Hello. I have a 3 stage RO unit which produces around 10-25 TDS water. Recently I have bought 2 different DI units and both times the DI failed to drop my water even by 1 TDS. Does any have any clue why is this happening? I am looking forward to reaching 0 TDS and I dont know what to do. Thank you for any help!
DI not dropping my TDS?
- Thread starter theofilos
- Start date
woz9683
Premium Member
Are you sure the DI cartridge is installed correctly, i.e. good seals and proper orientation not allowing water to bypass? How accurate is your TDS meter? Have you calibrated it lately? It seems like there's something wrong with your RO stage too (if it's not the meter), 10-25 coming out of your RO is a lot. What is your waste/product ratio?
As an example, my ratio was something like 2/1 right after I hooked up a new RO, and TDS after RO was 8. Changed the ratio to 4/1 and TDS dropped to 1-2.
As an example, my ratio was something like 2/1 right after I hooked up a new RO, and TDS after RO was 8. Changed the ratio to 4/1 and TDS dropped to 1-2.
woz9683
Premium Member
RO set with a proper waste/product ratio should be 98-99% effective, so with that kind of TDS in your system water, you should be seeing no more than 10 TDS post RO. Is the RO membrane fairly new as well, or is it older? With TDS that high in your system water, the membranes aren't going to last as long. Again, this is all assuming your TDS meter is calibrated recently and properly.
woz9683
Premium Member
Well, there's either something not hooked up correctly or your TDS meter isn't working right. Check the meter, check the waste/product ratio, check the seats on the RO membrane and on the DI cartridge, check all the fittings and make sure the water is following the proper path through the unit, etc.
The meter is a new one. So far its tested correctly a lot of times. The ro unit is performing quite good actually. 24 tds from 450 in tap water is good I believe. With both DI units I tried I have the same problem. The fittings are correctly fit. I dont see a problem with my RO unit. What do you mean check the waste/product ratio?
woz9683
Premium Member
the waste/product ratio will effect the TDS of the product water coming from your RO membrane. In a standard unit you should be running 4-5x as much water out of your waste line as is coming out of the product line. Like my example above, I'll explain more fully this time. My system water is about 125-150 TDS. When I first hooked up my most recent RO membrane the waste/product ratio was 2/1 and the resulting TDS of the product water was 8. When I properly adjusted the waste/product ratio to 4/1 it dropped the TDS of my product water to between 1 and 2. Basically, restricting the product line slightly allows more free flow of water through the waste line and keeps small particles from being forced through the RO membrane.
I'm pretty sure the standard rejection rate for RO membranes is around 98-99%. So, if your input water is 450 TDS and the product 24, that's still only 94-95%. So, that tells me you need to restrict your product water line more to get the rejection rate higher, so you see maybe 5-10 TDS product water. This will make your DI resin last longer. And the reason I keep saying to check the meter is because DI resin is extremely effective. If everything is hooked up correctly, and the resin is new, it should drop that 24 TDS water to 0 with no trouble, although this isn't good because it will deplete the resin faster than normal, which is why I said you should probably adjust the waste/product ratio. Adjusting the waste/product ratio isn't going to fix your problem, but it looks like a secondary problem that should also be fixed.
Bottom line though, if you're reading 24TDS into DI and 24TDS out of DI, there's only 3 possibilities. 1. Bad DI resin 2. Bad/wrong connections 3. Bad TDS meter
I'm pretty sure the standard rejection rate for RO membranes is around 98-99%. So, if your input water is 450 TDS and the product 24, that's still only 94-95%. So, that tells me you need to restrict your product water line more to get the rejection rate higher, so you see maybe 5-10 TDS product water. This will make your DI resin last longer. And the reason I keep saying to check the meter is because DI resin is extremely effective. If everything is hooked up correctly, and the resin is new, it should drop that 24 TDS water to 0 with no trouble, although this isn't good because it will deplete the resin faster than normal, which is why I said you should probably adjust the waste/product ratio. Adjusting the waste/product ratio isn't going to fix your problem, but it looks like a secondary problem that should also be fixed.
Bottom line though, if you're reading 24TDS into DI and 24TDS out of DI, there's only 3 possibilities. 1. Bad DI resin 2. Bad/wrong connections 3. Bad TDS meter
