Buckeye Hydro

We'll, we're a little biased in this regard. If we pointed you towards the vendor and systems we prefer, it would undoubtedly be considered a commercial post and I suspect the forum gods wouldn't like us advertising our business or products here!

Russ

What is it that you are filtering?

I know this is an old thread, but the original post includes a timeless question that we typically answer for customers at least a couple times a week. If you check the specs on your RO membrane, you'll see something refered to as the "rejection rate." In many cases, the rejection rate will be something like 98 or 99%. This means that the membrane will "reject" (not allow to pass through to the purified water side) 98% or 99% of the TDS in the feedwater.

If you do some experimenting with your TDS meter, you'll note that your sediment filter and carbon filter do very little to remove dissolved solids. So with your tap water at 400 ppm, for example, you can measure the water at the “in” port on your RO membrane housing and you'll see it is still approximately 400 ppm.

The RO membrane is really the workhorse of the system. It removes most of the TDS, some membranes to a greater extent than others. For instance, 75 gpd Filmtec membrane has a rejection rate of 97 to 99% (i.e., they reject 99% of the dissolved solids in the feed water). So the purified water coming from your 75 gpd membrane would be about 8 ppm (a 98% reduction). To test the membrane, measure the TDS in the water coming into the membrane, and in the purified water (permeate) produced by the membrane. Compare that to the membrane’s advertised rejection rate, and to the same reading you recorded when the membrane was new. Membranes also commonly produce purified water more slowly as their function declines.

So if someone has feedwater of 50 ppm, and you have feedwater of 650 ppm, you'll see there is not a specific number everyone can shoot for - it all depends on the tds of your feedwater and the specs your membrane.

Russ

You could easily do much better (in several regards) than that RO system.

Russ

GE made a great small RO system called the Merlin, it has since been replaced by the Evolution model (I have not used an evolution) retail is US$ 800.00; it is supposed to do 700-1500 US gallons (3.785 liter = gallon) per day depending on your water conditions. They are tankless, on-demand and require no pump or external power other than water pressure. The new model needs 60 psi inlet pressure which I think is higher than the older Merlin version.

Be careful however in that the Evolution - which is just a name given to a system by a vender, includes a next to useless carbon prefilter. If you use the system much at all, you'll very quickly burn through the capacity of the carbon block, and you'll expose the membrane to chlorine. We just replaced one of these with a system that uses the identical membranes, but we installed a backwashing carbon tank ahead of it to handle the chlorine.

Russ

If your water is hard, make sure you get a softener before the RO. Will save the filters and extend the life of the system by a lot. Good RO filters are pricey, water softener salt is cheap.

In general I think this is good advice, but it may be overkill if we're talking about a residential scale RO system (<150 gpd). Membranes for these systems are pretty reasonably priced. When yu jump up to commercial RO units, the membranes get pricier, and the cost/benefit of using a softener starts to look better.

Russ