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Readers Question: Do I need UV?

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Question from Reader:

I have build a house with a metal roof, a rain water collection system which eliminates the first run off and which then drains into a 25,000 gallon double cistern under the garage. There are a few filters which progressively smaller diameter , the last one down to one micron absolute. As most bacteria or spores are > 1 micron, I assume that a UV light is not necessary (viruses are not a real issue for rain water).

As there will be no or a very small amount of minerals, I believe I won't need a carbon filter either. Are my assumptions correct in your opinion? To clarify, the water is for domestic and for garden use.

John, first thank you for the question.  Unfortunately rainwater will have active bacteria in it in almost 100% of the cases.  This bacteria needs to be taken care of either with filtration or purification.

The norm for rainwater systems is UV or Ozone.  Chlorination, RO, and distillation are also options however, not commonly used in rainwater systems.  A 1 micron filter will not remove all bacteria,  but most.  I have read publications by the CDC that it needs to be a .3 micron to remove almost all bacteria.  Of course this would not remove viruses.

The linked article below gives a good overview of the various options.  I have also linked a book below that is very useful, with great charts and tables.


As far as carbon you are correct.  I only recommend them when more removal is required.

I recommend .25 micron (sediment removal), a 5 micron (most bacteria and dust), a granulated carbon filter (chemicals, chlorine, bad smells and taste) and finally a UV light (disrupt DNA of bacteria making it harmless). This combination removes bacteria, most viruses, and cysts.

Removing Algae for dripheads

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Algae is good for the plants, but very bad for the emitters so that is the dilemma. So it is a matter of balancing the filtering. Algae can be as small as .5 micron. The smaller stuff in algae is bacteria and beneficial to plants. A filter is recommended with every drip irrigation system because it removes sediment and other particles that are large enough to clog the emitters. Most tape and drip emitters can not handle up to anything large than this. So at least a 200 mesh filter MUST be used on any T-Tape system. There are, of course, smaller micron filters (i.e. a 200 mesh equals a 73 micron filter) that will take out much more. However, these are generally used in drinking water systems and not irrigation systems. Most irrigation filters you get will be this matter out without a problem.