Zero-Waste Water Purification

[Rivka]

So are you no longer suggesting a DI system?

[Ray]

A DI system capable of handling the volume of water you'd be likely to need will be very expensive - probably many multiples of the equivalent output RO system, plus a lot more expensive to maintain.

Reef-keepers will put a DI column after an RO system to get the purest water possible, but you likely wont find a DI-only system this small.

[DavidCampen]

I operate the RO system that I built at a reject ratio of 1.5 gallon waste water to 1 gallon of produced RO water (actually 12 gallons per hour of waste water for 8 gallons per hour of RO water) this is a significantly lower proportion of waste than you will get from an off the shelf consumer grade system. But then, I spent much more on my system than you would on an off the shelf system. I use an electric pump to boost my water pressure so that it is at about 90 psi going into the membranes, I use a pair of Filmtec residential grade membranes operated in series and I collect the product water in a low pressure tank from where it is boosted by another electric pump into a higher pressure storage tank; these features are intended to maintain a high rate of flow (velocity) along the face of the membranes to reduce fouling. I also have the system instrumented with flow meters and pressure gauges so that I can detect membrane fouling.
The municipal water that I use as input is about 250 ppm TDS and my produced RO water is about 10 ppm. I have had the system in operation for over a year now and have certainly produced at least a couple thousand gallons of RO water with no sign of membrane fouling.

[Rivka]

wow, would love to have your system!

[Ray]

I recently built a 200 gpd system for myself. It started out at 2.5 gal flush/ gal pure, but I recently modified it and am getting about the same 1.5:1 ratio, relying on line pressure only. PPM in = 210, RO = 10.

The simple fact is that lowering the flush water ratio must increase the fouling rate of the membrane, but David's experience is most encouraging.