[BuckeyeFS]

I just got a 4-stage 50 gpd RO/DI unit. Can I put a 75 gpd RO membrane in it when I replace it, and then it is suddenly a 75 gpd unit? Or is there more to that than just the membrane?

Assuming your system accepts stand-sized membranes, you can add a higher (or lower) capacity membrane very easily. Pull the old one out, slip a new one in, and flush the new membrane appropriately. You'll also need to replace your flow restrictor. Flow restrictors are matched to the capacity of the membrane, and serve to partially plug the waste line that exits the RO housing. By partially blocking flow in the waste line, they serve to pressurize the RO housing. It’s this pressure that allows the reverse osmosis process to work. So - all systems have flow restrictors.

There are three common types of flow restrictors. The plug-type restrictors are just that - a small plastic cap, or plug with a very small hole that is inserted in the waste line of the RO (sometimes these come inside the fitting at the waste port of the RO). To find this type of restrictor, remove the waste line from the waste port on the RO housing, and look INSIDE the tubing. Remove the restrictor and replace it with a restrictor sized appropriately for your new membrane.

Some systems use a capillary flow restrictor. These restrictors look similar to the plug type, except they have a long, thin (several inches to a foot or so) capillary tube extending from the cap. Again - this type of flow restrictor can be found INSIDE the waste tubing.

A third type, sometimes called block or tube restrictors, look like a short length of 1/2 –inch pvc tubing with a quick connect fitting on each end. Some of these restrictors have an integrated flush valve.

I have read that RO membrane that are over 75 gpd are less effective. Is that true?

Filmtec 50 gpd = 98% rejection rate
Filmtec 75 gpd = 98%
Filmtec 100 gpd = 90%
BFS 150 gpd = 98%


I have a psi meter right on the unit. It reads 45 psi when I turn my tap on full blast (just cold). When that drops I should replace my sediment filter, right? How much can it drop before I should replace it?

Your pressure, even with clean filters, is low. So I wouldn't think you'd want it to go any lower than 40 psi.

My understanding is that the carbon block is usually changed at the same time as the sediment filter. I understand that the purpose of the carbon block is to remove chlorine as chlorine will damage the RO membrane. Is that right?
In general, yes.

So in order to know when it is time to replace the carbon block, I assume it may be wise to purchase a chlorine test kit and test the water coming out of the carbon block. Good idea?
That would be an unusual approach. More standard approach would be to replace the block BEFORE you get chlorine break through - carbon blocks come with a specified chlorine capacity - change the clock when you get to about 75% of the chlorine capacity or a 6 months, whichever comes first.

How often and for how long do I use my flush kit? I haven't found a definitive answer for this. If you were in a lab setting you would use it ever time you shut the system down. That's not practical for most reefers, so once a week is better than once a month is better than once a year.

I really don't like all that waste water going down the drain. Considering the chlorine has been removed, is the waste water good for drinking water and for use as waterchanges on my freshwater tank? Potentially - remember that the tds in the waste water will be about 20% higher than in your tap water. If you have low tds to start with, it may be fine to drink. If you have high tds to start with, you may not like the flavor. You can use it for laundry, watering the plants, etc... You'll have to specifically look at the quality of the waste water to know if it is ok for your fresh water tanks.

Feel free to call if you have questions... my fingers are cramping!

Russ