Seasonal change to TDS in water maker output

[silver-fox]

The membrane and seals are never perfect so a small amount of salt gets into the product water. The amount is dependent on the initial salt concentration.

Yes but why? If you have sieve it will let through anything small enough to go through the holes in the sieve. A mebrane or filter is the same as a sieve.

Surely the quality of output depends solely on the fineness of the filter or membrane in this case and not the salinity of the input?

 

[noelex]

A small quantity of raw sea water gets past the membrane and seals and into the product water. The saltier this raw water is. The saltier the product water will be.

 

[silver-fox]

Ahh! So we are looking at leakage around the membrane rather salt passing through it?

 

[Sybaris]

Calibrating TDS meter

Does anyone know if it is reasonable to assume that distilled water would have a tds value of (zero) 0 ppm for calibration?

 

[jdc]

Molecular processes

Yes but why? If you have sieve it will let through anything small enough to go through the holes in the sieve. A mebrane or filter is the same as a sieve.

Surely the quality of output depends solely on the fineness of the filter or membrane in this case and not the salinity of the input?

I don't think the concept of sieve applies here: generally our assumptions built up on large scales fail at a molecular level, and NaCl are small, order a few Angstroms, so 1/10000th of conventional filter pores. At this size matter is mostly empty, each atom consisting of a tiny nucleus with a cloud of electrons, each tiny beyond imagination, vaguely (and probablistically) in 'orbit' - it's quantum mechanics innit! The concept of a 'hole' which a molecule will or won't fit through is simply misleading.

Membranes work on electric forces at molecular level: polar molecules or ions like Na+ or Cl- are held attached to the membrane by electric forces, while non-polar molecules like water can go through. The selectivity isn't due to 'size' difference, but to whether they stick to the membrane's surface. This sticking is however not 100%, and given enough ions and enough pressure some will come through.

I hated thermodynamics anyway, and was a mathematician not a chemist, so I'm probably wrong in detail, but I think right in principle.

 

[AntarcticPilot]

I don't think the concept of sieve applies here: generally our assumptions built up on large scales fail at a molecular level, and NaCl are small, order a few Angstroms, so 1/10000th of conventional filter pores. At this size matter is mostly empty, each atom consisting of a tiny nucleus with a cloud of electrons, each tiny beyond imagination, vaguely (and probablistically) in 'orbit' - it's quantum mechanics innit! The concept of a 'hole' which a molecule will or won't fit through is simply misleading.

Membranes work on electric forces at molecular level: polar molecules or ions like Na+ or Cl- are held attached to the membrane by electric forces, while non-polar molecules like water can go through. The selectivity isn't due to 'size' difference, but to whether they stick to the membrane's surface. This sticking is however not 100%, and given enough ions and enough pressure some will come through.

I hated thermodynamics anyway, and was a mathematician not a chemist, so I'm probably wrong in detail, but I think right in principle.

You raise a point I hadn't thought of - NaCl isn't that in solution, but Na+ and Cl- ions. So the action of the membrane certainly isn't comparable to a sieve; a Na+ ion is actually smaller than the OH- ions that water splits into. The Cl- ion is "larger" than the OH- ion, I think, so there must be something more complex going on.

 

[silver-fox]

You raise a point I hadn't thought of - NaCl isn't that in solution, but Na+ and Cl- ions. So the action of the membrane certainly isn't comparable to a sieve; a Na+ ion is actually smaller than the OH- ions that water splits into. The Cl- ion is "larger" than the OH- ion, I think, so there must be something more complex going on.

OK, Let me ask a simpler question.

In the opinion of our expert panel......does the salinity of the input water have any significant bearing on the the salinity of the output/production water

I am expecting answers like

"yes because..."
"no because......"
"Dunno and dont care...."

 

[BeBe]

Filmtec membranes specification

OK, Let me ask a simpler question.

In the opinion of our expert panel......does the salinity of the input water have any significant bearing on the the salinity of the output/production water

I am expecting answers like

"yes because..."
"no because......"
"Dunno and dont care...."

I expect answers like that but never receive them.

Yes, because this is exactly what the specifications state on the Filmtec (saltwater "SW") membranes. The specification is that "Stabilized Salt Rejection is 99.4%." It therefore is obvious that as you increase the salinity of the input, the salt content of the output will increase.

See attached specs.

Bill

 

[silver-fox]

I expect answers like that but never receive them.

Yes, because this is exactly what the specifications state on the Filmtec (saltwater "SW") membranes. The specification is that "Stabilized Salt Rejection is 99.4%." It therefore is obvious that as you increase the salinity of the input, the salt content of the output will increase.

See attached specs.

Bill

Bill

Thank you, its amazing the effect some facts can have on a debate

To introduce some more here is a paste from Wikipedia

"Seawater is water from a sea or ocean. On average, seawater in the world's oceans has a salinity of about 3.5% (35 g/L, or 599 mM). This means that every kilogram (roughly one litre by volume) of seawater has approximately 35 grams (1.2 oz) of dissolved salts (predominantly sodium (Na+) and chloride (Cl−) ions)."

"Although the vast majority of seawater has a salinity of between 3.1% and 3.8%, seawater is not uniformly saline throughout the world. Where mixing occurs with fresh water runoff from river mouths or near melting glaciers, seawater can be substantially less saline. The most saline open sea is the Red Sea, where high rates of evaporation, low precipitation and river inflow, and confined circulation result in unusually salty water. The salinity in isolated bodies of water (for example, the Dead Sea) can be considerably greater still."

So! If this information is correct a Filmtec membrane is having to deal with between 31 to 35 grams per litre. Assuming it performs to specification and filters out 99.6 % then the production fresh water will contain 0.6% of the salts of the input salt water as illustrated below.


Lowest 31 grams/litre x 0.006 = 0.186 grams per litre
Highest 38 grams/litre x 0.006 = 0.228 grams per litre
Average 35 grams/litre x 0.006 = 0.21 grams per litre

Which then leaves one remaining question........

Is that small variation detectable to the average liveaboard palate?

I have read elsewhere that the salinity of the Med is 3.8 % (38 grams per litre) and I can report from my own experience that the output water from my own humble Katadyn water maker in the Med tastes amazingly good and I would compare the taste with tap water in the London area.

My conclusion to all this is that whilst input salinity does affect the production output salinity as evidenced by Bebe (Bill), the variations are so small that they will have little or no effect on a typical, liveaboard, tea drinker, like myself

QED