RO/DI or Just DI?

We made this tool recently to help give you some math behind the RO/Di or Just DI only question.

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That’s awesome Chris!

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People often think they really need an RO system… We wanted to show folks that DI only can be cost effective in the right areas and situations.

This is something I just posted about on the other forum. Some may find it helpful in translating the vendor information into practical number that may help someone decide what they need:

Ok folks - time to resurrect this post. I kept track of my usage of one 1/2 cube tank of mixed bed resin and figured I would pass on my numbers:

Started use in 10/14/2019.
I did 20 homes total for a total of $5290.00 which averages to 264.50 per house.
That total is before sales tax and includes the portion of invoices that cover the outside.
My tds averages around 130-140 ppm
Last used this tank on 4/8/2020 finishing at 2ppm.

Tank exchange will be 55.00 including tax.
So in summary 5290.00 came with only 55.00 overhead on the purification front. Add in 10 for a carbon prefilter and its 65.00
I anticipate doing 3 exchanges every 12 months as this just covers the winter months
This gives me a baseline to figure out whether an R/O Unit is worth the investment for me.

Also, may help others to decide where to begin. If it costs me roughly 200/yr to run a Di tank, how many years with RO would I need before I had a return on that investment. This makes the numbers a bit more tangible for someone entering WFP work as opposed to just estimating say 300 gal on one 1/2 cube tank. What does that mean in practical terms? This gives me a better idea of what 300 gallons may mean for me and may help others who are in similar position.


The other thing that this fails to address is the fact that if you run 2 di tanks in series you can extend the usefulness of resin more than if you simply ran 1 DI tank.

Just in case anyone doesn’t know, it works like this: Buy 2 DI tanks of the same size. But, start off only using one. When the water being produced from that tank starts reading above 10 TDS, the resin is no longer suitable for window cleaning. DO NOT DISCARD THE RESIN!

Now, add your 2nd DI tank AFTER the original one. Now, your old tank is “The Workhorse” and your new tank is “The Polisher”. Your Workhorse tank will continue to pull out minerals from the water, reducing the wear and tear on your Polisher tank. As usual, keep monitoring your TDS.

Once your TDS starts getting above 10 ppm, you now know that your Workhorse resin is fully spent. Clean that tank out, and replace with new resin. Now swap tank positions. Your tank with new resin becomes the Polisher, and the other one goes BEFORE it as the Workhorse.

Repeat this swap and resin refill each time TDS gets above 10 ppm, and you will always max out your resin.

If you run on one tank only, you will never get the full value out of your resin.

Neat! I want to ask what the source of the TDS information is from. I know the two locations that we live/lived in had different TDS readings compared to the tool’s data. I know there are way too many variables as well. Just providing some feedback.

Or talk to me. I have the most dynamic, real-time calculator in the industry. You will see multiple scenarios, including the extent to which projected growth will affect your decision.

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I’d be interesting to add a RO obly category. We use RO membrane only for at least 50% of our work and save tons of $$$!!


What’s your tds reading your looking for before deciding to go RO only?

In our area it’s fluctuates from 150 ppm down to 60 ppm. At 150 we can get 004-008 ppm and with 60 we can get down to 000-001 ppm at 80 psi incoming.

For sure I’m running my RO’s hard which hurts the lifespan so I’d have to take that into consideration. But, it’s lighter (bike trailer), and saves time changing out di.

Also to note, something I’ve noticed over the last almost ten years of experimenting, di water just cleans slightly better. I can’t quite explain it. 000 DI water, compared to 000 RO water just has this glide to it.


Sorry I missed your question. The data was sourced as an average water reading in something like 47,000 USA zip codes. Its just an average and not a guarantee of course. You can have wildly different TDS readings in the same zip code.

I have around 150 coming out of the tap at my house and 10 minutes down the road at work its 250+

We figure some rough data is better than none.

No worries and thanks for the update. Isn’t it nuts how you can get such differences from just a few miles away? Here, many are on a well, so yeah, that messes with everything.

But the principle of calculator is great! Thank you. It helped us figure out which system to build.