RO water during flush

[albertski]

I have the 5 Stage Premium Plus 150 GPD Water Saver RO/DI System. To flush my RODI unit I turn on the house and have my flush valve set to flow. During this process, I get some water come out of my clean water hose. Is this water considered to be good or should I put my blue hose in the waste bucket during flushing?

 

[gbroadbridge]

I have the 5 Stage Premium Plus 150 GPD Water Saver RO/DI System. To flush my RODI unit I turn on the house and have my flush valve set to flow. During this process, I get some water come out of my clean water hose. Is this water considered to be good or should I put my blue hose in the waste bucket during flushing?

I believe that as it is on the clean side of the membrane and after the DI it is clean.

 

[Shirak]

The first pint or two out of the good RO line is going to be high in TDS if the RODI unit has been sitting for a day or more, due to what's called TDS creep. Ideally you should have a valve between the RO and DI filter to allow the first quart of RO water to be collected and discarded. You can run it through the DI however it will use up your DI resin much faster. I usually collect a half gallon or a gallon of just plain RO then switch the valve to run through the DI for the aquarium use. That RO only water is really good for houseplants and coffee machines :)

So if you are talking about water that has gone through the DI while flushing the RO membrane then yes it is fine for the aquarium. BTW you shouldn't need to flush the system every time you use it. Especially if your prefilters are clean and your source water is decent.

 

[flyfisher2]

I installed 2 shut off valves between the membrane output and the DI canister. Water flows to drain after I flush for a couple of minutes then I close the valve and run the water to the DI. Saves on DI and allows you to collect drinking water post membrane.

 

[flyfisher2]

I installed 2 shut off valves between the membrane output and the DI canister. Water flows to drain after I flush for a couple of minutes then I close the valve and run the water to the DI. Saves on DI and allows you to collect drinking water post membrane.

 

[PeterC99]

The first pint or two out of the good RO line is going to be high in TDS if the RODI unit has been sitting for a day or more, due to what's called TDS creep. Ideally you should have a valve between the RO and DI filter to allow the first quart of RO water to be collected and discarded. You can run it through the DI however it will use up your DI resin much faster. I usually collect a half gallon or a gallon of just plain RO then switch the valve to run through the DI for the aquarium use. That RO only water is really good for houseplants and coffee machines :)

So if you are talking about water that has gone through the DI while flushing the RO membrane then yes it is fine for the aquarium. BTW you shouldn't need to flush the system every time you use it. Especially if your prefilters are clean and your source water is decent.

Curious why you say you don’t need to flush RO membrane every time you make RODI water? All manufacturers say you will get extended RO membrane life with the pre and post flush.

Thanks!

 

[LeftyReefer]

I flush my RO/DI before and after each use. I installed a 3 way valve after my RO membrane so that I can divert high TDS water away from the DI during each flush. I wait until the TDS meter coming out of the RO shows 1 or 2, before I shut the flush valve and and my DI bypass valve and start sending the water to the DI canister again.

Without the DI bypass valve (between the RO and DI stages) you are sending all that high TDS water through your DI resin each time.

BRS has the DI bypass valves for $20. Or they are a few bucks more at McMaster Carr.

 

[Shirak]

Curious why you say you don’t need to flush RO membrane every time you make RODI water? All manufacturers say you will get extended RO membrane life with the pre and post flush.

Thanks!

Just from personal experience using RO membranes for the past 30 years. I have never noticed any difference in longevity or output with clean source water. A properly sized restrictor will flush all the stuff that is blocked by the RO during it's operation. I replace the membrane when TDS is higher than I want and few points over a new filter or output has fallen, so maybe every 4 or 5 years. That's with flushing .. or when I flush once a month. I did have a situation using well water with silt. Even with 1micron prefiltering some got through it was so fine. I would flush every week or so and the initial flush water would look like mud for a few seconds.

If it makes you happy to flush then a few second flush at the end of each use won't hurt anything, just a waste of water IMO. As noted above since we are using DI post RO, a valve to divert the RO for the first quart or two will save a lot of DI resin since that first bit of RO water is high in TDS and flushing doesn't change that since it is on the 'good' side of the membrane and flushing is on the waste side.

 

[PeterC99]

Just from personal experience using RO membranes for the past 30 years. I have never noticed any difference in longevity or output with clean source water. A properly sized restrictor will flush all the stuff that is blocked by the RO during it's operation. I replace the membrane when TDS is higher than I want and few points over a new filter or output has fallen, so maybe every 4 or 5 years. That's with flushing .. or when I flush once a month. I did have a situation using well water with silt. Even with 1micron prefiltering some got through it was so fine. I would flush every week or so and the initial flush water would look like mud for a few seconds.

If it makes you happy to flush then a few second flush at the end of each use won't hurt anything, just a waste of water IMO. As noted above since we are using DI post RO, a valve to divert the RO for the first quart or two will save a lot of DI resin since that first bit of RO water is high in TDS and flushing doesn't change that since it is on the 'good' side of the membrane and flushing is on the waste side.

Really appreciate the response! I also have good quality input water and am diligent about changing sediment filter. Going on 2 years with RO membrane and still get 0 TDS output. Was thinking about changing RO membrane but now will keep monitoring TDS and wait till it goes above 0 before changing.

 

[LeftyReefer]

A proper DI bypass valve is more important than a flush valve if you ask me....
Really, you need both, but having a flush valve without a DI bypass valve is kind of pointless. And yet, most RO/DI systems come with only a flush valve installed, and are missing the more important (in my mind) DI bypass valve.
Not sure why that is.

 

[KStatefan]

Just from personal experience using RO membranes for the past 30 years. I have never noticed any difference in longevity or output with clean source water. A properly sized restrictor will flush all the stuff that is blocked by the RO during it's operation. I replace the membrane when TDS is higher than I want and few points over a new filter or output has fallen, so maybe every 4 or 5 years. That's with flushing .. or when I flush once a month. I did have a situation using well water with silt. Even with 1micron prefiltering some got through it was so fine. I would flush every week or so and the initial flush water would look like mud for a few seconds.

If it makes you happy to flush then a few second flush at the end of each use won't hurt anything, just a waste of water IMO. As noted above since we are using DI post RO, a valve to divert the RO for the first quart or two will save a lot of DI resin since that first bit of RO water is high in TDS and flushing doesn't change that since it is on the 'good' side of the membrane and flushing is on the waste side.

This. The flushing that the manufacture (Dow) recommends is to prevent scaling of the membrane but that is for high tds hard water where it is possible for the hardness reach the insoluble point at spots on the membrane. I doubt many if any home water systems are that high. It does not remove TDS creep.

My opinion that at some point one of the companies selling RODI units added a flush valve to make there product look better and then other companies followed.

https://www.dupont.com/content/dam/...olutions/public/documents/en/45-D01504-en.pdf

 

[flyfisher2]

A proper DI bypass valve is more important than a flush valve if you ask me....
Really, you need both, but having a flush valve without a DI bypass valve is kind of pointless. And yet, most RO/DI systems come with only a flush valve installed, and are missing the more important (in my mind) DI bypass valve.
Not sure why that is.

I totally agree. Without the DI bypass the water coming out after flushing the membrane goes straight to DI and depletes it that much quicker then you need to buy DI cartridge or resin.... But we all know they wouldn't want us to have to buy anything prematurely.

 

[KStatefan]

Really appreciate the response! I also have good quality input water and am diligent about changing sediment filter. Going on 2 years with RO membrane and still get 0 TDS output. Was thinking about changing RO membrane but now will keep monitoring TDS and wait till it goes above 0 before changing.

If you have low TDS water and you keep the carbon filters changed as needed your membrane will last a longtime. I would just monitor the rejection rate of the membrane and when it goes down replace it.

 

[gbroadbridge]

A proper DI bypass valve is more important than a flush valve if you ask me....
Really, you need both, but having a flush valve without a DI bypass valve is kind of pointless. And yet, most RO/DI systems come with only a flush valve installed, and are missing the more important (in my mind) DI bypass valve.
Not sure why that is.

Mixed DI resin costs what? $5 per lb from a filter supplier?

Hardly seems worth the trouble of installing and remembering to use a bypass valve for the tiny amount of leakage on starting the filter..

 

[LeftyReefer]

Mixed DI resin costs what? $5 per lb from a filter supplier?

Hardly seems worth the trouble of installing and remembering to use a bypass valve for the tiny amount of leakage on starting the filter..

Everytime I turn on my RODI the TDS coming out of the RO is 40-50 due to tds creep. Basically it's about half the TDS of my tap. Running that 50 tds water through my DI rather than 2 TDS water will exhaust my DI resin 25 times faster.

I only run My RODI once or twice per week, so TDS creep is an issue for me. Maybe not if you run your RODI every day.

 

[gbroadbridge]

Everytime I turn on my RODI the TDS coming out of the RO is 40-50 due to tds creep. Basically it's about half the TDS of my tap. Running that 50 tds water through my DI rather than 2 TDS water will exhaust my DI resin 25 times faster.

I only run My RODI once or twice per week, so TDS creep is an issue for me. Maybe not if you run your RODI every day.

If you continually ran the 50 tds water through your resin, I'd agree that it would exhaust resin 25 times faster.

But in reality you only run a few ounces of the high TDS water through before the tds drops to almost zero, so the difference in resin life is insignificant.

 

[Shirak]

If you continually ran the 50 tds water through your resin, I'd agree that it would exhaust resin 25 times faster.

But in reality you only run a few ounces of the high TDS water through before the tds drops to almost zero, so the difference in resin life is insignificant.

Have you measured how much water? It takes mine about 32 ounces to go from around 100 tds to 3 or 4. Eventually it will get down to about 2ppm. True it's not all at 100ppm but it's not an insignificant amount either, at least for me. Years ago I ran tap water through mixed DI because I had a tiny aquarium and needed very little water and didn't have a big RODI system. Tap water around 200ppm would burn up one DI canister after about 40-50 gallons. So not a whole lot of water vs RO at 2 which will make hundreds and hundreds of gallons for me these days before I need to swap DI canisters.. PLUS I really don't like messing with packing the DI canister. Stupid beads go everywhere! So no not a huge huge expense but saves on a PITA job for me LOL