RO Pressure/Leak Problem

[Rudy]

The system pressure on my RO fell to the point where I could only maintain 140 psi. The quality of the water was still so-so....about 6 ppm...so I let it go for a while. Yesterday, I changed the membranes. The system pressure now will not go above 100psi, and the quality of the water is 3 ppm. Usually, when I put new membranes in, the quality goes to 0 or 1 ppm.

I CAN get the system pressure to hit 200psi, but only if I set the reject rate very low....which is not good for the membranes.

I'm wondering if there's an internal leak ???? When I replace the membranes, there are two "extenders" which fit on the ends of the membranes to connect the centers into the end plugs. There are O-rings involved. If these were to leak....wouldn't that cause a drop off in pressure, and also contaminate the output water?

It's my understanding that these "extenders" were used on pressure vessels a long time (15 years) ago, and that now.....modern pressure vessels seat the end plug directly onto the end of the membrane.

Am I on the right track here, or is there some other reason my system can't run enough pressure, while, at the same time there appears to be some contamination with brand new membranes??

 

[MEP001]

Your pump may be failing - a drop in pressure does not usually indicate a problem with the membranes. As they clog the pressure would increase, not decrease, unless chlorine is getting to them.

I assume that your meter is reading "times 10" since 6 ppm is very good. If it's 30 and due to a leak around one of the "extender" o-rings, I wouldn't think the amount of flow around it that's causing such a small increase in TDS would cause the pressure to drop so drastically.

Test the pump. Measure the product flow and the reject flow, which on a 4 x 40 membrane should be about 2 GPM product for RO and 2 to 3 times that for reject per membrane. Your pump could be failing, and insufficient flow could be causing the increase in the TDS.

 

[Rudy]

We've got very high quality water from the Utility Company. Yesterday, the feed water was 75 degrees, and with 2-4x40 membranes, I was getting 3.2 gpm of product, and 3.2 gpm of reject (we have been able to run at almost 1:1 for years with no adverse effects).

BTW....my "TDS" meter reads straight out, not x10 I think.....

Since my membranes are rated at 2600 GPD, my actual rate of 4600 for both isn't all that far off....No???

 

[MEP001]

Some systems were meant to run 1:1, quality of water and whether some of the reject is recirculated through the membranes determines how long they'll last.

If your meter reads true and not x10, then even 3 is good. higher than 30 (or 50, depending on who you ask) is bad - the true test is whether it spots.

4x40 TFC membranes will only produce 1800 GPD - did you use CA or high-volume ones?

 

[Rudy]

They're HF (High Flow), Low Energy (Model HF4. I got them at Wood Brothers. Here's the link: http://www.woodbrosind.com/PDFs/membraneelementshousing.pdf

 

[BayWatch]

Have you thought about replacing the gate valves that control the pressure on the membranes. I have to routinely replace the reject needle valves, and about a year ago we had a similar issue and replaced the gate valves which solved the issue. I wanted to find a stainless valve but no luck. I would love to have 6 ppm, but live with about 25 ppm.

 

[Rudy]

It's turns out that my replacement membranes are the "Low Energy" type....which, if I understand it correctly, output the same on less pressure. Instead of needing 200psi, these membranes can output 2600 gpd at 100psi.

I'm concerned however about the "recovery" rate. In the past, I was able to get 9 years out of my first set of membranes running at a 1:1 ratio....that is, I dump a gallon down the drain for every gallon of spot free I make.

From what I read, this is an "aggressive" setting. I've never had a fouling issue, but now I'm wondering if these new membranes are more sensitive. Some places recommend a 5:1 ratio...but with water and sewer so high, it would be cheaper to buy new membranes. What type of recovery rate do you use????

 

[MEP001]

I've replumbed a number of systems to make them 1:1 - all it takes is recirculating some of the reject back to the inlet of the pump. It has to do with the speed of water flow through the membranes.

 

[Rudy]

My unit has a "recirc" needle valve. In the summer, when the water is warm, it's usually fully closed in order to have enough system pressure. In the winter though, the recirc valve is needed to keep the system pressure within limits. My question would be, how do you calculate (or determine) whether or not the system can be operated at 1:1. And if not 1:1, then at what ratio?

 

[MEP001]

It seems like the nine-year lifespan of your last membranes tells that you already have a very good system. I wouldn't mess with it - I rarely see them last that long.