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I currently use half Reverse Osmosis (RO) water, half tap water & dechlorinator for my aquariums (probably closer to 75/25) but it takes a lot of water to generate enough RO to do water changes every week. For each unit of RO I create, I throw away at least 2 units. I know that I could use it for other purposes (such as water plants etc) but I still have to pay for that water.

I live in the UK so there's no shortage of rainwater falling out of the sky. As I am removing the total dissolved solids and, I think, the bacteria, there should be no reason that I cannot generate RO water with rainwater that I can see.

Can I safely make RO water using rainwater as the input and is my reasoning above correct?

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  • Not an answer, but have you considered put the discard water in toilet tank? There should be a shut off valve on the line going to the toilet tank, turn it off when you get up in the morning (before first use). As waste RO water is generated fill the toilet tank with it. If you have an older model toilet it probably holds 5 gallons (19 liters) even the newer ones use 1.6 gallons or 6 liters per flush. This applies even if you do use rain water as your source supply. You might even use a two barrel solution, one for rain, one with an on demand pump for the toilet.
    – James Jenkins
    Commented May 16, 2018 at 14:19

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It all depends on your pre filters too.

My RO/DI unit has a sediment filter -> carbon filter -> RO membrane -> DI resins.

If you have the carbon filter you should be ok filtering rain water through your RO unit.

Also keep in mind Ro filtering works if you have proper pressure: 60 PSI is best.

So you will need a booster pump for it.

This article might shed some more light on the bacteria removal (paragraph 5): http://www.purewaterproducts.com/articles/carbon

Seems between carbon and the RO membrane you should be fine.

This seems like a good choice: https://www.bulkreefsupply.com/brs-universal-carbon-block-filter-1-micron.html

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