PDA

View Full Version : Reverse Osmosis Membrane - Any Pros/Cons besides GPD Output?


SanFranTodd
10/07/2008, 03:24 PM
I need to replace my Filmtec RO Membrane for the first time - GPD doesn't really matter to me so I was wondering if there's any unintended benefit (i.e. rejection rate, lifespan, etc.) between the 24GPD, 36GPD, 50GPD, 75GPD, and 100GPD models?

Fig
10/07/2008, 04:11 PM
http://www.pure-aqua.com/brands_dowfilmtec.htm

The 75gpd is the choice among most reefers here. It offers a 98% rejection rate compared to the 100gpd at 90% . This doesn't seem like much but I assure you that your DI resins will last longer. Price for all range in the same area for the most part. Most membranes will last up to 2 years plus depending on your water. A TDS meter will give you a good indication when its spent.

AZDesertRat
10/07/2008, 07:28 PM
It depends on the brand of membrane as well as the GPD rating.
Dow Filmtec rates all their membranes output at 50 psi and 77 degree water temperature. GE Water(Osmonics) and Applied Membranes and others rate theirs at 65 psi and 77 degrees so you don't quite have apples and apples. Companies like Kent use membranes from one of the two latter companies so they are also rated at 65 psi.

The significance of the above is that what GE calls a 100 GPD is basically identical to Dow Filmtecs 75 GPD in every respect. Lay the graphs over each other and you will see a 75 GPD produces 90 GPD at 60 psi and 100 GPD at 65 psi, by the same token a 100 GPD GE membrane produces 90 GPD at 60 psi and 75 GPD at 50 psi, same difference.

Where you run into trouble is with the Dow Filmtec 100 GPD which is the wors tpossible choice, its not even an RO membrane but is a step below and called a Nano Filter. Nano filters are only 90% efficient while RO membranes are 96-98% efficient. The difference is really apparent when you are following RO with a DI system since for every 2% you increase the membranes efficiency you DOUBLE the life of your DI resin. The 6-8% difference can mean hundreds of gallons difference in your DI life.

The 24, 36, 50, 60, 75 and 90(Spectrapure) will all be true RO membranes. I added Spectrapure to the mix here for a reason, they are the only RO/DI manufacturer/vendor that individually hand tests and guarantees their Select Series membranes to be greater than 98% rejection. They offer these membranes in syatems like the MaxCap or you can buy them loose and install them in your system. They ship their membranes wet in a proprietary solution which further improves both the GPD output and the rejection rate or efficiency, again no other vbendor offers this service or written guarantee.

The GE or Applied 100 wil be a true 96-98% rejection rate RO membrane but will not produce the rated GPD unless you have 65 psi which is much higher than the national average pressure of 50-55 psi and also have 77 degree water temperature.

I would stick with the 75 GPD Dow if price is all you are looking at. If quality comes into play the Spectrapure membranes, even their non Select series come wetted and perform better than an off the shelf Dow, or their hand tested Select are the better choice. You will pay a little more but you will soon make the cost difference up in DI replacement savings and then its money in the bank after that.

THE GIMP
10/07/2008, 11:30 PM
Question: what should the TDS reading be after the RO, before it hits the DI?
I've been adjusting the rejection valve but I am not sure if there is a direct relation due to the fact that I replaced all my elements. It was at 98 TDS, and now it is at 68 TDS before the DI.

I don't mean to hi-jack, perhaps all could benefit...

Thankyous.

AZDesertRat
10/08/2008, 07:30 AM
Waste ratio and rejection rate are not related. What you are calling the "rejection valve" is probably the flow restrictor and regulates the amount of flush wate rthat passes over the membrane to keep it free of solids or contaminants. This needs to be kept at 4:1 or 4 gallons, pints, cups, oz. or whatever of waste water to 1 gallon, pint, cup or oz. of product or good water. never adjust it any lower since it will greatly shorten the life of the RO membrane.
TDS is a function of the membranes rejection rate or efficiency. The waste ratio really has nothing to do with TDS other than if you restrict the waste too much, as in 2:1, 3:1 etc instead of 4:1 your membrane will eventually plug and fail. TDS can be affected by water temperature and water pressure though. higher pressures make better, lower TDS water, 50 psi will work but 80 is better and 90 is better yet. 77 degree water is what they all rate the membranes at but colder water actually makes lower TDS treated water. It will cut the GPD production rate due to its viscosity but thats why we have storage containers and make water over time and not spur of them moment.

The TDS from a new 75 GPD Dow membrane should be 96-98% less than the incoming tap water TDS. If your tap water is 200 then your RO only water should be between 4 and 8 TDS. Sounds like you either have extremely high tap water TDS, your membrane is bad or its not seated in the housing all the way.

THE GIMP
10/08/2008, 03:39 PM
Thanks for the reply, A.J. My incoming Tap is 481 TDS so it should be 10 -12 on the out. I'll check the membrane that I replaced no too long ago.