RO efficiency options: less waste and more good water...?

redfishsc

redfishsc

New member
I have a standard 75 GPD RO/DI unit (AquaFX Barracuda) that I'm eyeballing to modify for a more efficient (ie, less wastewater) output.

My tap is around 110 TDS, on average.

I know adding a second membrane supposedly reduces waste water. If you run tandem membranes, how is your water output, waste water compared to good water?


Could adding a more strict flow restrictor on the exhaust also work?
 
I would ask some of the filtration professions for help with this issue. Forcing more water through the membrane might increase the TDS of the output and reduce the life of the membrane. What's the waste ratio now?
 
Hard to tell, but I just figured out how to test it (timing how long it takes to fill a glass of water)--- wish I had thought of that before.


If I had to guess, I'd say about 20-30% good water, the rest exhausted.
 
That's about the right range. I'd ask some experts for more help, or perhaps consider using DI resins that can be regenerated, and not using RO, although that's more time-consuming.
 
Remove the restrictor from the reject line, that line feeds the next membrane. Install the restrictor in the reject line of the second housing. The two membrane product lines need to be connected together, then run to your DI housing.
 
In other words, if I add a second membrane, the restrictor should follow after the two of them? Makes sense.

I'll be sure to remember that if I order a second membrane.


How much more efficient would a second membrane make the RO unit, if both membranes are flowing into the same DI canister?
 
The better rejection membranes are typically 100 gpd or larger. The efficiency does not improve withy size and is typically about 5 to 1, ie 20 percent efficiency. Larger RO systems have a large pump at the inlet (no not a little booster pump), say 1/4 hp or larger, that pushes 200 to 250 psi water through the membrane and they are also set up to divert part of the waste water back through to the pump to be added to the new incoming water. Usually about 50% is diverted but up to 80% percent is diverted with low TDS water and water that previously was run through a water softener. I divert 70 percent back through as my water was lime softened at the treatment plant and also runs through a home water softener. Sodium is easy for the RO membrane to remove and it does not normally ever clog membranes. Your tap water is a very low TDS for tap water in general. The large motors and pumps at are not cheap, nor are divertor valves. Your cheapest and most productive option would be to feed your RO system from a large water container not a house hold tap. You would need a pump, but a cheap booster pump would work. The waste discharge of your RO system should be sent directly back to your initial water chamber. Use a simple float valve to keep the container full. Check the TDS of your water and change part or all of the water when the TDS gets alarmingly high. I would just change say half the water once it climbed to over 500 or 600 TDS. I would run Alkalinity and hardness tests on your water. You might just have one of those water sources with mainly silicates and phosphates to remove rather than calcium and calcium carbonates/bicarbonates.

A tandem run could be set up but with standard household pressures your outlet pressure from your first RO filter would be to low to feed a second membrane therefore requiring a booster pump. You would not need prefilters on the second RODI syatem and other than your costs your savings would only be reduced by the cost of the pump rather than water costs. The first suggestion would be much cheaper and more efficient but it would require you occasionally checked to TDS of the water in your feed container.

The first option would likely lower your output water costs by about 70 to 75 percent. The second option would cut output water costs by about 25 or 30 percent. The second option would produce the water twice as fast though.

DOW and Applied membranes is probably the only manafacturers of 75 gpd RO membranes with 98% rejection rates at 20% efficiency. Applied Membranes is a supplier of Dow membranes.

http://www.appliedmembranes.com/

Just call or email them, tell them what you have and what you want for an output. Your set up could easily be upgraded to a 100 gpd membrane just by changing you waste water fitting/insert and the membrane.
 
You dont need to buy another restrictor, just relocate the one you have to the new membrane reject line. Dont use a smaller restrictor either, you may over flux the membranes and shorten the life of both the membranes (potential fouling or scaling) and DI resin due to lowered permeate quality.
 
therealfatman has some good points. I am going to set up a multistage RO for myself employing a feed tank as he mentioned. I operate an industrial RO that utilizes a similar system, but I cant elaborate in any great detail due to the proprietary nature of the system and all that legal mumbo jumbo.

But to the original question, you will produce more water with less waste by installing another membrane as mentioned above.

Edit:
If you go feed tank route, keep hardness <500 ppm and SiO2 <200 ppm
 
Careful with this issue guys. If you add a second membrane, plumbed in series, then what you have in effect is a single longer membrane with double the capacity to produce permeate (purified water). BUT - the old 4:1 rule of thumb still applies.

You CAN over-restrict any membrane (or series of membranes)and lower the amount of waste water produced. BUT, you will reduce the useful life of the membrane in most situations.

Russ
 
Realfatman, what a great idea! Unfortunately I'd have to use a 5 gallon bucket for the pump container/reservoir since I live in a pretty small apartment. However, I'll save that idea if we move out to a larger area.



Russ, thanks for the input---- I will keep that in mind as I consider my options (which may end up being, just stick with what I have)
 
<a href=showthread.php?s=&postid=14941621#post14941621 target=_blank>Originally posted</a> by BuckeyeFS
Careful with this issue guys. If you add a second membrane, plumbed in series, then what you have in effect is a single longer membrane with double the capacity to produce permeate (purified water). BUT - the old 4:1 rule of thumb still applies.

You CAN over-restrict any membrane (or series of membranes)and lower the amount of waste water produced. BUT, you will reduce the useful life of the membrane in most situations.

Russ
Click to expand...

True the old 4:1 waste applies but as the 4 parts waste water is now free (no cost water) it is merely the wattage cost of boosting that water up to a usable pressure that is figured into the water cost. Therefore we have 4:1 of free water and the water cost is the primary cost in RO water production in a home where the water is delivered already pressurized. I would not recomend using RO waste water for a secondary membrane directly from a primary membrane without boosting the pressure back to a usable range nor would I recomend the use of just one flow restrictor on a series membrane set up. I will correct myself by saying if for some reason you should have a household water system with extremely high water pressure where the waste water produced by t your first membrane was in excess of 50 to 60 psi I would use that water without a booster pump.
 
fatman, I think you are referring to your suggestion above of running the RO/DI from a vat of water utilizing a pressure pump and exhausting the waste water back into the vat (thus producing some wickedly high TDS water near the end if you don't intervene)---- correct?

I don't think Russ was referring to your comment, then. I think he was referring to someone else's, perhaps my idea of running two membranes alongside each other.


Thanks for all the info though! I still love the vat/recycle idea. I am staring at an empty 25g I may use for that.
 
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