Why is initial product water from watermakers saline?

[mikereed100]

This has been bothering me for quite some time and I can find no answers online. On the product side of the membrane the water is "desalinated" to one extent or another. However, when I start up my watermaker the first gallon or so is still quite saline. As far as I can figure the water left in the system after last use is all fresh, on both sides of the membrane. When the system is started up again any water flowing from the product side has passed through the membrane and should be "desalinated", so why is the initial water saline?

[bigreelsc]

Look up capillary action in fluids

[RaymondR]

The desalinating process is called reverse osmosis for the reason that without the high pressures involved the osmosis is normally from the higher salinity side to the fresh. To get fresh from saline one must reverse the natural process using high pressure. Without pressure on the membrane the natural osmotic process draws saline fluids through into the fresh increasing it's salinity.

[mikereed100]

Originally Posted by RaymondR

The desalinating process is called reverse osmosis for the reason that without the high pressures involved the osmosis is normally from the higher salinity side to the fresh. To get fresh from saline one must reverse the natural process using high pressure. Without pressure on the membrane the natural osmotic process draws saline fluids through into the fresh increasing it's salinity.

With respect, I think you have it backwards. Without pressure, fluid will be drawn across the membrane to the high salinity side until equilibrium is reached.

[mikereed100]

Originally Posted by bigreelsc

Look up capillary action in fluids

I understand capillary action, but how does that transport salt across the membrane?

I guess my question here is "How does a membrane, impermeable to salt, allow salt across initially, and then cease to allow salt across?"

[RaymondR]

Originally Posted by mikereed100

With respect, I think you have it backwards. Without pressure, fluid will be drawn across the membrane to the high salinity side until equilibrium is reached.

On pondering upon it I think you are right. During my driller days we added salt to the drilling mud to draw water from shales so they would not swell.

Maybe the residue of salt in the produced water is concentrated as a portion of it returns through the membrane into the salty feed water??

[mikereed100]

Originally Posted by RaymondR

Maybe the residue of salt in the produced water is concentrated as a portion of it returns through the membrane into the salty feed water??

Wow, excellent thought! I was all in for a moment, but the thing is, after making water we flush for several minutes, leaving RO water on both sides of the membrane so there should be no return of product water across to the feed side.

[Hartleyg]

Hi Mike,

Do you have a Spectra watermaker? I ask because our Spectra (200T) does the same 2-minute flush at the end of a cycle. In ours, we've never had any salty water produced (and I do check) - initially or even after the 5-micron filter got a bit clogged.
With our setup. there isn't any saltwater in the system until the pump is re-energized to start a new cycle and make more water.

Hartley
S/V Atsa

[LByers]

Millions of dollars have been spent studying how the semi-permeable membrane in your watermaker works. The pores of the membrane aren't truly small enough to act as a filter for sodium by themselves. The charge of membrane helps to repel sodium, (part of salt), ions. As well, the pressure and flow of the salt water help. When you turn off your watermaker the pressure and flow go away allowing the sodium to move back across the membrane into the desalinated water. This is called diffusion. When you turn it on again the pressure and flow are re-introduced, the sodium-infused "fresh" water is rinsed out and the process starts again.

Very interesting topic. I hope this is clear.

[Bean Counter]

It is interesting. Ours is a Desalator from France, very straightforward device, with cat pump and 3 * 21" membranes. When we start it up we turn the pressure valve up slowly until in the Green Zone. At this point the pressure is appropriate to produce fresh water. However the salinity indicator does not go green for another 30 seconds. At that point it switches the output to the tank. It seems to do this irrespective of whether we have fresh flushed or not.

[Tellie]

I like LByers response. But in a properly set up watermaker you really shouldn't have this issue unless you stop your watermaker without fresh water flushing after and then start it up again. This will show a higher initial salt reading when first starting it up for the second time. If you are getting higher than 1000PPMs readings on your initial start up after a fresh water flush then your fresh water flush in not set up properly. It can be a bit tricky to do this right because even with identical watermakers the setup is probably going to be different with different boats. The NewPort400 you have is designed to use your fresh water house pump to feed it fresh water from your tanks for the flush cycle. If for example your NP400 demands 3gpm input and your house pressure pump can only deliver 2.5gpm the watermaker is going to look to make that .5gpm difference up and it will do this by drawing salt water up from the intake thru hull past the check valve. Thus giving you higher PPM readings on initial start up. Do you ever have any foul smells when you first start your watermaker? This difference in demand in input of the feed pump and the capabilities of the house pump is adjustable by either slowing down the feed pump so it demands less input to match the house pumps output, or on an older system it is possible to set the program to start and stop the feed pump motor several times during the fresh water flush to give the house pump a chance to keep up with demand. Setting it up properly is going to assure there is plenty of fresh water on both sides of the membrane. In fact the PPM should read lower on initial start up and work it's way up to the membranes rejection capability on a properly set up fresh water flush cycle.
You've got my number.

[mikereed100]

Originally Posted by LByers

Millions of dollars have been spent studying how the semi-permeable membrane in your watermaker works. The pores of the membrane aren't truly small enough to act as a filter for sodium by themselves. The charge of membrane helps to repel sodium, (part of salt), ions. As well, the pressure and flow of the salt water help. When you turn off your watermaker the pressure and flow go away allowing the sodium to move back across the membrane into the desalinated water. This is called diffusion. When you turn it on again the pressure and flow are re-introduced, the sodium-infused "fresh" water is rinsed out and the process starts again.

Very interesting topic. I hope this is clear.

Lare,

I like your explanation. I suspected it may have something to do with the membranes not being fully functional without pressure.

[mikereed100]

Originally Posted by Tellie

I like LByers response. But in a properly set up watermaker you really shouldn't have this issue unless you stop your watermaker without fresh water flushing after and then start it up again. This will show a higher initial salt reading when first starting it up for the second time. If you are getting higher than 1000PPMs readings on your initial start up after a fresh water flush then your fresh water flush in not set up properly. It can be a bit tricky to do this right because even with identical watermakers the setup is probably going to be different with different boats. The NewPort400 you have is designed to use your fresh water house pump to feed it fresh water from your tanks for the flush cycle. If for example your NP400 demands 3gpm input and your house pressure pump can only deliver 2.5gpm the watermaker is going to look to make that .5gpm difference up and it will do this by drawing salt water up from the intake thru hull past the check valve. Thus giving you higher PPM readings on initial start up. Do you ever have any foul smells when you first start your watermaker? This difference in demand in input of the feed pump and the capabilities of the house pump is adjustable by either slowing down the feed pump so it demands less input to match the house pumps output, or on an older system it is possible to set the program to start and stop the feed pump motor several times during the fresh water flush to give the house pump a chance to keep up with demand. Setting it up properly is going to assure there is plenty of fresh water on both sides of the membrane. In fact the PPM should read lower on initial start up and work it's way up to the membranes rejection capability on a properly set up fresh water flush cycle.
You've got my number.

Thanks Tellie, I was hoping you would weigh in. However, I do not have a Spectra, rather a unit that I put together myself. Looking at my schematic it does not appear that salt water could be drawn into the flush flush water.

[Tellie]

Originally Posted by mikereed100

Thanks Tellie, I was hoping you would weigh in. However, I do not have a Spectra, rather a unit that I put together myself. Looking at my schematic it does not appear that salt water could be drawn into the flush flush water.

Man I must be tired. Very long days. I went back and re-read the thread and how I came up with you having a NewPort400 Spectra I'll never know. Tired, I'm sticking to it.

J.T. Halden (aka Tellie)
(954) 515-7077
Spectra watermakers
Blue Water watermakers
Rainman watermakers.

[Captain Bill]

Originally Posted by mikereed100

This has been bothering me for quite some time and I can find no answers online. On the product side of the membrane the water is "desalinated" to one extent or another. However, when I start up my watermaker the first gallon or so is still quite saline. As far as I can figure the water left in the system after last use is all fresh, on both sides of the membrane. When the system is started up again any water flowing from the product side has passed through the membrane and should be "desalinated", so why is the initial water saline?

My view on this is that all membranes leak a little salt as the specs vary from about 98% salt rejection to 99.4%. I have experience with a couple of watermakers and all of them seem to do this. My theory (which I can't prove) is that the amount of salt leaking through is not as pressure dependent as is fresh water production. Your initial fresh water flush passes through the membrane almost instantaneously because the fresh water on the brine side of the membrane requires almost no pressure to pass through the membrane whereas sea water will not begin passing fresh water through the membrane until the mechanical pressure exceeds the osmotic pressure (about 450 psi). My theory is at this pressure the amount of salt leaking through is because of leaky membranes as the salt does not have to overcome osmotic pressure and is governed by the size of some pores in the membrane that allow salt ions through, but because of the low flow rate of fresh water at about 450 psi it is not diluted as much as when the pressure is higher and the fresh water is flowing through the membrane at a much higher rate. I have a manual pressure valve on my watermaker and slowly increase the pressure. I get a bit of high salinity water at a bit over 450 psi but the flow rate is very low.. As Increase the pressure the flow rate jumps rapidly and the salinity of the product water falls very rapidly. By the time I get to 800 PSI I am getting close to the membrane specification and am always within spec in less than a minute.

Well that's my theory and I'm sticking to it until some one shows me a better one.