Dual membrane RO questions, anyone done this?

[superman]

I am interested in adding on another 75 gpd RO membrane to my RO/DI system, but first need to know if it is worth it. I get between 45-50 psi water pressure, and I am confused on whether I simply split the water from the carbon filter to the two different RO membranes or if one RO waste line feeds the other RO input. I have seen set ups with it plumbed both ways, looking for any input. Thanks guys!

[drake1]

you can take it off the last set of prefilters and split it but you can't use the waste water or do one after another. back pressure on the ro membrane will give you high tds and destroy the membrane. the membranes work by water migrating through them since there is no pressure on the membrane and only H2o can go through, this is why they have such low gpd. but if you back up pressure on the waster water or you put another membrane after the first one you increase the presure on the first one forcing larger particales thru it and opening the pores on the membrane
jeff

[superman]

interesting points Jeff, I wonder why thefilterguys.biz have a system they sell that feeds a second membrane with the wastewater from the first. Check out the page explainging it http://thefilterguys.biz/water_saver_ro_di.htm

[David Grigor]

I bought all the parts to do this just haven't gotten around to it. Yes, you take the output from the 1st RO membrane to feed the second so they are inline. Not only does waste water go down but you also get even more throughput.

But sounds like you may need the pressure pump. 50psi is the absolute minimum.

[drake1]

you can do it but it will cut the life of the first membrane that the waste water is comming from, unless your using a step pump, sorry i should have added that in my first post or you could use a back pressure waste gate.

[drake1]

funny you should bring this hole thing up since desalinization plants use thousands of RO membranes. Saudi Arabia is in the process of installing a new system that can reuse the waste water back through 2 more membranes and also uses it to make power cutting the cost to desalinize by 30 percent. they are using a simple system of firing the waste water through a small hole in a wheel that has an angle to it which speed up the wheel which makes it a back pressure regulator, i doubt we will see this in the home ro system though. we have looked at installing the new process at work though and it is simple but the patents make it very spendy

[superman]

Have you heard that it is hard on the first membrane as drake has stated?

[David Grigor]

I have never heard of issues with first membrane and don't really see how that one would be at all comprimised as long as pressure is good. The water input of the second membrane is going to be about 20% higher TDS than the first so I would expect it to wear a little more but since using 2 membranes vs. one it will take longer to wear out than the usual 3 years IMO.

I'm not a RO / water treatment guru though and could be wrong. If it were a problem for our purposes I'm sure it would be talked about much more frequently as this dual membrane config have been out and people running it now for quite a while. If in doubt, ask Jim ( Fahz ) he will give you the straight up answer if there is a downside.....

For now, the main stubbling block for you will likely be the investment in the booster pump as it isn't cheap and really need 69psi ( 65 psi plus 4psi loss after carbon ) to function best.

[superman]

I will look into the cost of a booster pump and see if it is worth it to me to spring for. I have been trying to get a hold of Fahz, but have not had much luck.

[FAHZ]

Hi guys I'm here don't get much board time lately. Dual membranes work very well I have been running one since last June on a booster with no issues. Superman you will need a booster as you do not have enough pressure. My system which we also sell, Ocean Reef Miser and dual conversions kits, produces 7.5 gallons per hour with 8.75 gallons of waste which is 1.25 gallons of waste to 1 gallon of product water. Standard waste is 4:1 so water savings is big. So far there is no downside to either membrane and what you are making is a 20" membrane instead of a 10". I also have contact with someone who has been running the same dual membranes for 2 1/2 years with no issues so it is a proven system.

Jim

[superman]

Thank you so much for replying Fahz! I guess that I will be in the market for a booster pump!

[FAHZ]

We have pumps, transformers, switches, and autoflush valves in stock.

Jim