Adding Second RO membrane to Unit to reduce Waste water

[bmwaaron]

Just like the title says I'm considering adding this kit to my RO unit.
I am going to either get a second 75 GPD membrane and have the Waste water line from the first feed the second or Just buy a 150 GPD membrane. I just wanted to know if it is really more efficient to have the wastewater from the first feed the second like one of the retailers is selling?
This would seem to cut down on waste water if it works which is my Question does this work?
Also I will add that I am getting a booster pump either way so I will have plenty of pressure.

 

[tufacody]

I think it depends on what your TDS of the source water is. If it is already high, I doubt your second membrane will get the waste water to zero.

 

[One Dumm Hikk]

Yes, the water saver setups do work. The key to them is the pressure and with a booster pump you will have plenty of pressure. Personally, I would go to the 150gpd membrane first. Make sure you change the flow restrictor at the same time though. That means fewer membranes to change each time they get changed. Then if you need more water production later on, you can go to dual 150gpd membranes.

 

[bmwaaron]

So you dont think that the water it saves doing it with 2 membranes vs 1 is worth it?
I wonder if the lifetime of the membrane (2-3 years) it could save the $40 in water that the extra membrane would cost?
Does anyone have any experience with this?

 

[yardboy]

Generally all industrial setups use a 2-pass system, some even more. They look at "recovery rate" and expect >75%, so <25% is waste. You will definitely save water with a first and second stage 75gpd RO over a single 150gpd membrane. Just remember that you should figure costs on how often you need to replace the membranes.
By the way, in an industrial setup, using the exact membranes (well, same type, just bigger) we get 10 years out of them, but then we use antiscalents injected into the feedwater (we have high calcium) and we clean them periodically.
Did you know it wouldn't hurt to clean your membranes? Just remove them from the housing and soak in vinegar overnight. Reinstall, and dump the output to waste until the vinegar is flushed (tds stabilizes).

 

[Buckeye Hydro]

Remember that when you plumb two membranes in series, they function as a single membrane. You should have 4 parts waste to every 1 part permeate - if you have less it will likely shorten the life of the membrane.

Can you install a flow restrictor that gives you less than a 4:1? Sure. And you don't need two membranes to do that. But again, be prepared for scale buildup on the membrane faster than would otherwsie be the case.

Russ

 

[cmm1970]

Aaron,
I am running dual 75gpd membranes with a booter pump and I am pleased with the production rate.

In deciding between 75 vs 150gpd membranes, look at the rejection rate of the membranes. I believe the rejection rate is better with the 75s.

Here is my setup:

Mike Meek
Austin

 

[cmm1970]

Well poop,
I took this photo before adding the second membrane. So much for showing off!

 

[yardboy]

<a href=showthread.php?s=&postid=15071733#post15071733 target=_blank>Originally posted</a> by BuckeyeFS
Remember that when you plumb two membranes in series, they function as a single membrane. You should have 4 parts waste to every 1 part permeate - if you have less it will likely shorten the life of the membrane.

Can you install a flow restrictor that gives you less than a 4:1? Sure. And you don't need two membranes to do that. But again, be prepared for scale buildup on the membrane faster than would otherwsie be the case.

Russ

In parallel, they would work as one membrane, then two 75gpd's would not do anything any different than one 150gpd.
In series, the first membrane would have no "waste", as the waste line would run through the second RO membrane. It would still produce a 4:1 ratio of waste to permeate, but adding the first membrane to the mix of permeate would give you a 4:2 ratio, now 50% recovery, instead of 25%. Yes, the second membrane would scale somewhat faster as it's seeing feedwater with a 20% higher load of contaminents, but not at the same level as trying to decrease the waste on a single membrane to increase permeate. And asmentioned above, scale can be cleaned from a membrane if not allowed to be completely clogged frist.