Filter Systems for Rainwater Harvesting and Collection

Rainwater storage systems are becoming very popular in areas of the west coast where there is plentiful rain in the winter months and minimal rainfall in the summer. The warmer climate allows the use of large outdoor storage tanks with no worry of freezing. Rainwater collection is a year around activity. Many residential users have tanks with up to 15,000 gallons capacity. The average roof can easily collect 20 – 30,000 gallons per year on the west coast. Rainwater has many advantages over drilled wells:

1) Rainwater is naturally soft, has no iron, odor causing sulphur, or other problems associated with well water

2) A rainwater collection system is often less expensive than a drilled well, deep well pumps, and associated water treatment systems

3) Rainwater collection systems can be set up to operate on gravity flow and use no electricity

Below: Typical collection of dirt after winter season on the West Coast in a gutter.

Rainwater as it falls to the earth is very clean, having been naturally distilled by the sun and its heating action causing evaporation from the surface of the earth, lakes and oceans. Rainwater will collect dust on its way down thru the atmosphere and is usually slightly acidic in nature. If you look at a clean car that has been rained on by a short shower you can appreciate the amount of dust left behind once the raindrops evaporate. Once rainwater hits the surface of the roof it will dissolve or physically carry dirt and debris that is sitting on the roof and carries it down to the eaves troughs.

Your roof is a large flat surface that collects a variety of debris including dust, airborne soil, leaves, pollen, deteriorating roof shingles, leaves, twigs, mould, fungus, and any wind borne items. Your roof and eaves troughs are also home to a population of insects, which includes sow bugs, spiders, mosquitoes, flies, beetles, bees, wasps, and other exotic life. A large variety of birds – seagulls, crows, robins, finches, etc, use the roof for rest, target practice or as a source to find insects for an easy meal. It is not unusual to find bird nests and bats living on roof structures and eaves troughs. Naturally all these birds leave their calling card behind. Rats, mice, raccoons, squirrels are also frequently found on rooftops.

Below: Dirt collected from 10 ft. of trough, note the peanut shell – squirrel has been here!

The end result is that the largely organic and fecal collection of material on the roof finds its way into the rainwater collection system and into the holding tanks. The best strategy is to remove as much of this material as possible prior to the collection tank with strainers and roof washers which divert the first flow from a rainstorm to drain, followed by a variety of devices to intercept leaves and debris. The problem with these strainer and diverter devices is that they have a very large sieve size and are not capable of removing the smaller harmful bacteria, virus and parasites that can be found in surface water. Intestinal parasites can be infectious for over a year once they leave the host body carried by fecal excrement. The nutrients from the dissolved and suspended solids will set up a reaction similar to a septic tank, where large numbers of bacteria and algae flourish and quickly use up any available oxygen in the system. This will change. The color of the water and result in an unpleasant odor in the water system. Bad odor is a definite sign of contamination and this water should no be used. Unfortunately no one wants to collect thousands of gallons of rainwater over the winter and then have to dump it when they need it the most.

Below: Used and dirty cartridge-style water filters.

Each filter begins as pure white in color. The best strategy to clean rainwater is to use a BioSand filter in the system. The BioSand filter has a capacity to remove large amounts of contaminants and will also remove 100% of parasites, 98% ob bacteria and virus. It is imperative that the water treatment system has at least 2 barriers to remove disease-causing organisms. Cartridge filters are not a barrier to disease unless rated by NSF as 1 micron absolute. All rainwater systems should have a main filter such as the BioSand as well as a good quality UV – ultraviolet disinfecting light.

Below: A typical BioSand filter set-up in a pump house.

Many people are content to rely on inferior cartridge style filters, as they are small and inexpensive. I have seen many systems with 3 – 4 filters installed in series, starting at 100 microns and working down to 5 microns. The problem with the use of cartridge filters is that they will give you a false sense of security. The only type of cartridge filter that will actually remove parasites and bacteria are NSF rated 1.0 micron absolute filters. These filters plug quickly due to their fine blocking action and are expensive ($120.00 for a 4.5 x 20 absolute NSF rated) to replace. The superior alternative is to use a BioSand filter that uses a multi layer of quartz that never needs replacement and has a self-cleaning mechanism built in. The BioSand filter cannot be bypassed because if you neglect to clean the filter it will stop operation and shuttle the water to drain so it is relatively fool proof. When inexpensive cartridge filters plug the water will bypass the filter around the top of the seal, allowing untreated water to flow into the household.

The best method to assure clean rainwater is to:

1) Keep the roof and eaves troughs clear as possible.

2) Do not collect rainwater in the spring pollen season.

3) Use a strainer or system to remove leaves and large debris.

4) Collect the rainwater in a tank that is then channeled to a BioSand filter.

5) The water from the BioSand filter is stored in a clean water storage tank, clean water will store for long periods, and dirty water will go bad very quickly.

6) Use a small air bubbler to keep the water aerobic in the storage tanks.

7) Use a whole house cartridge filter rated at 5 microns and UV light leaving the clean water storage tank prior to entering the home.

8) Add bleach to the storage tanks to prevent growth of algae on a regular basis.

9) Clean the BioSand filter monthly, change the UV bulb yearly, and clean the roof and troughs every 3 months if possible.

10) Do not rely on cartridge filters to protect your health unless preceded by a BioSand filter

Remember nothing is free – there is some effort required to operate a successful and safe rainwater collection system. Keep it clean use the correct equipment and your water supply will be clean and safe!