[Ian Lancaster]

This is one of those stupid situations that just doesn't seem to make sense.

One of our 300gpd systems has suddenly jumped from about 14ppm to 70ppm.  New membranes about 4 months ago, new prefilters 3 weeks ago.

I phoned June at Gaps (filters and membranes are from them, system from Pure Freedom), she suggested I check the restrictor valve (working at 2.5:1, so OK) and then check each membrane individually.

I connected each membrane to the mains by itself, one after the other and tested the product water.  One was producing at 33ppm, one at 35ppm and one at 38ppm.  I know this isn't brilliant, but it's much better than 70 and DI gets it down to less than 10.

Connected them all up again in parallel (seperate supply of mains to each membrane), connected the 3 product water outlets together so I can feed it through the DI and the 3 waste together and then into the restrictor.

CHECKED ALL CONNECTIONS TWICE, THEN GOT SOMEONE ELSE TO CHECK so I know there's no mistake there.

Ran the RO - the product is coming out at 70ppm!!!  Even after running it for 20 minutes to let it settle down.

Can't think of any logical reason why this should be.  Any suggestions?

If each membrane produces in the 30's, how can connecting the outlets together make the TDS 70?

[elite mike]

hi ian

what micron sediment filter have you got fitted ?

i know it sounds daft ,but i fitted some new filters about 18 months ago

and i was having similar problems with my ro  :

i bought a new ro and when i took the old one apart ,on the sediment filter which i took out

imprinted on the filter itself it said 30 micron    i did not notice at the time when i fitted it originally .

i am not sure if this caused my old ro to go kaput ,but it seems a coincidence .

mike

[Ian Lancaster]

Thanks for the input Mike,  we run Gaps prefilter sets, there are two sediment filters first one is 0.5. second one 0.1, so no problem there.

[elite mike]

yes i usually use the same size micron filters ian

but when i fitted the last ones on the old ro ,i should have checked on the packet  

lesson learnt  

[andyM]

Have you checked the tds meter calibration?

[Ian Lancaster]

Thanks Andy,

Yes I have, and I used another as a control.  The bit I can't understand is if you had three barrels of water at (say) 10ppm and you put them all together into your van tank, the water in the tank wouldn't suddenly become 30ppm it would still be 10ppm.

So if three membranes are producing at 35ppm each, how come when I mix the product together it becomes 70ppm?

[Dave Willis]

How come the di doesn't get it to zero? Is your resin spent too?

[Dave Willis]

I think we need a photo, sounds really odd, almost as if the waste from one is mixing with the pure from the other two.

[andyM]

Does sound a very strange situation.
For starters if the membranes and pre-filters have been changed recently I would of thought depending on your input tds you would get a much lower reading than 35ppm?
I have only had one similar situation where the tds had risen alarmingly and that was due to a kink in the waste pipe. 

[craig21t]

You say that you connected them up in parallel, I believe they should run in series.

The waste from the first membrane should feed the second and the waste from the second should feed the 3rd membrane, then the waste from the 3rd should go to the flow restrictor.

By running them in parallel, you are splitting the feed pressure & flow 3 ways. So unless you have very good pressure & flow, the membranes will not come close to the correct operating envelope and would explain your poor results.