Is it just my RO membrane?

[IslandCrow]

OK, it's definitely about time to order a new RO membrane, but I was caught a little off guard when my TDS shot up from 0 to 20something and is now hanging around 40-50. I figured even if the RO wasn't doing its job, the DI stage would be just as effective, but get depleted more quickly. I replaced the DI, carbon and micron filters just a few months ago, so I'm pretty sure they're still good. Could an old RO membrane be enough to cause this problem, or am I having issues with my DI phase as well? I went ahead and ordered the full set just in case, but I'm rather curious. Was I just wrong in my understanding?

[smleee]

What unit are you using, what is the configuration? Membrane, horizontal DI, etc?

Please post these readings:

Input TDS
RO Only TDS
RO/DI TDS

[IslandCrow]

Oh, sorry. I'm using the 5 stage, 50gph Aqua FX RO/DI unit. It's a micron filter (can't remember what micron off the top of my head), carbon, RO (horizontal), and then DI.

Input TDS is around 150. I'm afraid I've never looked at the RO only TDS. I don't have a tap between the RO and DI. RO/DI TDS is usually zero. As of yesterday, it was around 50 when I first started, then I flushed the RO for about 15 minutes and was able to get it down to 20. It could be my TDS meter went wacky overnight, but the input TDS is what I'm used to seeing, and I even tried switching around the input and output lines to make sure there wasn't an issue with just one of them (it's an inline model).

[AZDesertRat]

If the membrane is shot the DI will be exhausted very quickly. 16 oz of DI resin will treat approx 2000 total TDS so if your membrane is putting out water with a TDS of 20 you can expect to get 100 gallons out of the DI before it is exhausted. If the RO is putting out a TDS of 50 it would only be good for 40 gallons.
You really need to get a tee and ball valve and test the RO only output more often. You wallet will appreciate it as will your corals.
You are probably doing more harm than good right now as exhausted DI resin releases weakly ionized substances in huge gulps, much higher than the incoming levels are. Some of the weakly ionized things are nitrates, silicates and phosphates so get it fixed soon or discontinue using it.

[IslandCrow]

Very good info, thanks! My replacement membrane came in today, so it's been replaced. The TDS is now at 4, so I'm wondering if I've prematurely exhausted my DI resin (fortunately, I ordered a replacement for that as well). It should be no problem to do a little splicing and hook my TDS meter up to the RO output. I just never realized that it was important. My plan right now is to stop monitoring the water coming into the filter from the tap and just hook that connection to the line coming from the RO into the DI. Does that sound like a good plan, or should I get a second TDS meter?

By the way, when the TDS shot up to 50, I immediately stopped using the water. Even when it was 20, I only made enough water to for top-off and to replenish my kalk drip. I've just been waiting for the new RO membrane to come in.

[Misled]

You can get the t and ball valve from HD. only about 10 bucks.

[AZDesertRat]

Although I own two dual inline TDS meters I do not like nor trust them. I use my handheld HM Digital COM-100 for best accuracy. The inlines are not temperature compensated and they cannot be calibrated easily so I just use them as a rough guide. I would do as you suggested and move the probes to post RO and post DI though for best results. Tap water TDS does not change significantly unless your utility uses multiple sources.

[IslandCrow]

Yeah, In about a year, I haven't seen any significant change in the TDS of my tap water. I'll give some serious thought to getting a handheld TDS meter. Maintaining water quality is certainly key. I think it may have had something to do with a RTN problem I had not too long ago.