RO DI What is it really removing

[trueperc]

Hello,

I have been in the hobby for a long time. What has always bothered me it using an RO DI unit and having to deal with so much waste water. Now I know people have found uses for waste water or adding dual membrane for better rates. We base the health on flow rate and TDS. I do understand that in a perfect world using super clean water is the best case. That being said. Other than a number in TDS, what is really in the water we are removing by the membrane? I can get sediment filter and carbon filter for sure. I am not sure why we have not added GFO in the mix as I believe in fresh water is excellent at removing silicates and phosphates. Also the stuff being removed by the membrane, are we just readding back with the salt mix? I have talked to a few that just use prime and others that use an RO DI unit but the waste water goes into the same bucket as the good water. Boils down to, what exactly are we worried about that is being removed? With today filtrations methods and for me cost of the water bill, its come time to ask this question.

[mcgyvr]

In general its location/water source specific..
Someone from New York may be able to use tap water with nothing more than some prime.. While another guy a state over may have other "impurities" that can/will cause problems..

In general "TDS" is composed of inorganic and organic substances..
These may be calcium, phosphates, sodium, potasium, chloride, silicates, etc...

A sediment filter removes..sediments like sand,etc...
Carbon removes organic chemicals/chlorine,etc...
RO can remove bacteria, viruses, metal ions, salts, copper, lead, nitrate,phosphate,etc...
DI stages can remove more "ions" of cal/mag/iron/nitrates/silicates,etc...

And yes.. the salt mix may put some of this "back" into the water but in a "balanced" way and hopefully not so elevated its "too much" in an attempt to replicate natural sea water levels...

In general we try to start with a "clean slate" then the salt mix adds back as needed..
You can't get to the end without knowing what you are starting with.. So its best to just remove everything (as best as possible) and start with "FRESH" water (aka low/zero TDS,etc..) and go from there..

Can some get by using tap water... sure...
Can tap water cause problems in other tanks.. absolutely..

[Rilelen]

I use RO/DI because of the elevated copper and heavy metals in our tap water here. Is the tap fine for people/most of my tank inhabitants - yeah (and I use it in my FW tank), but there's a noticeable difference when I get lazy and top off my SW tanks with tap water vs RODI water. My xenia stop pumping after adding a few gallons of tap water and start to "melt" if I add more than that.

[tmz]

These articles note many of the potential reef toxins and other undesireables including PO4 in tap water vs ro/di water and explain how an ro/di system works and what it removes:

http://reefkeeping.com/issues/2005-05/rhf/index.htm

http://www.advancedaquarist.com/2004/1/chemistry

[trueperc]

Thanks. I just wanted to start the discussion a bit. I will check out those links for sure.

[oseymour]

In general its location/water source specific..
Someone from New York may be able to use tap water with nothing more than some prime.. While another guy a state over may have other "impurities" that can/will cause problems..



I live in NYC and our water is great. I still use a RODI. The water that leaves my membrane is 0 tds, my DI resin is now showing small signs of being depleted after a year. My sediment filters get blocked very easy though and this changes from day to day. I guess it's because our infrastructure that carries water is old 100 years plus. Who knows what types of pipes and the condition in my building.

I changed this sediment filter recently. About 50 to 70 gallons of good water later.

[ComforablyNumb]

"I am not sure why we have not added GFO in the mix as I believe in fresh water is excellent at removing silicates and phosphates."


Its because a properly working RO membrane removes them even better.

[tkeracer619]

Dual membranes don't improve ratio rates. This is a misconception. 1 or 2 membranes ideally should be ran at 4:1.

If you are really concerned about waste water adding a water softener will allow you to significantly reduce your ratio.

You can also add a zero waste system that pumps waste water back into the hot water heater.

The simple answer to your question is that you just don't know what is in the water so you do what you can to remove it all and start from as close to a known zero as you can.

Cation DI resin removes ammonia, carbon blocks cannot remove it.

[fishchef]

Just curious, if your carbon is saturated and allows chlorine to pass onto the membrane, will the DI remove the remaining?

[tmz]

Not much if at all ; it may also damage the membrane. See previously cited article.

[fishchef]

Not much if at all ; it may also damage the membrane. See previously cited article.

Over the years I noticed the smell of chlorine in my RO/DI at times, but my TDS was zero. We always kept a close eye on media usage length, but this seem to indicate that chlorine did not read on a TDS meter. Being a soluble gas seemed to be a reason.