Rainwater Filters

rainy dayRainwater harvesting can be used to supplement your household water with the use of rainwater filters. When filtered, it can be used for laundry, washing, and flushing toilets. If proper steps are taken to insure it’s purity it can even be used for household drinking water.

Even if your house is connected to a main water supply, rainwater can ease the burden on the main water supply as well as cut down on your water bill. A rainwater harvesting system collects water from your roof. It is stored in a tank and saved for future use. If you are collecting rainwater for garden use the system can be as simple as a drum connected to a gutter or downspout.

However if you want to connect your rainwater harvesting system to your household plumbing you will need a permit. Some cities will require an annual inspection of your system. You must take steps to insure that the collected rainwater is kept separate from the main water supply. This is done by the use of a dual check valve system or a backflow prevention device. This must be installed by a professional plumber.

Roofing Materials and Pipes

Certain roof materials are not suitable for rainwater harvesting. Uncoated metals should be painted with a roofing paint that is labeled suitable for rainwater collecting. If your roof contains lead, chromium, cadmium, soldering, or flashings the roof should not be used for rainwater collection. PVC has come under scrutiny because of concerns over chemical contamination.

Rainwater can also contain many harmful bacteria, including salmonella and e. coli. It can also contain animal droppings, chemicals from agricultural spraying, residue from vehicle emissions and other harmful contaminants.

Filtering and Purification

To prevent organic matter from contaminating your rainwater use a first flush diverter or a sediment trap. This device diverts the first rainwater that is most likely to be contaminated into a pipe or drain, and then into a garden area or some other suitable soakage space. Make sure that the tank is sealed tightly. A screen over the inlet pipe will filter out solid matter and keep out insects, birds and other small animals.

A simple, yet effective filtering system can be made using inexpensive materials. The most important aspect of the filtering system is that the water is allowed to flow through a series of filtering agents. The most common agent is activated carbon. It is available at any fish aquarium supply store.

The molecular structure of activated carbon is such that micro-organisms and contaminant particles are absorbed into the carbon as the water flows through it. The carbon must be rinsed of all dust. Run water through it until it comes out clear.

A layer of sand is also an effective filtering material. You can use a layer of carbon on top of a layer of sand. Effective filtration is achieved by allowing partial filtration at the top layer of filter material but as the water seeps through the filter material it becomes trapped by the porosity of the material. If all particulate were filtered at the top level the filter would become clogged almost immediately.

This type of filtering utilizes the force of gravity to power the water through the filter material. It can be quite slow. There are vacuum filters that force the water through paper filters that can remove fine particulate, as small as a few microns in size.

Another common filter material is 100% polyester quilting padding. This is available at any fabric store. Don’t use cotton because it will grow mildew. The best filtration system will employ several filtering mediums.

In order to safely drink rainwater it needs to go through some sort of purification process. This involves the use of chlorine and boiling. Ten drops of bleach per gallon in addition to adequate filtering should make most water safe to drink. There are many water purification kits available on the market today.

There is also the option of ultraviolet light purification. You should have your purified water tested by a laboratory on occasion to ensure its safety. There is no filtration process that can filter out contaminants completely dissolved in water so never assume that any rainwater that has only been filtered is harmless.


Regular maintenance is vital if you plan to use harvested rainwater for household uses. This includes regular cleaning of your gutter system for buildup and debris, removing sludge from the water tank, and cleaning the first flush diverter.

Replacing the filter material is important because when the filter material becomes saturated with contaminants, it will begin to release them into the water instead of removing them making the water more harmful to drink than before the filtering process.

The type and size of system that you choose should be dependent on the amount of your average rainfall, the surface available for the capture and the amount of water that you anticipate to supplement. If your harvesting system becomes overloaded, make sure you have adequate overflow precautions in place to direct excess water into a safe area. If the overflow is not safely diverted it can cause damage to your property or your neighbors.

Photo by laffy4k, Creative Commons Attribution License