permeate question

[pathfinder22]

sure it has been asked before,,,,,,,
how long does permeate last if kept cold?
I mean it is pure water and should not spoil,,,right.
I would like to hang on to some from week to week between runs
for flushing the ro and mid week clean up.

[maple flats]

If you in fact have absolutely no sugar passed, it will last a few months. My testing using a long sap hydrometer (more accurate readings) after boiling the permeate down to 1/10 the original volume I still can't get a reading on sugar in it, but when I empty a gallon jug of distilled water and then fill it with permeate and cap it, after 8-9 months I see a growth, thus there must be a slight amount of sugar passing is my guess.
In season I keep my excess permeate until the end, however if I start running low, I take that time to drain it into another cleaned tank and then use it to wash the permeate tank.
Since I also grow blueberries I actually save that water for mixing with my insecticide sprays during the season. Each time I use just over 50 gal of permeate starting as soon as the berries first show any blue and then every 7-10 days thru the season, which usually means about June 8-12 to start and about Aug 20 to end. In that time the permeate still looks clear just looking into the 1000 gal tank I use.

[Lanark]

Thanks Maple Flats.

With a tds meter I would want the reading to be 0 ppm? And with a ph meter I would want the reading to be 7, which is neutral?

[maple flats]

You will likely seldom if ever attain either of those readings. Dr Tim might have an answer but my guess is that permeate, while it should have zero sugar, it will not generally have a PH of 7.0 nor will it have no TDS. I don't know for sure but I doubt there are no TDS in permeate, and while the PH might be slightly off the 7.0 mark. An RO removes water from the sap because the sugar molecule is too big to pass thru the membrane, other disolved solids might be small enough to pass. Also, passing thru the membrane won't likely neutralize the water, but if it removes some of the causes of the PH it could bring it closer to neutral.

[DrTimPerkins]

Generally you want your sugar to read as low as possible, but some membranes sacrifice a small amount of sugar passage for higher processing rates. Depends upon your membrane/strategy. Note that refractometers often don't read accurately down that low, so even if you see 0.1, 0.2, 0.5 Brix, it might still be zero and either your refractometer isn't calibrated or just not sensitive enough.

pH of permeate is typically a bit acidic, 5.5-6 is not uncommon, but it'll depend on the starting sap chemistry and your processing (membrane properties, etc.).

For dissolved solids, typically you want it to be very low, under about 50 μS/cm. Again, some membranes trade-off leakiness for speed.

For those uber geeks among you, https://file.scirp.org/Html/4-3002254_89798.htm

[pathfinder22]

Yeah thanks, Like I said, I would like to keep it for flushing between weeks of boiling and stuff.
sounds like it is ok to keep if for a while and just add to it each time I run the RO.
I have 1 barrel for raw sap, one for concentrate and one for permeate.

[maple flats]

What size is your RO. In general, you want at least 2x the hourly rate (for sap input) in permeate storage. Thus if the RO does 25 gph, of sap input, you need at least 50 gal for permeate storage. Mine does 250 gal/hr, so I need 500 gal storage, in fact I have 1000 gal tank, extra is not a bad thing.
I think some states have a max allowed permeate storage but I'm not aware of any such in NY state.

[TapTapTap]

I think some states have a max allowed permeate storage but I'm not aware of any such in NY state.

Why would the state set limits on permeate storage?

[johnallin]

Why would the state set limits on permeate storage?

because they can...

[maple flats]

I'm not sure why some do, but I've heard they do. John Allin is right.