RO/DI water same TDS as tap?
 

Hello all.

I've had the same RO/DI setup for about 12 years. It's a standard 5 stage. I recently (last couple of months) changed out the entire set of filters and DI resin. As usual, I check the TDS every once in a while. On average after 8 months or so, I start getting 1 or 2 TDS on the meter and I change filters.

Recently I noticed a slight algae build up and I got out the magnifying glass. I'm currently getting the same TDS out of the unit as I am out of the tap!!! In 2 months? I have a 5 micron sediment, 1 micron carbon block, another .6 micron carbon block, the RO cartridge, and then then DI. After passing through all of that after 2 months it's entirely depleted? I've even ran a filter set an entire year before and only got to 4 TDS. What happened?

My water supply is not THAT bad - it's only ~146. I checked all of the connections to make sure I didn't bonehead something and everything looks fine. There are no leaks and the unit is shut completely off after each use. Could my flow restrictor be causing this in some way? I've got it configured as a 75 GPD. Do the restrictors go bad? How could I have burned through all of this filtration in such a short period of time with a source of 146 TDS?

I'm honestly amazed you were able to go 8 months & only get a TDS reading of 1-2. Even so at that point, the only thing you should be changing is the DI resin & possibly the sediment/ carbon block if flow is dropping.
So are you on City supply or well? If a City supply, it is possible there was a water line break & the City flushed the line with chlorine at high concentration, which has probably shot your RO membrane. If you're on a well, then maybe something has leached into your ground water. Is your house water pressure the same as always? Low pressure will allow more impurities to pass by the RO filter.

Maybe they changed from chlorine to chloramines as a disinfectant and that destroyed your membrane and other filters.

I'd try pulling the membrane and re-seat it if your tds in is the same as out. Maybe a gasket failed.

If you've been using the same R/O membrane for 12 years, then my first guess would be that the membrane is shot. I would measure the tap water TDS and then measure the TDS of water coming out of the membrane (not through the DI). You should see approximately a 90% reduction in the TDS. If not, your membrane is not working and should be replaced.

Thanks. These are all good suggestions. There was, in fact, a water main break right up the road from me 2-3 weeks ago. I didn't even think about that. Now I don't know if I should just replace all the filters again and hope for the best or try and determine if the water is going to cook the system before I spend another $100+ on filters. I assume all of that super-treated water has passed through the system by now.

I will check the seating of the membrane, but I'd suspect the casing would leak if it weren't sealed. Not to mention that the RO membrane isn't the only thing removing solids from the water in the system. It seems that even the sediment filter would remove a small amount. The carbon blocks should remove substantially more, followed by the membrane reducing things to almost 0. Then the DI kicks in a finishes things off.

At this point these filters are so new, the sediment filter is still bright white. For now I'll have to go with distilled water until new filters come in. I'll update as things progress. Thanks again for the ideas.

your ro membrane does most of the work as far as tds. I'd be looking there for problems. water could be bypassing the membrane without leaking out the housing.

I was able to pull apart the canister for the membrane tonight and can verify something isn't right. There's barely any water being led to the waste line. It's like a 2% rejection rate instead of 98%. The membrane itself seemed dry. The water is definitely bypassing the membrane somehow, but I'm not sure how. When I pulled it out it was seated firmly and gave a nice suction sound when I pulled it out, so I think it's sealed well. The seal around the membrane seems in order and fits nice and tight, and the o-rings at the bottom are not damaged. What else can I do?

The only thing I can think if is this. Originally since day one I've used the Dow Filmtec BW30-1812-75 membrane but they phased that part and replaced it with BW60-1812-75. This is the one that I'm currently using. The only physical difference that I see is the length of the membrane on the sleeve. This one is slightly shorter.

My gauge reads ~60 PSI. I don't think pressure is a problem, it never was before. I'll also admit that the accuracy of this pressure gauge is likely an estimate at best, but it's always read the same. Bad membrane perhaps? How can I check without just buying another one? Now that I've done further research on this model, it seems that some are having issues with higher TDS readings. I've yet to find a claim that they are having a bypass issue.

If the membrane is dry its shot. They can't dry out once wet.

I recommend the spectrapure 99% membrane. Its awesome.

So you have replaced the membrane? When? How old is your current membrane?

For your output TDS to be the same as your tap water, the water must be completely bypassing your membrane (or blowing right through it), although the way membranes are made and normally plumbed, I'm having a hard time visualizing how that could even happen.

Are you sure there are not valves open (or closed) somewhere that cause the water to bypass the RO membrane? Who is the manufacturer of the original RO/DI system?

LOL. Yes. I haven't been using the same RO membrane for 12 years. I've changed it quite a few times, most recently being this past June. The only difference is that before I had always used Filmtec TW30-1812-75. Now they want me to use BW60-1812-75. For all intents and purposes it's the same membrane dimensional-wise, but with a new type of membrane with a higher salt rejection. Almost every vendor that sells reef products/water purification products sells this membrane to go into a standard RO housing. That's what I have - a standard RO housing. There's nothing strange or unusual about it. The membrane slips right in and fits like glove.

There are no other valves or bypasses installed on the system. It is a generic unit. The water is just simply not being forced through the membrane properly.

In the meantime I've purchased a new housing and restrictor (550ml). A small investment for replacing 12 year old components. The housing arrived and was almost exactly the same as the one I received 12 years ago. Same with the restrictor. Once hooked up, same results - barely any water from the waste line. A bit more through the clean water line, but still the same TDS as tap.

My conclusion at this point is that I have a defective membrane. As strange as it sounds, I have to only assume that even Dow lets a bad one get through from time to time. There is simply no other thing it can be. Unless my original ro housing went "bad" somehow and the new one I replaced it with is "bad" in the exact same way. It has to be the membrane. The good thing is that this model of membrane tends to be half the price as the original one. Since I'm long past the return period, I'll just have to swallow the loss and move on.

I'm going to try and locate the original TW30-1812-75, but I couldn't find it before. I'm just hesitant going with the same model again after having this problem.

If you are barely getting any water through the waste line then either there is a blockage on said waste line, or perhaps a hole in the RO membrane. I had my waste line freeze up once, preventing any waste water flow and the TDS out of the RO stage rose to equal that of the incoming raw water.

I would check the waste line for some kind of blockage first.

My guess is your missing the restricted in the output line. Did it end up in the waste water line by mistake, or did the lines get switched?

The restrictor should be in the waste line.

That is correct. My mistake. I still think it is missing or in the wrong place

Give us a call when you are in front of the system if you need help getting this squared away!

Russ