Salt water pools: how salt electrolysis works

How chlorine is made from salt

Pool salt, i.e. sodium chloride, is dissolved in the water. After the filter, the water passes through the electrolysis cell, which has titanium plates with a special coating. A low-voltage current splits the chloride ions and forms active chlorine, the same disinfectant that chlorine chemicals provide. Once it has done its job, it turns back into salt and the cycle starts again.

That is why the salt is not used up day by day. It is lost mainly through backwashing, splash-out, rain diluting the water and partial water changes, and it is topped up from time to time.

How salty the water is

Most units need a salinity of a few grams per litre, and there are also low-salt models. Seawater has many times that, around 35–40 g/l. So the water feels soft, does not sting your eyes like the sea, and the saltiness is barely noticeable.

• Use salt made for pools, with no iodine or anti-caking agents.
• Spread it over the whole surface with the pump running and brush the floor so it dissolves, without leaving piles sitting on the liner.
• The unit usually warns you when salinity drops; confirm it with a test.

Why the pH rises

Electrolysis gives off by-products that raise the pH, and aeration of the water pushes it up as well. In salt pools the pH tends to climb steadily, so you need regular doses of pH minus (pH−) or an automatic dosing pump. Keep the pH at 7.2–7.6, because above that chlorine works less well and scale forms. See pool pH and automatic pH and chlorine control.

Compatibility with the liner and equipment

A PVC liner copes with the low salinity of these systems, and the panels sit behind the liner, with no contact with the water. What really protects the liner is balanced chemistry: correct pH and free chlorine of about 1–3 mg/l, not permanently higher. Take care with:

• metal fittings in and around the water, such as ladders and grab rails, which must be a suitable grade of stainless steel;
• heating, which needs a salt-resistant heat exchanger, usually titanium;
• some natural stone paving around the pool, which should be checked for resistance to salt water.

What maintenance it needs

Most cells clean themselves by reversing polarity. In hard water, however, as on many islands and with borehole water, they build up calcium scale and need cleaning as the manufacturer specifies. The cell has a limited lifespan and will need replacing at some point.

The unit only produces chlorine while water is circulating, so the pump must run enough hours. In cold water output drops and many units shut off, so check the chlorine more carefully in spring and autumn. For a comparison with chemical chlorination, see salt or chlorine.

Questions

See also