Bladder tanks and more
I saw it in the product brochure sheet for the AEWP water purifier unit – “Bladder tank”.
They aren’t the same as the idea I had, but for a second I was hoping they were.
I was thinking of using rubber surgical tubing as the “bladder”, with clean water on one side, and dirty water on the other. The tank is sealed, and there should be no or little air. What happens is, the clean water is under pressure (exerted by the bladder as it tried to contract like a balloon), and the dirty water is not under any pressure. Suppose you have a faucet and drain connected to the supply and dirty water sides of the bladder. You turn the faucet on, and the bladder starts to put out water, but because the bladder shrinks, dirty water is sucked back in the other side. Handy. It would eliminate the need for a drain pump. It could also eliminate the need for the supply pump, so all told it could cheapen the system. The system still needs one pump, but it could be the small (and therefore cheap) filter pump used to put water through the filter system. Water comes out of the filter system under pressure, and that pressure is used to fill the bladder tank, storing most of the energy.
It would also cut the size of the storage tanks in half for the same amount of water storage, as opposed to a separate dirty and clean water tank, because you don’t have to make room for air.
Silicone surgical tubing would last longer, as it would not be degraded the way rubber sometimes does (though I don’t know if nitrile suffers from that). To increase the pressure inside the bladder, you could use 2 layers of tubing, one inside the other, so it takes more force to force water into the “bladder”.
The water pressure might not increase as the bladder is filled as much as you might think, so it could be used as the pressure storage vessel after all. If you pump the water in with a small cheap pump, you can take it back out again at a high rate, without the pressure dropping all that much as you run the faucet (which would suck).
With the bladder tanks mentioned at that link, the water pressure would change, because the air pressure goes down as the tank is deflated.
To see why the bladder could be better, just suppose you had a sort of cubical bladder, the sides are flat and stay flat as it is filled (somehow), but the cube must get bigger. The elastic force that results from the stretching of the rubber is pulling the left side and the right side of the cube together. Suppose the force exerted to pull the sides together is proportional to the width of the cube, W. As the width of the cube is multiplied by x, the (f)orce to bring the sides together is multiplied by x, but the (a)rea of the side of the cube is multiplied by x^2. P=f/a, so the pressure stays the same, regardless of the level which the bladder is filled to. This same sort of logic applies to a spherical or tubular bladder
Hooke’s law, which a metal spring obeys, says the force is indeed proportional to the stretchedness. Rubber doesn’t obey this, but the force still goes up as the inflation does.
Another interesting possibility, I think, is the possibility of using a hand pump for a water recycling shower and sink of some sort, that doesn’t need electricity, for remote camps etc., actually I was thinking of burning man after Michael janzen’s recent post, where they have a lot of water issues since it is out in the desert, and they cannot dispose of water on the playa. Suppose you had 1 tank, but 2 bladders. One bladder has thinner walls than the other, so the water pressure in it is lower than the next. The highest pressure bladder is the pressure one, from which water flows to the tap. The lowest pressure bladder is the dirty water one, and the region of the tank that is not within a bladder is the clean tank, or you could have a third, lowest pressure bladder as the clean tank, with the unbladdered space unused, for reasons I’ll explain in a minute.
There is a hand pump, maybe a cheap rubber bulb type, connecting the clean bladder and the pressure tank. There is the filter system, like the nasa system minus the volatile removal thing (so the water quality isn’t perfect, but it might be enough for showering etc.), in between the dirty and clean tank (it has to be a system that can be sealed from the air to maintain the pressure, and there is no electricity to power an MBR). You pump the system up before use, then turn the faucet on. The pressure bladder shrinks, and dirty water is sucked into the dirty bladder. When you are done you just walk away. There is a pressure differential between the dirty and clean tanks, so the water is forced through the treatment system.
That’s only a rough outline of a system, though, the water treatment system would still have to be worked out, as usual… You could use chemicals though, I recently found out about Fenton’s reagent that could be used for the oxidising stage. Sounds like nasty stuff it it’s original form, but iron is not very toxic and would be removed by the rest of the treatment system, and the peroxide of course breaks down to water and oxygen, and would also be removed by the filters. You could either find a way to dose it automatically, or after the contaminants build up, have provisions to dose the water manually then send it through the filters again.
The reason you might want pressure in the clean tank is to prevent any intrusion of dirty water, since if there was a hole in the dirty bladder that would be a problem. If you are willing to make the system that much less compact, you could just put the clean water in a separate tank