What I’m finding is if I have the waste coming out slightly higher then the TDS readings are higher going into the resin, why’s this
What we all have to do is balance our waste output so we get the r/o working as efficiently as possible. This is very much input pressure related.
On the old 450gpd r/o we had we had a preset waste restrictor which was a 3 waste to 1 pure ratio. We couldn't change that. Our water pressure back then was 40psi and our tap water tds was 250ppm. With no booster pump the tds of the pure from the r/o membranes was 4ppm.
With our current 4040 the water pressure is up slightly and our tap water tds has dropped to between 99 and 147. Its 123 atm. My r/o is set to a 50/50 waste to pure and the output tds from the membrane will fluctuate between 2 and 3.
A 7 liter di vessel needs a resin change about every 12 months. It could go a little longer as the tds seems to stay at 1 after di for a long time. (I pushed it a little before I did the last resin change and it still sat a 1ppm for a further 6 weeks before I changed the resin. Interestingly I think my inline tds meter is slightly 'stricter' than my handheld one as the water in the van's tank continued to read zero during this time. The hand held tds meter was calibrated a while back when the batteries were changed.)
If I restrict the waste further to say 40 waste to 60 pure then the tds of the pure goes up. Doing this means I save water but my membrane won't last as long and my resin costs will be slightly more.
I can change the pure to waste ratio the other way. I know that if I choose to set the ratio at 60 waste to 40 pure I get the same result when the ratio is 50 waste to 50 pure (1 to 1), but I use more water. If I increase the waste ratio to about 65 waste to 35 pure, the tds of my pure increases as the pressure on the membranes reduces, and I will be using a lot more water.
So for me, my 4040's sweet spot (maximum membrane efficiency and less water usage) is 50/50. I also have 13lpm of water available to my r/o. If that flow was restricted but with the same water pressure, then I might see my pure tds output increase even at a 60/40 ratio. (ATM we have sufficient water flow available to sustain our water pressure over a wider spectrum. At current settings we produce 2lpm of pure and 2lpm of waste. We are using 4 lpm of the available 13lpm from the tap.)
This could all change in the next year with a new housing estate being build on the side of ours with 150 new houses. The only silver lining is that all these houses will have water meters and people with water meters seem to be more frugal with their water usage.
So finding your r/o sweet spot is all about water pressure and how much water (flow from the tap) you have. This applies to all r/o's to one degree or another.
