Considering if I really need to run RO

Skydvr · Mar 4, 2019

This is a discussion on wheteher I actually need to run an RO cartridge, not if I need to use RO/DI.

My well water usually sits somewhere around 147TDS. My RO waste water is at 70TDS with a rejection ratio somewhere over 8:1 (I know TDS is only a measure of how effective the unit is and is a general guide for maintenance). I was thinking about running a pump to run the waste back through the system or maybe running a second stage to incease the output. Then I began questioning whether or not I even need the RO stage.

I ran a Zero Water pitcher for a few years with no issues untill I got a larger tank and it became too tedious. The website claims that each filter removes 18,000mg of dissolved solids. I’d get 47 gallons or more before starting to register anything on the TDS meter.

I’m wondering if a sediment filter, carbon block, and DI resin (possibly 3 stage) would be sufficient and more cost effective, or cost effective enough to offset the waste. It would have more prefiltration than the Zero Water pitcher. I’d have to run some calculations for a cost comparison, but I’m not sure how fast resin gets used up.

Does anyone have any input on how long DI resin would last with 64TDS or so water, assuming CO2 isn’t too high? (I haven’t run the RO/DI unit long enough to know if CO2 is an issue)

Does anyone see long term issues with DI over RO/DI other than the increased rate of resin exhaustion?

laverda · Mar 5, 2019

You have given numbers that do not make sense. You water starts out at 147, but your waste water is only 70 TDS. Typically your waste water is going to have higher TDS. Unless your well water has a lot of large particulate mater that the pre filter would catch I would expect the waste to be higher than 170. What is the TDS coming out of you RO unit? It should be less than 4 depending on the rejection ratio of your membrane.
You mention water of 64 TDS going to you DI resin. With your TDS that high you will go through resin pretty quickly. I would not plan to bypass your membrane unless you plan to rejuvenate your resin your self. I run duel membranes to get more good water for a given amount of waste. I have also run my waste back through my system a second time in the past. Both were effective.

ckalupa · Mar 5, 2019

Be careful on the Zerowater system. A lot of times I will see my water coming out of the unit with something in the 6-13 for TDS - so zero is not always the end result in my experiences. And it is very slow in making water.

As others have stated, DI does not require RO to begin the process but your depletion rate for the DI resins will be significantly higher without it.

Skydvr · Mar 6, 2019

The pitcher is extremely slow, but was suitable when I was running a pico. Collecting water over a couple of days (refilling when I walked by or thought about it) and storing enough for a 100% water change plus a week of top off was a somewhat effortless task. I did check the water frequently to verify that the TDS is below 3. It would slowly climb and hold at 3 for quite some time, but once it started climbing from there, it would jump quite quickly. You also need to keep the filter flooded, once it dried or got air pockets, it would channel and TDS would climb rapidly.

The 64 TDS is a rough estimate based off of an 8:1 rejection ratio and and 1/10th of the total TDS passing through. I'd have to break the system apart to be able to measure what is coming out of each stage, which I will likely do at some point this weekend.

I am assuming that my RO waste is lower than what is coming out of my faucet due to the particulate filter and the carbon block. I suppose I should see what is coming out between each of the stages so I actually know what I am dealing with. When I removed the faucet aerator to hook up the RO unit, there was quite a bit of particulate on the back side of the screen.

Running through 90 or more gallons of water a week to collect 10 gallons of water for water changes, top off, and cultures is a bit excessive. The typical re-use methods don't work for me as I'm in the woods and don't get enough sun to garden for the 5 months a year that it isn't too cold or too wet to water and I most definitely don't do enough laundry to make a dent in the 80+ gallons of waste per week.

With the Zero Water pitcher, I was paying 11-23 cents per gallon, depending on my ability to find the cartridges on sale. Those cartridges can't be cheaper than bulk resin. Basically, I'm tossing ideas around trying to figure out the most cost effective way to produce water with the least amount of waste without investing a significant amount in the setup.