How do I lower the water level in my pool without removing salt at the same time?

Every year to winterize my pool I lower the water level below the skimmer by pumping water out of the pool.  Since I switched to salt, this means I will be pumping about 120kg of salt out as well which I would like to save for economic and environmental reasons.

How do I drop the water level but retain the salt in the remaining water in the pool?

Reverse Osmosis?
Large black tarp that will allow evaporation of the water and trap the salt?

Get creative.

And AfAIk, the extraction pressure in a reverse osmosis filter climbs with slainity gradient, so the concentrated brine is going to be ludicrously hard to pump the lower the water gets.....
derekmjenkins (author)  steveastrouk3 years ago
Anyone know the extraction pressure for a reverse osmosis filter with 4000 ppm salinity? Remember the pool runs a 1.5 HP pump that can provide pretty significant pressure as it already pumps the water through a large sand filter.

Well, running the maths from my limited understanding, using D'Hoft's equation yields 400 PSI.

That's a LOT more than I thought the pool pump can deliver.
orksecurity3 years ago
Evaporation, or an osmotic filter, are the only thoughts I have. Why are you using salt water in the pool?
It makes it much easier to float, and there's a whole school of belief that saltwater immersion has health benefits.
Granted, I've seen saltwater pools before... I'm just a bit surprised to see someone _switch_ from freshwater to salt. I'd be really worried about whether all the components (pumps and pipes and valves) are rated for the additional corrosion hazard.
derekmjenkins (author)  orksecurity3 years ago
All piping and fittings are PVC.
And pump body/shaft/impeller/seals are also rated for salt water? Valves likewise? (If yes, that's fine. I'm just a bit paranoid about that issue.)
I live in az, and I agree with Derek. Most pools now are saltwater, even large attendance like community pools are making the switch in az. Being that we spend almost 7 months out of the year in the pool I agree that saltwaters much better, especially on the eyes for kids...
derekmjenkins (author)  orksecurity3 years ago
Almost all pools sold today are salt-water pools. Chlorine is not used much anymore, Bromine is still prominent in Hot Tubs. Pools are however starting to move toward inline UV disinfection.

Remember that salt itself is not the disinfectant. The pool water runs by an electrode which uses electrolysis to break down the salt (NaCl). The resulting chemical reaction eventually produces hypochlorous acid(HOCl), and sodium hypochlorite (NaClO), which are the sanitizing agents already commonly used in swimming pools. The advantage is that no chloramines are used.

We run at 4000 ppm salt.

I haven't been seeing salt-water pools in my area (Northeastern US), though I have seen the move toward bromine and UV. Then again, I haven't looked closely recently. I'm willing to take your word for it, for now.
Does that mean that free chlorine is never produced, because AFAIK, chloramines aren't "used" so much as produced as reaction products of chlorine and organics.
Or distillation, which is essentially assisted evaporation. But the energy required to evaporate that much water would be pretty substantial... For ecological reasons, I highly approve of not pumping out more of that salt water than you must. But I don't see a way to get the results you want in the time you have available at the end of the season at a reasonable cost. I would be delighted to be wrong.
aeray3 years ago
The salt will stay behind as the water evaporates, no need to trap it.
Evaporation's a neat idea.