Sounds like a broken doo-hickey in the filter. Or dirty or defective screen.

Who made it, and what do they say? Have you called them, since they would know what you have as a filter, which we don't.

We can't diagnose and fix your problem with nothing to go on.

FAQ on Reverse Osmosis System, Water Filter System, Pure Water ...

SCREEN FILTRATION TECHNOLOGY AS APPLIED TO ...

shooting from the hip, I would say that one or more of the membranes failed, or the interconnects.

Alot more information is needed?

To start with:

on your second stage, what is the MWCO of your membranes? Might as well state the first stage as well.

What is the Δ P accross Stage 2?

And what is your flux rate?

You may have a bad membrane on that second pass, or a bad o-ring somewhere. Check each membrane product tds.

Since you have two different pressures, I presume you have two high pressure pumps.

If it were me, I would consider putting the 'worst' membranes on the low pressure system, and the 'best' membranes on the high pressure system.

I also presume all the feed is from one well. The reject from the first machine goes to the second machine. I can't imagine it working out perfectly, there must be a reject tank after the first machine that the second one draws from, right? The first one makes a little more reject than the second one draws, so some reject is overflowed to somewhere.

If the plumbing is different from this, let me know, it sounds interesting.

Yes its interesting,

like to interject about the higher pressures, it could be a polishing stage for the reason of higher pressure.

The higher the tds of the feed, the higher the pressure that is necessary to get fresh water.

Under-sink ro units operate off of line pressure, but the incoming tds is real low.

Membrane in the first stage must have been damaged. How old these membrane are? Check the scaling and consider proper chemical dosing to avoid scaling.

in 1 st ss tage feed pr is only 28/27.5 also permeate tds is only 300 ppm but in e2nd stage pr is 49/48 only permeate tds is 4600 .

both combined tds is 900 .how to control the second stage tds

Why are you taking the dirty water from the first stage through the second? My interpretation of the system is that the clean (300ppm in your case) should be sent to the second stage to further reduce the solid content. Plumbing check required??

Anything up to 35,000 tds, the usual tds of seawater is ok for RO membranes. Many places have what we call 'sweet' wells, or brackish water. This brackish water screams through a RO system, giving good product at a good rate, like an under-sink system.

The reject from such a system has already been pumped, filtered, and treated with anti-scaling chemicals. It is still low enough tds to feed to a separate RO system.

I operated a RO system where the return was right in the plumbing, the feed water passed through the membranes twice.

The OP has a minor problem, maybe just a failed o-ring on the product pipe allowing concentrate or reject into the product line. Careful testing will find the culprit.

Sometimes a membrane fails completely, bypassing bad water into the product.

I had a separate setup to operate one membrane alone to test a membrane one last time before I tossed it. I could also use this setup for aggressive cleaning procedures to try to save a membrane. It required a small high pressure pump and some plumbing, but was very handy.

From the figures, it looks like the second stage is operating on the first stage reject stream. If this is correct, then rather than blending the second stage permeate with the first stage permeate to produce a worse-than-tap-water blended stream, it would be better to use the second stage permeate as a blended feed to the first stage to reduce the ionic load on the first stage membranes, which would in turn improve the first stage permeate.

Some idea of the intended performance and the history before the issue arose that prompted a posting in CR4 would be of value.

Without knowing how this system is currently connected up versus how it is intended to be connected up, it is not possible to advise further remotely. Try turning off Caps Lock and contacting the original equipment supplier directly by telephone for advice. A site visit by the membrane supplier's technical representative may be worthwhile after all the "quick-fixes" have been discussed.