Should Rain Water Be Filtered Before Drinking? 4 Key Points

Heavy rainfall in an urban area

Rainwater is a common source of drinking water, I grew up drinking nothing else for the first 14 years of my life. The question is, should we filter it before drinking it? is all rainwater the same? These queries got me thinking about the topic and were what pushed me to write this article to help anyone else who has wondered the same thing.

It is a good idea to filter rainwater before drinking, but not essential, particularly if in rural areas and drunk shortly after collection. However, rainwater’s drinkability is heavily influenced by geographic location, the collection surface, the storage medium and the time between rainfall and consumption.

If you are certain that you want to filter your rainwater, you are just chasing the easiest option, have a quick read of my article on the topic.

Location: High Pollution = Poor Rainwater

Unfortunately, a large amount of the world’s population can not drink their rainwater due to airborne chemicals contaminating water during precipitation.

Acid Rain

A common phenomenon in areas of higher pollution is that air quality dramatically improves after rainfall, particularly heavy rainfall. This improvement in air quality is caused by a subsequent reduction in rainwater quality, the rain collects pollutants like Nitrogen dioxide (NO2) and Sulphur dioxide (SO2) as it falls to the ground. This rainfall constitutes what is commonly known as acid rain, with Nitric acid (HNO3) and Sulphuric acid (H2SO4) being present in elevated amounts. Although acid rain is certainly not the ideal source for rainwater, it is generally not as bad as it sounds, with the pH usually being well within drinkable limits and nowhere near as low as drinks like Coca-cola or soda water which are made deliberately acidic by large amounts of CO2 dissolving into Carbonic acid (HCO3). A simple way to test the drinkability of rainwater if you are worried about its acidity is with a simple pH test kit. In order to ensure acid rain is not an issue with your water it is recommended that it is filtered through an activated carbon or charcoal filter to remove such contaminants.

Industrial areas are often highly polluted and have poor rainwater quality

Forever Chemicals

A lesser-known but perhaps more influential group of contaminants known as polyfluoroalkyl substances (PFAs) and more commonly called “forever chemicals”, are man-made substances which can last in the environment for thousands of years and are easily spread in the atmosphere. Generally comprising plastic materials from products like food packaging and waterproof clothing, these substances are linked with a plethora of side effects with governments around the world decreasing their recommended limits in water over the last twenty years. Although technically present all over the world due to their ability to spread in the atmosphere, they are much more concentrated in industrial areas or areas of high population density. There are three ways to remove these forever chemicals from rainwater:

  • Distillation: Returning water to vapour and then its pure form, removes the water from the PFAs rather than vice versa.
  • Reverse Osmosis: A common desalination technique will also remove inorganic compounds like PFAs.
  • Activated Carbon/Charcoal Filtration: The activated carbon attracts and bonds to the PFAs allowing clean water to pass through.

Collection Surface: High Chance Of Contamination

Dirty collection surfaces or surfaces composed of heavy metals can lead to the contamination of rainwater. The collection surface could be the roof of your house, the watershed into the local reservoir or practically anything else that can capture rainfall! When water runs across the collection surface it will pick up and carry with it anything that happens to be on that surface, be it animal faeces, dirt or even trace amounts of lead. The collection surface section should be divided up into two sections, the cleanliness of the surface and the material itself.


If a collection surface is unclean, this can lead to the collection of organic material which will then provide the basis for bacterial or pathogen growth. A particular contaminant is the aforementioned animal faeces which leads to the growth of the harmful bacteria E. coli, this is particularly prominent in the first rainfall after a dry period where organic material has had the chance to build up. Fortunately, common treatment systems like UV, distillation, RO, chlorination, ozone and organic carbon can all remove these harmful bacteria from rainwater.

This moss-covered rooftop with lead flashings beneath the window is an example of a poor water catchment.


The material of the collection surface plays more of a role in heavy metal contamination, particularly the use of lead flashings to seal gaps around roof structures and gutters. Lead is a heavy metal that is highly soft and malleable which explains its extensive usage in roof flashings for centuries, unfortunately, it is also quite soluble and extremely harmful to the human body. When rainwater runs down a roof of an older home it may come in contact with lead flashings and tract amounts will dissolve into the water on their way to storage or consumption. Fortunately, common treatment systems like distillation, RO and organic carbon all help remove lead from rainwater.

Storage Material: Bacteria & Heavy Metal Contamination

In a similar manner to the catchment surface mentioned above, the material where the water is stored can heavily influence the drinkability of rainwater. You could be in the pristine countryside with rain falling on a clean roof with no heavy metals, only for it to be contaminated in the storage tank itself. Common tank materials are plastic (polyethylene), rounded fibreglass and concrete, with some less common materials like copper & other metals also used. There is no clear winner when it comes to minimizing contamination of water when in storage:

  • Plastic generally presents the strongest case as they don’t suffer from rust or corrosion like concrete or metal options which degrade over time.
  • Copper has the best antibacterial properties however it is soft, expensive and not practical for large-scale storage.
  • Galvanized metal tanks were popular in the 20th century however without regular upkeep they are very susceptible to corrosion and subsequent contamination of water.
  • Stainless steel is a great material however it is extremely expensive for household use and is generally reserved for industrial applications.
  • Concrete is susceptible to the generally slightly acidic nature of rainwater, which leads to its slow degradation over time and subsequent contamination of water.
Galvanised metal tanks will slowly leech metals into the water as they age.

Storage Time: Less Storage Time = Better Water

Taking into account what we have read in the above paragraphs, it may be obvious that the longer rainwater sits in storage, the more likely it is that contamination will be worse. This is particularly prevalent in warmer climates where increased water temperatures assist bacterial growth, it is even further exacerbated when the water tank is not fully emptied after each use, meaning the residence time of some water in the tank can be uncomfortably long! One way to lessen the issue of excessively long residence time is to have two separate tanks, where you fully deplete one before switching to the other. You then flush the empty tank cleaning it out in preparation for the next batch of water.

Generally speaking, if your water is going to sit in storage for any length of time it is recommended to apply some form of water treatment. Reverse-Osmosis presents the most comprehensive option for on-demand water treatment as it will remove all contaminants from the rainwater, however, it does require a large capital investment. A more cost-effective option is to buy an activated carbon filtration jug or water dispenser which will remove most contaminants and ensure the water is of top quality. Distillation will also work, however, this is generally less user-friendly and time-consuming, hence its relatively low adoption for day-to-day use.

If you are interested in how to purify rainwater for free, I put together a quick guide with two clever techniques here.

Can You Drink Desalinated Water: The 3 Most Common Queries

What Are The 6 Different Desalination Types?


Recent Posts