RO vs RO/DI

[Mr31415]

What are the benefits of using a DI stage in addition to the normal RO unit? My TDS of my tap water ranges between 170ppm and 280ppm. The RO unit brings it down to < 20ppm. I have recently added a sediment chamber in the output line of the RO phase, which seems to lower the TDS to about 3-4ppm.

Is this good enough to not cause unwanted algae growth due to phosphates leaching in to the system? Or *must* one have a DI stage as well?

 

[boxfishpooalot]

the di portion is a safegaurd to be sure that all the water is pure. Ro does not remove everything by itself. Thats why we need di.

 

[AZDesertRat]

First off I would say if you are getting in the neighborhood of 20 out of your RO it is in need of replacement. With a tap water TDS oif 170 to 280 you should be seeing 3 to 6 TDS from the RO not 20.
Why would you put a filter after the membrane that is rated at maybe 10 microns when a membrane should remove anything larger than 0.001 microns? Prefilters and carbons are used before a membrane to filter the larger stuff out so it does not plug the membrane.
DI is a final polishing filter that is almost a must, it will get the phosphates, silicates and nitrates the membrane misses.

 

[drummereef]

RO doesn't get rid of everything. DI is the last stage of totally pure water. My tds out of the DI is 0. That's what you should aim for.

 

[Mr31415]

the sediment filter is 1 microns according to the web site:

http://www.thewaterboy.co.za/marineRO_main.php (Reefmaster Plus)

I was told that the RO unit is in good working order if you get at least 10x less TDS than what you put in. Since mine takes it down from 280 to about 18-20, it is more than 20x less.

I'll try and find a DI stage. Thanks for the info.

 

[AZDesertRat]

Depending on the RO membrane you should be getting a 90 to 98% reduction. If you have a 75 GPD or less RO membrane it is probably a 98% rejection rate so you should only see between 3 to 6 TDS. Sediment filters should Always be placed before the RO membrane, it does no good after the membrane as the micron size is too large.
If it has a Dow Filmtec 100 GPD membrane which I hope is not the case, you will see only a 90% reduction so 17 to 28 TDS would be about right. If it is new and you can return the 100 GPD if that what it is I would highly encourage you to do so as it will extend the life of DI resin by a factor of 5x.
3 to 6 TDS going in to a vertical refillable 24oz DI filter will make the resin last somewhere between 600 and 1000 gallons per cartridge but a TDS of 17 to 28 will only get you 100 to 175 gallons of treated water.

 

[Mr31415]

AZDesertRat, I am using the Reefmaster Pro from here http://www.thewaterboy.co.za/marineRO_main.php. Maybe I got my terminology wrong - the cylinder I added is that clear cylinder depicted at the top of the unit... Not sure what is in it. Would that work adequately?

 

[auntdeb]

Just for reference, we have tap water that reads 550-650 and after RO/DI we are almost always at 0. When the filters start getting old, then we start to see readings from 1 to about 5 or 6.

 

[Mr31415]

Mine is 3 months old...

 

[AZDesertRat]

It appears the clear unit on top is a horizontal DI chamber. If you are getting a TDS of 20 out of the RO only without the DI your DI will have a pretty short life. I am guessing that DI holds about 16 oz of resin so you will probably need to change resin about every 100 gallons or so. It would be a good idea to contact the manufacturer/vendor and find out what membrane you have an what is efficiency should be, I am sure it should be better than what you are getting. As an aside, a 2% increase in RO efficiency doubles DI resin life so if you can get the RO TDS down to about 5 or 6 TDS where it should be that same DI resin replacement will then last about 350 gallons.

 

[Mr31415]

Thanks!