75 and 150 gpd RO membranes used in a series

[bozoreefer]

Hi!

I'll get straight to the point.

My TDS is 500 from the wall.

PSI: 80

1: three 1 micron sed filters
2: two 0.5 micron carbon filters.
3: 75 gpd RO membrane
4. 150 gpd RO membrane (fed by 75 gpd waste line)(550 ml restrictor)
5. Both clean water lines joined together
6. DI

Questions:

1. What does my PSI have to be at to run the 75 + 150 membranes in a series?

2. What would my rejection rate be at with that achieved PSI?

3. Will this be beneficial for me at all if I'm using it for both my drinking water system and for DI? I only have 2 people using this RO unit for drinking water with a 5 gallon RO tank and a 25 gallon reef tank lol.

Thanks!

 

[LQT]

Running in series means that you only use one flow restrictor on the waste line of the second membrane. I’m not sure that running two different rated membranes would work at optimal efficiency as different membranes require different rated flow restrictors. Also, at a high TDS reading of 500, running in series would trash the second membrane pretty quickly as well as eat through a lot of DI resin.

Check out around the 8:30 mark in this BRS video discussing running dual membranes where they warn about running RO membranes in series in high TDS situations.



Perhaps running just the 150 gpd membrane with the proper flow restrictor would be the most efficient solution in your high TDS situation.

 

[mann1139]

150 GPD sounds like overkill for you.

Heck, the 75GPD is probably overkill.

Assuming 95% rejection to be cautious, 500 * (1 - .95) = 25 out of the first RO membrane.

Assuming a 4:1 ratio, you would be producing 1 gallon of 25, and 4 gallons of (100 / 95 * 500) = 526 water).

Assuming the same conditions again, the second membrane would produce .8 gallon of 26, and 3.2 gallons we don't care about.

So basically you would produce ((1 + .8) / 5) = 36% output at ~25 TDS.