Reverse Osmosis Water System Efficiency

Water analysis, treatment, and mineral recipes for optimum taste and equipment health.
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#1: Post by sbenyo »

I got a new 5 stage RO system. My water at tap shows ~400-440 PPM. The RO system get the water as low as ~20 PPM sometimes a bit less. Never saw it get lower than 13.

Is this how it should be? I am not getting clear answers from the company support. First they told me it needs more time. Now they tell me it can't get lower because of the high PPM. It does not sound right.

I need to know if I should expect to see around 0 PPM or at least lower then 10 PPM. I thought that how it should be.
I want to use TWW or rpavlis methods but I am afraid it will be too much as they probably been measured for pure water.

Hope someone can advise. For now I am very disappointed with the RO system and not sure what to do with it. Now sure if it's safe to use like this or how to mineralize the water.

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#2: Post by lessthanjoey »

Yes, that's pretty good, a normal non-high-pressure-pump RO system will only end up around that point, and will actually get quite a bit worse due to TDS creep (you may find if you use small amounts and it tops up the numbers will creep into the 30s). This is because the RO membrane is much less efficient with a low pressure differential across it. Making water when you're almost full will always have higher TDS than water with no output pressure, even with a permeate pump although that does help.

People getting ~5ppm have much softer starting water (or pump systems).

Having said that, all of this is totally fine! You have ~20ppm starting, so what? Frankly at that level with normal recipes you could just use them as is and know you'll be a little high on general hardness, alkalinity, or both. I used an API aquarium kit ( ... B003SNCHMA) to test my residual RO water (which is "worse" than yours) - you can use a larger test-tube and dilute the drops into more water to get more resolution. Normally it measures in 18ppm/drop. You can use 10ml instead of 5ml and get roughly 9ppm resolution for KH/GH which is plenty good for our purposes.

Basically, this is normal and not an issue. If you want to first-order compensate for it in your water recipes you can.
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#3: Post by sbenyo (original poster) »

Thanks lessthanjoey. I guess I will have to find the right measurement to compensate on the TDS with rpavlis or TWW. I will give it a try.


#4: Post by Reefguy »

For what it's worth, the best you will get from any reverse osmosis is 5-10 tds, they are not engineered to go down to zero tds. 75-80 psi is normal home pressure, while ideal for the tfc membrane is 100 psi, hence a booster pump being used for optimal performance. The cheaper tfc membranes are usually the reason for higher tds. Dow membranes cost more but last longer and perform better. If you want zero tds go for dionization for final stage.

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#5: Post by sbenyo (original poster) »

Reefguy, thanks for the clarification.
I understand RO won't get me as low as I wanted. I will need to find how to compensate for it with the mineralization.
I hope someone is already doing it, so I hope to get feedback how to measure it correctly.

I may as well add DI if this can help to get lower.


#6: Post by Reefguy »

Di is only way to get to zero. We used this type of filtration for saltwater Corals holding facilities.
I have an Ro di at home, but I'm somewhat lazy and just mix 30% zero tds with 70% 150 tds tap water.

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#7: Post by Almico »

Coffee tastes best with 50-75 TDS water. I have a commercial system in my coffee bar and blend some mains water into the permeate to bring it up a little.

For my home system I added a 6th stage remineralization filter to add back some magnesium and calcium. Makes coffee taste better.

Get a bottle of distilled water and make a pour over with it. Yuck. ... -standards

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#8: Post by sbenyo (original poster) »

Thanks for the information. I need to check the TDS with RO + TWW or rpavlis. I want to get tasty, good drinking water but the whole idea of RO is to completely eliminate scale so I hope it will still be like this. I will check it and see what the results are.

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#9: Post by sbenyo (original poster) »

Started using TWW and rpavlis. TWW gets me around 160 TDS. rpavlis around 70.
I think 160 TDS is too high and I want to be on the safe side and with zero scale. I will keep using rpavlis. It's also much cheaper.

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#10: Post by Nunas »

It is important to differentiate between overall TDS and hardness minerals. For example, if you run hard tap water (say 400-500 ppm TDS with lots of calcium and magnesium in it) through a standard ion exchanging water softener, the overall TDS will not change a lot. However, the softened water will be nearly devoid of harness components, as they will have been exchanged for sodium or potassium ions (depending on how the softener resin is flushed). Thus, the original water will build up hard scale quickly, while the softened water will not.