Reverse Osmosis Pressure Drop

[Benito]

For the last 2 years I've been periodically measuring the psi of my RO system to determine when it is time to change filters.

This is what I'm seeing. The pre filters were changed 1 year ago. Water quality is still ok.

1 year ago 6 months ago Now

Incoming Pressure 80 80 80
Pressure after pre filters 80 80 80
Pressure after RO membrane 50 43 41

What do you think is going on?

 

[WANG]

The purpose of the prefilter is to protect the RO membranes and reduce the frequency of maintenance on the membranes. They will still require attention at some point. Does your system provide a means of servicing the mebranes in place, or do you have to replace them?

 

[Benito]

Yes everything is serviceable and I have done so regularly. I started measuring psi because that's a better indication than just replacing filters every x months.

The pre filters I have are rated for very long life and I have soft water coming into them. They are 5 micron combo sediment and carbon, followed by a 1 micron sediment, followed by a 0.5 micron carbon.

My question is more to do with the psi I am getting after the membrane.

Is the membrane getting fouled? Is there a point where the membrane pressure differential indicates replacement?

 

[WANG]

Yes, the membrane will eventually be fouled. With such a great prefilter rig, and high quality raw water like you have it will take a long time but it will happen.
Regarding my serviceability question, is there a manufacturer prescribed procedure for flushing the membranes with Tergajet or similar for your system?

 

[Benito]

I haven't heard of any procedure. I changed the membrane years ago when I inherited the unit and don't have any instructions at all for the system.

About a year ago, in an effort to save water, I significantly reduced the waste restrictor. It's now a 100ml/m restrictor on a 24gpd membrane. I figured that my pre filters would allow that plus the infrequent use of the system means it is constantly topping up the tank which uses more water since the tank pressure is higher and needs to be overcome.

But I have probably accelerated the wear on the membrane so was wondering if the psi readings would tell me anything.

Thanks for your help!

 

[WANG]

I would expect the spec for maintenance to reference pressure at reject control valve with respect to volume flow rate of product water.

Out of curiosity, what happens to the reject water in your system?

 

[Benito]

Goes straight to drain unfortunately.

 

[WANG]

Oh man, that's unfortunate. It would be one thing if you were purifying ocean water or some other foul raw water source, but with a presumable potable water source it's a bummer that it goes to waste. Then again, it probably isn't a huge amount of reject water considering the pre RO water quality and the rate of production being measured in gallons per day(!) (Guessing that was GPD).

Very nice to have an excellent system like that for residential application!

 

[Benito]

I have it on my to do list to reroute the drain line to flow into the sink - maybe to another water dispenser locked into the open position. At least we then have the option to collect the water and if not it will simply run down the sink hole.

I suspect we are "wasting" about 1500 to 2000 gallons a year at the moment.

The one downside of our setup is the soft water. So I added a calcite filter after the membrane which adds just enough hardness to make tea taste perfect and it also gives some taste to the water.