RO membrane issue

[Silverwhisp]

Over the last few weeks, I’ve noticed that our RO system output is falling. I’ve got a 3-stage system and an in-line 5-micron sediment cartridge. The membrane is a Pentaire GRO 50 water saving membrane (1:1 drain to permeate ratio.). The membrane is less than 9 months old.

We tested the system yesterday, and water gushed out the upper components, but when we got to just downstream of the membrane, it only dripped out. I tested the TDS and it was 8.

Our water is good - Lake Michigan water treated by Milwaukee waterworks. Medians: TDS = 180; calcium = 34; hardness (as CaCO3) = 136.

The chlorine -as total chloramine residual— is 1.57mg/L . Could this be the cause of early membrane failure?

 

[Ray]

Modern membranes are less sensitive to chlorine and older ones, but they still need protection from it. Their spec is a max of 0.1ppm free chlorine. What cartridges or filters are in-line before the membrane?

What was the output TDS when you first installed it?

By the way, if you decide to replace it, I'd be happy to work up a price for you. I believe I can beat the prices I've seen online.

 

[Silverwhisp]

Thanks, Ray.

The order before the membrane is:

In-line sediment cartridge
MatriKX CTO 5u filter cartridge

I didn’t record / don’t remember the output TDS when we first installed it in late April, but I know it was single-digit. The actual usage of the membrane is less than 9 months, as I used rainwater from ~June - October. I did run a small amount water through the system on a weekly basis during that time, maybe 2 gallons per week.

The seller is sending me a free replacement membrane, and I’m going from the recommended 150 ml flow restrictor to around a 300 ml flow restrictor.

 

[Ray]

Most suppliers recommend a second carbon filter for use with chlorinated water supplies: sediment>carbon>carbon>membrane

With a 5µ carbon filter and a municipal water supply, you don't really even need that sediment filter (it's probably a 5µ filter anyway) - just put a second carbon cartridge in the holder.

I experimented with flow restrictors a while back in consultation with a membrane manufacturer, and we both agreed that a pure:flush ratio lower than 1:2 is asking for fouling. Going to the 300ml/min (114 gpd) restrictor will definitely reduce fouling for you.

Sounds like you're on the right track!

 

[Silverwhisp]

Most suppliers recommend a second carbon filter for use with chlorinated water supplies: sediment>carbon>carbon>membrane

With a 5µ carbon filter and a municipal water supply, you don't really even need that sediment filter (it's probably a 5µ filter anyway) - just put a second carbon cartridge in the holder.

I experimented with flow restrictors a while back in consultation with a membrane manufacturer, and we both agreed that a pure:flush ratio lower than 1:2 is asking for fouling. Going to the 300ml/min (114 gpd) restrictor will definitely reduce fouling for you.

Sounds like you're on the right track!

Great. Thanks for you input, Ray!