Rainwater can be an excellent source of clean water when filtered through the proper channels.
For outdoor uses, simple pre-filtration devices such as mesh screens and downspout diverters are great for removing large debris particles.
For indoor uses such as bathing, cooking, and drinking, one would need a filtration system designed to thoroughly remove harmful contaminants.
This article will discuss the following:
- Potential contaminants collected in rainwater
- Pre-filtration methods
- Post-filtration methods
- How to set up a rainwater filtration system
- Reviews of the best rainwater filtration systems available
Isn’t rainwater essentially distilled? In short, the answer is yes. Pure rainwater, through the hydrologic cycle, undergoes a natural distillation process as it evaporates, condensates, and eventually precipitates as pure water, free of any minerals or microorganisms. The concern with drinking pure rainwater comes from the external elements it comes into contact with on its way down.
Water that flows through a rainwater harvesting system isn’t readily available for consumption due to the numerous potential contaminants the rain can come into contact with. Harmful microorganisms on the rooftop and pollutants in the air can render the water supply dangerous for drinking.
Here are some of the potential contaminants that may end up in your rainwater supply:
Particles such as sand, dirt, or small pebbles, and larger debris such as leaves and twigs can be present on rooftops or in gutters and can end up in the rainwater collection tank if not filtered beforehand. Twigs and leaves can be blocked by adding mesh screens to the gutter or to the mouth of the rain barrel. A downspout diverter kit can also be installed to reroute larger pieces of debris from entering the collection tank.
For smaller particulates such as sediment, a post-filtration device can be installed at the point the water supply enters the household.
Harmful microorganisms such as bacteria, viruses, and parasites are some of the more concerning types of contaminants due to their dangerous health implications. Waterborne diseases such as Norovirus and Legionella can render one extremely ill if untreated properly.
While rooftops and gutters are excellent mechanisms for collecting and transporting rainwater, they are also effective at harboring harmful diseases, as is any consistently damp surface. In order to render dangerous pathogens harmless, a disinfection system such as ultraviolet light, chlorination, or carbon filtration of .02 microns or less will be needed.
Small critters such as squirrels and birds can leave traces of feces on rooftops and in gutters which rainwater can move into the collection tank. Animal feces are often known to carry bacteria such as Salmonella and E. Coli and various types of fungi.
Algal blooms can form under certain conditions where sunlight and consistently wet conditions meet. Certain types of algae can produce toxins which can be very dangerous to humans and animals if consumed.
Contaminants in the air caused by smog, industrial chemicals, and other forms of pollution may be absorbed into the rainwater. Unwanted chemicals and VOCs must be filtered on the chemical level via a robust whole house filter or reverse osmosis system.
Rainwater filtration system components:
Pre-filtration components are designed to capture larger pieces of debris such as leaves, pebbles, and twigs, before they enter the collection area, making it easier for the post-filtration system to further remove unwanted contaminants.
Gutter screen guards
The conventional means for transporting water from the catchment area on rooftops is via the rain gutter. Rain gutters are notorious for collecting twigs, leaves, and bugs as well as rainwater, and a mesh screen or cover is an effective way of blocking much of the debris.
The downspout is what connects the rain gutter to the collection tank and is often wide enough to allow unwanted debris particles to come through along with the rainwater. A downspout diverter is a device which redirects debris away from the rain barrel or collection tank, allowing for the entrance of water only. Be sure to read our article that explains the benefits of using a downspout diverter as well as how to install one.
Rain barrel cover
Something as simple as a rain barrel cover will protect the water supply from debris and from mosquitos entering and laying eggs. Most rain barrel covers come with a mesh screen which allows water to come through yet blocks large particles. Mosquito tablets can be added to the rain barrel which prevents bugs and larvae from inhabiting.
For rainwater harvesting systems intended to supply the interior of a household for clean water needs, a filtration system will be needed to remove harmful contaminants such as pathogens and volatile chemicals.
Disinfection via ultraviolet light is one of the most effective methods for eliminating pathogens. Used in many residential and commercial applications, ultraviolet light makes water safe from harmful microorganisms by destroying the cellular body with UV radiation, rendering the bacteria, protozoa, or virus inactive.
For a rainwater harvesting system, a UV purifier would be installed inside the home, after the water line leaves the rain barrel but before it reaches the fixtures. UV purifier systems are generally low-maintenance save for the occasional UV bulb replacement.
UV radiation is one of the most effective methods for destroying harmful pathogens so one can rest assured that no active bacteria, viruses, or protozoa will be in the water supply of a UV purifying system. But keep in mind that other contaminants such as debris, heavy metals, and other minerals will not be removed via UV radiation.
Be sure to read our article that compares several different brands of UV purifying systems and explains how to choose the right size for your home.
Carbon is one of the most widely used materials for filtration devices due to its effectiveness in absorbing a wide range of contaminants including microorganisms, chemicals, heavy metals, and sediment. Because carbon filters act as a shield which block contaminants from coming through, the effectiveness of the carbon filter is dependent on its pore size.
A carbon filter’s pore size is measured in microns. The smaller the pores, the more effective the carbon filter’s filtration ability. For example, a filter with a .05 micron rating will block waterborne bacteria which range from 1 – 10 microns in size.
Because viruses are generally much smaller than the pores of most carbon filters, a UV purifier can be paired with the carbon filter in order to eliminate all pathogens.
Reverse osmosis works by passing water through a membrane that only allows the water molecule to exit on the other side. The end result is pure, uncontaminated H20. Reverse osmosis systems typically involve re-mineralizing filters to introduce beneficial minerals such as magnesium and potassium back into the water supply.
Because reverse osmosis is such an efficient method for purifying water, it is unlikely that an additional purification method such as UV disinfection or carbon filtration will be needed. Reverse osmosis systems are typically installed under the kitchen sink to provide clean water for drinking and cooking, but larger units can be installed for supplying clean water to the whole house.
Be sure to read our article which compares different brands of RO systems and explains which features to be aware of.
Whole House Filter
In instances where the rainwater supply will be used by multiple individuals within a household, a whole house filter may be the appropriate choice for filtration. Whole house filters typically incorporate several different methods of filtration into one robust system. Some whole house filters might include both a UV purifier and a carbon filter, others might expand on a reverse osmosis system. Either way, whole house filters are large, robust and can be expensive, but are the best choice for large home applications.
A water pump can assist in delivering large amounts of water from the containment area into the household and will be quite helpful for home that intend to rely on rainwater for much of their indoor uses. Water pumps come in varying sizes and strengths. Check out our an article that compares different types of water pumps for rainwater harvesting applications.
Pressure tanks help regulate the amount of water pressure by containing pressurized air and water in an enclosed area, releasing water as needed. The end result is adequately pressurized water coming from the faucets and fixtures rather than weak, trickling water. The water pump and the pressure tank work hand-in-hand to provide the desired amount of water pressure for the home.
Setting up a rainwater filtration system
The set up of a filtration system is but one component of the entire rainwater harvesting application. To find out how to set up the entire rainwater harvesting system, check out our article which gives an overview of the process: How to set up a rainwater harvesting system.
A filtration system within a rainwater harvesting application is similar to any filtration set up, with the main difference being the water source. A typical home’s water source might come from the city water line or from a private well, a rainwater harvesting system’s water source will come directly from a rain barrel or collection tank. A rainwater harvesting system can also be incorporated into an existing water system.
There is a bit of freedom allowed in designing one's own rainwater harvesting system when it comes to aesthetic details, location and size of components, and overall complexity of the system. But the flow of rainwater should generally abide by the following parameters and filtration components should be installed at the correct locations:
Catchment area (rooftop, tarp, etc.)
Conveyance (Rain gutter, pipe)
Pre-filters: gutter covers, downspout diverter, rain barrel cover with mesh screen
Containment area (Rain barrel, large water tank, underground cistern)
Post filter: UV purifier, Carbon filter whole house filter
Entry point to home fixtures
The best rainwater filtration systems
- 600k or 1M Gallon Capacity
- Flow Rate: 16 or 34 GPM
- Pressure Range: 25 - 80 PSI
- 40 or 70 Watts
- Lifetime Warranty
Pentair is a leader in the clean water industry, providing sustainable solutions to homeowners, businesses, and NGOs around the world.
The Pelican whole house carbon filter with UV purification is a complete water purification system for rainwater harvesting.
The large diameter of the tank holds a significant amount of water filtration carbon media capable of removing sediment and harmful chemicals, while the UV purifier eliminates all pathogens. Parts are affordable and easily replaceable.
The Pelican Carbon Series Systems use an up-flow design and do not waste any water while filtering and conditioning the water for your entire home. While other systems use a down-flow design requiring them to backwash once or twice a week wasting 100-300 gallons of water each time they backwash.
- Smart TDS monitoring panel
- 400 GPD Fast Flow
- 1:1 Low Drain Ratio
- 7-stage Filtration
- Automatic Flushing
Waterdrop was founded in 2015, addressing increasing water quality concerns in the United States.
Waterdrop’s G3 Reverse Osmosis system improves upon the traditional RO design by introducing many innovative features and solving common problems. A common issue with most RO systems is that there tends to be an excessive amount of water wasted as the RO system is used. The G3 addresses this issue with its 1:1 Low Drain Ration system, saving on water usage by 300%.
While most conventional RO systems are known for their relatively low output, the G3’s fast flow system provides an unprecedented 400 gallons per day, that’s 1 cup of water every 12 seconds.
The seven-stage filtration system includes a pre-sediment filter, a post-carbon filter, and 3-layer reverse osmosis membrane, all responsible for removing chlorine, sand, rust, solids, and other impurities. Reduces TDS, limescale, heavy metals, bacteria, viruses, and more than 1,000 other harmful contaminants.
The G3 water drop has a smart faucet design, automatic flushing, and digital TDS meter.
For those seeking a reliable reverse osmosis system for rainwater harvesting, the G3 by Waterdrop is an excellent choice.
- Iron, Sediment, Carbon Filter
- Ferrous / Ferric Iron < 3 ppm
- Silica < 100 ppm
- Manganese < 1 ppm
- pH > 7.0
- 15 GPM
- 100,000 Gallon Capacity
- 2-year Warranty
- Made in USA
Home Master’s 3-stage whole house filter is designed to provide an entire household with clean water for drinking, cooking, bathing, and cleaning.
With a combined 3-stage filtration system, this filter also reduces 95% of VOCs, TOCs, chlorine, sediment and other contaminants that create foul tastes and odors. The post-filtration system uses high-grade coconut shell carbon for maximum chemical contaminant removal and improved taste.
Home Master’s signature 1” ports allow for or the strongest water pressure possible at up to 15 gallons per minute.
- Whole House Water Filter
- Sediment Filter
- Copper-zinc for scale reduction
- Activated Carbon Filter
- Post Filter
- 7 GPM
- 5-Year Warranty
Aquasana is a manufacturing company based out of Austin, TX and specializes in innovative filtering technology.
The Aquasana Whole House Rhino is a robust filter that removes pathogens such as bacteria and viruses; heavy metals such as lead and mercury; organic chemicals such as herbicides, pesticides and VOCs, and industrial solvents.
The pre-sediment filter removes larger particles while the activated carbon filter removes finer particles and improves the pH balance.
The UV filter eliminates any trace of harmful microorganisms that may lurk in the water supply.
The Whole House Rhino by Aquasana provides all of the filtration needs for a rainwater harvesting system.
- Fits 2" x 3" Downspout
- UV-Resistant PVC Plastic
- 4ft. Hose
The Oatey Mystic rain barrel diverter kit is well-designed, easy-to-install kit for rainwater harvesting. Made of UV-resistant PVC plastic, the diverter kit fits onto standard 2" x 3" rectangle downspouts. It comes with a 4 foot hose which attaches from the inlet to the barrel. The kit comes in white and can be easily painted to match any color.
The built-in reservoir channels water from the inner walls of the downspout and through the hose. When the rain barrel fills up, excess water travels down the downspout along with leaves and other debris.
There are many ways a rainwater supply can be utilized. For indoor purposes, it is important to filter the water properly to avoid potential contaminants. Some of the contaminants that can end up in a rainwater supply are debris, pathogens, chemicals, and algae.
Pre-filtration devices focus on blocking larger debris particles such as leaves and twigs and are installed before the containment area. Pre-filtration items include gutter covers and downspout diverters.
Post-filtration devices are designed to eliminate harmful contaminants such as bacteria, viruses, and dangerous chemicals. These devices are installed just before the point of entry to the home.
Thank you for taking the time to read our article on rainwater filtration systems. We'd love to hear your feedback in the comments section below. If you've found this article to be useful and are interested in learning more, be sure to sign up for our newsletter.
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The 7 layers of filters you need for potable Rainwater. (2020, January 21). Practical Preppers. https://practicalpreppers.com/how-to-harvest-potable-rainwater/
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Rainwater - Purification and filtration. (n.d.). The Rainwater Harvesting Community. https://www.harvesth2o.com/filtration_purification.shtml
Solutions, R. M. (2018, January 16). Rainwater harvesting 101: Pre-filtration. Rainwater Management Solutions. https://rainwatermanagement.com/blogs/news/rainwater-harvesting-101-pre-filtration
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