Pure Water Annie’s FAQ Series.

Pure Water Gazette Technical Wizard Pure Water Annie Answers All the Persistent Questions about Water Treatment.

This week’s topic:  Reverse Osmosis Flow Restrictors.

What’s the purpose of the flow restrictor?

The flow restrictor, as the name suggests, restricts the flow of brine (reject water) in drain line leaving the membrane. It provides resistance, creating pressure against the membrane and  forcing some of the water, the permeate, or product water, through the membrane.  Without the resistance provided by the drain line flow restrictor, all the water entering the membrane housing would simply take the path of least resistance and exit through the drain line.  In short, without the flow restrictor, the reverse osmosis process wouldn’t take place.

Where is the flow restrictor located on my home RO unit?

The most common situation is to insert a tiny restrictor into the 1/8″ threaded fitting where the reject water leaves the membrane housing.  Better units now normally use larger capillary restrictors that are inserted into the drain line itself.  These are easily visible and have the advantage of having the “size” of the restrictor printed on the surface. This is especially valuable, because if you can read the output of the restrictor you can guess the size (output capacity) of the membrane.

In RO units larger than undersink, flow restrictors are often electronically controlled or variable-output hand-controlled needle valves that allow adjustment to suit different treatment challenges.

Tiny fitting-insert-style flow restrictor that inserts into the elbow fitting where drain water leaves the membrane housing.

Capillary-style flow restrictor.

Are flow restrictors all the same output?

No, the fixed-output flow restrictors used in undersink and countertop RO units are sized in accordance with the membrane output.  In other words, a membrane that produces 25 gallons per day product water does not need as much drain water to keep it rinsed as a membrane that produces 50.  The higher the membrane’s permeate output, the looser the flow restrictor. In small units, sizing is usually done so that the restrictor size is about four times the membrane’s permeate rating: a 25 gpd membrane is matched with a 100 gpd flow restrictor.

Are restrictors rated by their GPD (gallons per day) output?

Some are, but most manufacturers use MLM (milliliters per minute).  This leads to confusion.  If the restrictor size is stated in mlm, it can be roughly converted to gpd by multiplying by 0.38.  Thus, a 400 milliliter per minute flow restrictor would flow around 150 gallons of water per day to drain if it ran continuously for 24 hours.

Can I save water by reducing the flow size of the flow restrictor–for example, replacing  my 250 mlm restrictor with a 180 mlm?

Yes, you can, but in most cases it’s a bad idea.  Unless the water has very low TDS and little hardness, you’ll probably get poorer TDS performance, reduced production because of hardness scaling of the membrane,  and reduced membrane life. The water that flows to drain is not “waste.”  It’s an essential part of the RO unit’s operation.  Its function is to rinse the membrane, keep it clean, and to wash the impurities rejected by the membrane down the drain.

Do flow restrictors have to be replaced?

Sometimes–if they fail.  Some manufacturers say that you should replace the restrictor each time you replace the membrane, but most of us let them run until they have a problem.  Usually this is never.

What are symptoms of flow restrictor failure?

Either too much water or not enough water (which can be no water at all) flowing to drain.  If the restrictor stops up and no water goes to drain, the RO unit is in effect constipated and the water quality gets bad, then it stops making water completely.  If the restrictor is too loose, you waste water, and if the problem is bad enough, the unit won’t make enough water and it won’t shut off properly.

How do you know if the flow is too much or too little?

The best way is to pull the drain tube out of its fitting and measure the amount of water that comes out with a measuring cup.  250 mlm means that literally 250 mlm should be coming from the drain.  Catch the water from the drain tube in a measuring cup and see how much comes out in one minute. It won’t be exactly the rated figure, but it should be in the ballpark. Remember that the TDS of the water, the temperature, and the water pressure are variables that make it unlikely that you’ll come up with a perfect reading.

There’s a lot more about flow restrictors on the Pure Water Products website.