7 TDS city water -> RODI necessary?

[Potatohead]

FWIW if you are getting 7 psi after membrane, and you are getting decent pressure into the unit (say, 50 psi) your tap water is probably around 200 TDS or so, which is pretty typical.

 

[Trever]

@Potatohead excellent, thank you!

FYI, my pressure is around 53 psi.

Now I will abuse this thread further by throwing this separate topic out there... can one drink the "waste" water? I've read you shouldn't drink the 0 TDS, and although opinions on the 0 TDS seem to differ, my wife is a doc and confirms it's "too pure", it will suck stuff out of you, even if you do hydrate. It may not kill you or similar (don't assume this is proper medical advice please), but best not to drink.

Mr. Saltwater Aquarium says in a video that he waters his plants and lawns with the waste water.

Let's see... thinking out loud, so I know my waste water is 7 TDS? I thought I understood the plumbing but now I'm confused again- time to go look at this thing again for the 25th time today, and re-read some of the above. Probably not that straightforward...

 

[Potatohead]

Plumbing goes:

Tap > sediment > carbon > pressure gauge > membrane

Two lines come out of the membrane, one product water, one waste water:

Product:
membrane > TDS meter > DI > TDS meter again >good water out

Waste:
membrane > flush valve > flow restrictor > waste water out

The product water out of the membrane going into the DI is 7 TDS. The waste water out of the line is usually about ~50% higher than the tap TDS, so it's probably around 300ppm. I think that's about 1/3 normal Arizona tap water ;Joyful . I'd drink it, unless it tasted weird.

 

[WVNed]

 

[Biglew11]

I read in an earlyer post that you used s combination of hot and cold water? You definately don't want to do this the hot water dissolves things a lot easier, your source water will have higher tds, and the warmer the water going through the membrane the more stuff will pass through it. You'll have higher tds out of the membrane wich will use di resin faster.

 

[Trever]

@Biglew11 I'm just doing cold water.

In the first few minutes I was experimenting to see what happened to pressure, etc. But that was only for a minute or so.

 

[Biglew11]

@Biglew11 I'm just doing cold water.

In the first few minutes I was experimenting to see what happened to pressure, etc. But that was only for a minute or so.

Ok just making sure.

 

[Trever]

Another question...

The RODI owner's manual states:
Operate the system for ten minutes prior to testing for TDS, readings will always be higher when the system is first turned on.

What does this mean? Just that "readings" are higher but the "actual" TDS is lower (eg. 0)? Or that initially, TDS is higher?

I'm asking out of curiosity, especially since 0 TDS is so highly coveted, meaning we should let it run for 10 minutes every time before collecting water. But since we aren't told to do that (that I've seen), the meaning seems to be initial readings aren't accurate. This would also be consistent with automated water systems as I understand them (they collect the RODI water they create on demand immediately, AFAIK).

 

[homer1475]

The whole purpose of "flushing" is to get rid of the water thats been sitting in the unit from the last time you used it.

This water has a lot of TDS creep in it(higher TDS from sitting in the unit, kind of like stagnant, but not really stagnant). The flush valve just reduces the pressure on the membrane so water can pass over it cleaning the membrane prior to turning the valve, creating pressure, so the water now passes through the membrane.

For instance:

My incoming tap water has a TDS of 24. When I first turn on my unit the initial TDS is 24 from the tap, but after the RO membrane its around 30(TDS creep from the water sitting in the unit). After 10 minutes or so of flushing, I'm down to around 5. I then turn the flush valve to produce water which gets me down to 1 pre DI stage.

 

[jccaclimber]

1. These inline TDS meters aren’t great, and for some reason every one I’ve checked reads differently when water is flowing through it vs. sitting stagnant in a bowl. After getting one replaced I was told this is not uncommon, and that I should verify with a handheld if I wanted to test it in a reservoir.
2. For those rare people who do actually have REALLY low TDS water and know it stays that way year round, I know one person who runs sediment>carbon>DI and doesn’t actually have a membrane. Given that their tap water is similar to what most of us have coming out of the membrane, this isn’t entirely unreasonable. It does require making sure they stay on top of monitoring and DI replacement.

 

[Viking_Reefing]

@WVNed hmmm... Ok. So you're saying while IN is not the same as OUT, IN isn't really what's coming from the tap?

Re the "use it anyways" camp... if there's "danger" in 7, of course. But if 7 TDS is "harmless", then I'd much rather park my unit for when we move to someplace where the TDS is higher and therefore RODI is necessary, and in the meantime, in our current location, get my water at 1000 times the speed and not send 2/3d's of it down the drain!

The problem is that you have no idea what those 7 TDS consists of, it could be freaking arsenik for all you know. You also have no idea if the tap water will stay the same over time.
There’s just no good reason to risk it.

 

[KStatefan]

Another question...

The RODI owner's manual states:
Operate the system for ten minutes prior to testing for TDS, readings will always be higher when the system is first turned on.

What does this mean? Just that "readings" are higher but the "actual" TDS is lower (eg. 0)? Or that initially, TDS is higher?

I'm asking out of curiosity, especially since 0 TDS is so highly coveted, meaning we should let it run for 10 minutes every time before collecting water. But since we aren't told to do that (that I've seen), the meaning seems to be initial readings aren't accurate. This would also be consistent with automated water systems as I understand them (they collect the RODI water they create on demand immediately, AFAIK).

Running the system get you to a stabilized system to measure TDS.
When you shut off your RODI system the water that in in the membrane housing will be at 2 different TDS one higher than your source water and one lower than your source water as the water sits there the TDS will move to an equilibrium of equal TDS on both sides of the membrane. You want to send this water to the drain with a DI bypass valve and not to your DI resin. It does not have to run very long to remove the TDS creep. The flush valve on does not remove all of the TDS creep.

My automated system at home uses a high and low level float switch so I do not short cycle the system. It does not make RODI until I am down to about 10% of my storage container. When the system turns on it opens a valve to bypass the DI for 90 seconds then switches to DI and goes in the storage container.

 

[Biglew11]

Running the system get you to a stabilized system to measure TDS.
When you shut off your RODI system the water that in in the membrane housing will be at 2 different TDS one higher than your source water and one lower than your source water as the water sits there the TDS will move to an equilibrium of equal TDS on both sides of the membrane. You want to send this water to the drain with a DI bypass valve and not to your DI resin. It does not have to run very long to remove the TDS creep. The flush valve on does not remove all of the TDS creep.

My automated system at home uses a high and low level float switch so I do not short cycle the system. It does not make RODI until I am down to about 10% of my storage container. When the system turns on it opens a valve to bypass the DI for 90 seconds then switches to DI and goes in the storage container.

+1
the flush valve cleans the outside of the membrane, does very little for tds creep (high tds on the product water side). to get rid of tds creep you need to bypass the di for the first few minutes.
this tds creep is why they tell you to get your measurements after running for several minutes.

 

[Potatohead]

Correct this is why I mentioned earlier there are a couple kinds of flush valves. The waste line one is for flushing the membrane. It is also common to have another one on the product side after the membrane and before the DI, which is opened for the first 30-60 seconds when you start to make water. This way you don't get the extra TDS creep into the DI and will extend the life of the DI.

2. For those rare people who do actually have REALLY low TDS water and know it stays that way year round, I know one person who runs sediment>carbon>DI and doesn’t actually have a membrane. Given that their tap water is similar to what most of us have coming out of the membrane, this isn’t entirely unreasonable. It does require making sure they stay on top of monitoring and DI replacement.

Running a membrane is way cheaper than replacing DI more often. My tap TDS is 13-14, my DI is over three years old and only about 1/3 exhausted.

 

[Opus]

I will give my usual take on flushing the membrane. There is no proof that this actually extends the life of the membrane in any meaningful way. A membrane will last me 5+ years and my tap water has a tds of 400 to 450 during the year. The time and water waste over those 5+ years is not worth the extra month of use I might gain.

 

[PatW]

I would use an RODI system. The reverse osmosis would get you down to about .1 tds and so your DI resin would last almost forever.

 

[Trever]

1. These inline TDS meters aren’t great, and for some reason every one I’ve checked reads differently when water is flowing through it vs. sitting stagnant in a bowl. After getting one replaced I was told this is not uncommon, and that I should verify with a handheld if I wanted to test it in a reservoir.
2. For those rare people who do actually have REALLY low TDS water and know it stays that way year round, I know one person who runs sediment>carbon>DI and doesn’t actually have a membrane. Given that their tap water is similar to what most of us have coming out of the membrane, this isn’t entirely unreasonable. It does require making sure they stay on top of monitoring and DI replacement.

So I'm learning but I think I can offer clear information on #1.

I trust BRS on this. I don't have the link handy but they have a video explaining that the TDS meter on their RODI units (and presumably RODI units in general) is SPECIFICALLY designed to measure TDS with water flowing thru it. Furthermore, the meter's probes must be properly placed in the flow. The two probes need to each have water flowing straight into them- one probe should not be before the other probe in the stream. (There is only one of two ways the probes can be inserted into the plugs designed for them, and they need to go in the "perpendicular" way, or you also get an incorrect TDS reading.)

BRS is clear: any other use of RODI TDS probes/measurement is going to give you a totally invalid reading. So, sitting in a stagnant bowl will give you inaccurate TDS if you're using your RODI probes and readout for that bowl. For the bowl you need a handheld designed for this type of TDS water sampling.

I don't know how "great" the RODI TDS monitors are, but if you are comparing what they report while in use versus their readings of a bowl of water, that's not a valid comparison, and you wil think they are screwy. You'd need to see (for example) if the RODI TDS reports 0 out while the RODI is making water, and then test the water that has been made with a different type of TDS meter and see if that is also 0, for example. Presumably, for the purposes of aquarists, the readings are hopefully "good enough" and will match.