TDS Hell

[BBMarlin]

I have about a 6 month old RO/DI unit I purchased at BRS. I typically make around 20-25G of RO/DI at a time and store the water in 5G jugs. The water is used in around 1.5 weeks for water changes and top off and sits in my garage until I need it.

The RO/DI unit has one of those inline TDS meters on it and it always shows 0 TDS coming out of the unit. I’ve had some problems with diatoms, and I’m trying to narrow down the problem so I decided to purchase another TDS meter just to double check the water that ultimately goes into my display.

What I found was that the water coming out of the RO/DI unit is indeed 0 TDS. Once the water goes into the jug it jumped up to 4TDS immediately. I checked the jug this morning and it is now 12TDS. I checked a few other jugs of water that have been sitting for a week or so and they range from 10 – 50TDS. My question is why is this happening? I do use the same jugs for my water changes (in-out), could that be the problem?

[Elizzy]

Wow, waiting on responses, because I'm stumped!

[+Hydro]

Well there is obviously some leftover particles inside the jugs. Most likely the particles left are not going to turn in to food for diatoms IMO, most likely dust or minerals leftover from the water drying out inside the jug. We are talking about part per million, doesn't take much. My RO/DI comes out at about 4-6 ppm and I don't have any issues with it at all. I would most likely look somewhere else for what is feeding the diatoms.

[chrisp]

The de-ionization process, as the name implies, strips the water of all the dissolved ions. This leaves the water molecules begging for something to bind to. The CO2, dust and other molecules in the air will quickly bind to the water molecules. That is why if you have a drinking water tap it goes before the DI cartridge. The DI water can bind with some of the minerals in your stomach and they won't be as readily available for your body to use.

[offroadodge]

YES, RO/DI water absorbes everythng in the air it can. Thats why you need to keep it closed to air while sitting. If your using your jugs as duel purpose than YES that is and issue. Good luck

[Timfish]

What Hydro and Crisp said. I'm curious, what are the TDS readings on a bucket of your RO/DI before and after adding a very small amount of food or adding salt? In all my years of keeping saltwater I've never worried or bothered with TDS and actually have maintained a system for a client with just tap water for over 3 years (owners decision, long story) and not seen anything different than in my tanks I use R.O. or RO/DI water. When I started doing maintenance 15 years ago there were measurable nitrates and phosphates in the tap water and switching to R.O. eliminated probably 2/3 of the algae problems I had. More recently though I've started looking at the tap water and haven't seen any in Austin's water (haven't checked everywhere though) and the main reason I still use R.O. is I don't want to be surprised. (2nd is water pressure and flow can be surprisingly variable and it's actually quicker for me to bring water with me and I've never bothered with silicates either).

[BBMarlin]

Thanks for the info guys, I guess I will search elsewhere for my diatom catalyst. Tim, I did run a quick test to see what would happen if I added a small amount of salt mixture to some RO/DI (12oz) and the TDS went up like 20X. Given that little test, my guess is residuals from the salt mixture contained within the jugs is the culprit.