RODI units

[Turf]

any input on this; I've got a 100gpd rodi unit... what would the ratio of good vs waste water be (ball part)

[Turf]

also is it better to have cold or hot water?

[hypertech]

cold - hot will ruin your membrane

[kpete4]

I had heard room temperature but I have no idea why

[mtfatwork]

Room temp is best. Usually you get better production when the water is not ice cold.

[Turf]

Good to know and thanks for the input... I guess I'll just use my new spiffy TDS meter to figure out the good to bad ratio.

[David Grigor]

In general, it's about 4 to 1. Lower the water temp the less RO production so winter time will be even higher ration. By running multiple membranes inline you can reduce the ratio closer to 1 to 1.

Rejection rate is really the most important stat. You should strive for is 98%+.

For example if you incoming TDS is 250ppm and rejection rate is 98% then TDS out of membrane would be around 5. At 97% it is 7.5. and 96% is 10 so every % change in rejection can mean a significant increase in your DI use thus the overall costs to run. Using best rejection rate membrane possible is pretty important. 100gpd membranes are notorious for low rejection some as low as 90% which will have a pretty significant impact on your DI usage to maintain a 0 tds output from your filtration. So be careful on what brands and quality of membranes that come with prepackaged RO/DI units and replacement membranes. Low rejection can easily negate any initial savings you might get from lower end units such as ebay/water general systems for example.

[Turf]

so in short on the membranes your saying a 50gpd is better then a 100gpd?

[David Grigor]

No, whatever membrane has the best rejection rate. Nothing to do with gpd rating. A number of 100gpd are bad performers, yet there are good ones too. Just know what your getting. Some of those cheaper units don't actually mention what membrane brand they use in their units which can be an alarm bell because it if where a good one they would definately use it as a selling point. When you look at it from DI consumption standpoint it can be significant the difference between 90% and 98% rejection rate can be looked at in terms of requiring 5X the amount of DI to maintain 0tds. In general though, filtmec 75gpd are a pretty proven winner as far as performance and price outside of using handtested membranes that exceed the manufacturer ratings which usually are more expensive ( the spectapure max cap for example ).