Membrane sizing - increasing the rated gallons per day

[fireant911]

I have a basic question regarding reverse osmosis membrane sizing (i.e., 150 gpd, 200 gpd, 400 gpd…). Is there any disadvantage to using a larger 500 gpd membrane over using a 150 gpd membrane? Is the additional gallon per day for the higher rated membranes rating simply because of increased surface area? Assume that the system is using the Aquatec 8852 pump.

[DRoseum]

The higher gpd ratings mean they can remove more permeate / hr.

The only "downside" is to consider the effect on "recovery rate".

Recovery rate is defined as permeate-outflow / input flow (at operating pressure). Typically you want to keep this number as low as possible to prevent fouling of the membranes. Too low of input flow with high permeate rejection can essentially cause the sugars to stick to the membranes and there isn't enough turbulent flow to scrub them off.

The aquatec 8852 pushes about 15 - 18 gallons per hour at 100psi. A 500 gpd membrane will reject approximately 4.5 - 5 gallons per hour (due to operating temperatures). The rejection rate would be in the 25% - 33% range.

If you do multiple in series, you use a reduced inflow number (by the permeate out removed by upstream membrane). So 18 - 5 =13 (for 2nd membrane) and the recalculate the recovery rate for whatever size the 2nd one is.

Hope this helps.

[fireant911]

D. Roseum,
Excellent! Thanks for the explanation.

[fireant911]

DRoseum,
Regarding the 500 gpd membrane housings that you are using for your 2023 RO build - are those setup for 1/4" or 3/8" fittings? Can it be either? If so, is there any advantage of the smaller 1/4" or larger 3/8" fitting?

[DRoseum]

I used 3/8 everywhere except final concentrate outlet (1/4).

You you can just buy the 3/8 size w9th the same threaded sizecto fit the housing. Reason I used 3/8 is to reduce internal friction losses amd increase flowrate through the tubes. On permeate side you want as little restriction as possible to help create the necessary pressure differential