables pressurized domestic water to supply the irrigation system or indoor fixtures without using the rainwater pump. The reduced pressure principle device prevents rainwater from coming into contact with the domestic supply.

Source: WhollyH2O (By John Russell) Abbrev.

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