Hello Punatic ... Sorry for the delay on the road again .
Questions: What type of pre filtration are you using before the 5 stage RO?
When was your last filter change?
Do you use anti scalent?
What is the water temp?
How many membranes and what size?
It sounds like the membrane is scaling which will cause fouling, loss of flow, etc. Scaling is normally caused by calcium and magnesium. If you can give more info I can direct you to a solution.
The system has a two stage prefilter system; a multimedia prefilter that auto-backwashes at 350 hours of runtime, followed by a catridge filter with a 5 micron filter element. I changed the filter element about two weeks ago. It was not very dirty, the MMF catches most of the solids.
Yes, the system uses a phosphonate antiscalent (Avista Vitec 3000). Antiscalent is fed with a metering pump set to maintain ~3 mg/L.
There are five 4" pressure vessels configured in series. Each vessel houses one spiral-wound membrane. There is no concentrate recycle, or bypass blending.
Water temp from the well is constant at 76⁰ F.
I do not suspect there is scaling or fouling. The membrane feed pressure has been a flat line on the trend charts - 110psi. If there were scaling or fouling the pressure would increase to maintain the 4gpm permeate flow rate. Plus I keep the recovery down at 50%.
I suspect a bad seal in one of the pressure vessels, however when a seal has failed in the past the permeate conductivity spiked up quickly. This increase has been a gradual climb over the last two months. Maybe a tear in a membrane?
The 4" pressure vessels are a pia to work inside of; much tighter than standard 6" vessels. Since you offered (and I looked at your web site) I thought I'd get another mind thinking about what the problem might be, before I start opening up pressure vessels.
740 µS/cm is not the end of the world, but the water is also used for swimming pool make up. The woman who maintains the pool mentioned she has noticed a change in her chemical feed rates
Thanks for your help!