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Old 01/11/2008, 02:17 AM
jdieck jdieck is offline
Flea Slide
Join Date: Dec 2002
Location: Naperville IL
Posts: 12,458
Re: Have 2 RO/DI units, can I run Waste water from one to the other?

Originally posted by raynist
I have 2 ro/di units and was wondering if I can take the waste water from one and run it to the other unit to make more water with less waste?

The first unit is a 4 stage unit with a prefilter/carbon block/RO/DI. The membrane is a 150gpd filmtec.

The second unit is a 5 stage with a 75gpd filmtec membrane. If this could be done, I would think I would not need the prefilter or the carbon block since this is already done on the first unit.

My water pressure is sky high if that matters at all.

Let me know your thoughts.

Yes and no

To get any advantage you need to use two membranes of the same capacity.
If you use diferent capacity, then to be able to keep the membranes operating within their specification 4:1 to 5:1 you will need to use one restrictor for each membrane sized for their individual capacity, this means the waste of the first membrane will be at low pressure (after being restricted) Thus you would need to boost pump it higher to opearate as an input to the second membrane.
At the end you will get the same volume and waste as if you operate both systems independently but at the same time.

On the other hand if the membranes are of the same capacity and specifications (both filmtec, both same production rating, both same rejection) then they can share a single restrictor (sized for a single membrane) and the first membrane will not require one (See diagram below), In other words two 75 gpd membranes can share a single restrictor sized for a membrane of 75 gpd.

The advantage of this operation is that:
The life of the membranes is not afected, as a matter of fact the life of the first membrane is improved as it will operate at or close to a 5:1 ratio while the second will operate at the 4:1 ratio.
In this configuration the waste of the first membrane has almost the same input pressure (is only 2 to 4 psi lower than the rated 70 psi inlet)

The diagram below shows aproximate volume and purity balance, in summary for every gallon produced you will waste about half than before (2 gallons instead of four), you will be using 60% of the sediment and carbon filters that you used before because you need to filter less waste but you will be consuming 10% more DI resin per gallon than before as the delivery purity from the combined membranes will degrade by about 10% (In the example below purity goes from 6 ppm to 6.6 ppm)

Note that the cost savings in metered consumption lowering the wasted volume will not pay back for the cost of a second membrane but the reduction in the consumption of sediment and carbon filters will make it a wash over the life of the membrane.

I have been using the shown configuration with 75 gpd filmtec high rejection membranes for 4 years with tap water that fluctuates within 120 to 140 ppm at a pressure of 70 psi at the membrane inlet and I still get more than 99% rejection from both membranes combined and get depending on the temperature between 140 and 180 gpd.

Did I write what I wrote? What the heck am I talking about! Well..... Nevermind.