Salt water chlorination is a process that uses dissolved salt (2,500â€“6,000 ppm) as a store for the chlorination system. The chlorine generator (also known as salt cell, salt generator, salt chlorinator or SWG) uses electrolysis in the presence of dissolved salt (NaCl) to produce hypochlorous acid (HClO) and sodium hypochlorite (NaClO), which are the sanitizing agents already commonly used in swimming pools. As such, a saltwater pool is not actually chlorine-free; it simply utilizes added salt and a chlorine generator instead of direct addition of chlorine.
The presence of chlorine in traditional swimming pools can be described as a combination of free available chlorine (FAC) and combined available chlorine (CAC). While FAC is composed of the free chlorine that is available for sanitizing the water, the CAC includes chloramines, which are formed by the reaction of FAC with amines (introduced into the pool by human perspiration, saliva, mucus, urine, and other biologics). Chloramines are responsible for the "chlorine smell" of pools, as well as skin and eye irritation. These problems are the result of insufficient levels of free available chlorine, and indicate a pool that must be "shocked" by the addition of 5-10 times the normal amount of chlorine. In saltwater pools, the generator uses electrolysis to continuously produce free chlorine. It also burns off chloramines in the same manner as traditional shock (oxidizer). As with traditionally chlorinated pools, saltwater pools must be monitored in order to maintain proper water chemistry. Low chlorine levels can be caused by insufficient salt, incorrect (low) chlorine-generation setting on the SWG unit, higher-than-normal chlorine demand, low stabilizer, sun exposure, or mechanical issues with the chlorine generator. Salt count can be lowered due to splash-out, backwashing, and dilution via rainwater.
The chlorinator cell consists of parallel titanium plates coated with ruthenium and sometimes iridium. Older models make use of perforated (or mesh) plates rather than solid plates. Electrolysis naturally attracts calcium and other minerals to the plates. Thus, depending on water chemistry and magnitude of use, the cell will require periodic cleaning in a mild acid solution (1 part HCl to 15 parts pool water) which will remove the buildup of calcium. Excessive buildup can reduce the effectiveness of the cell. Running the chlorinator for long periods with not enough salt in the pool can strip the coating off the cell which then requires an expensive replacement, as can using too strong an acid wash.
Saltwater pools can also require stabilizer (cyanuric acid) to help stop the sun's UV rays from breaking down free chlorine in the pool. Usual levels are 20 â€“ 50 ppm. They also require the pH to be kept between 7.2 and 7.8 with the chlorine being more effective if the pH is kept closer to 7.2. The average salt levels are usually in the 3000-5000 ppm range, much less than the ocean, which has salt levels of around 35,000 ppm. In swimming pools, salt is typically poured across the bottom and swept with the pool brush until it dissolves; if concentrated brine is allowed into the return-water system it can cause the chlorinator cell to malfunction due to overconductivity.
Salt water chlorination produces an excess of hydroxyl ions whilst releasing chlorine from salt, which makes the pool alkaline (sodium hydroxide, NaOH, caustic soda). This requires the frequent addition of hydrochloric acid (HCl, also known as muriatic acid) to neutralise the alkalinity and convert the sodium hydroxide back into sodium chloride (i.e. table salt), which can be split by electrolysis again. Thus the consumable supplying all the chlorine is in fact the hydrochloric acid, with the salt acting as a non-consumed intermediary, being split and reformed. The whole process is exactly balanced by equivalents: the amount of chlorine produced is directly related to the amount of hydrochloric acid used, other things remaining equal. This could be represented roughly as follows (with the ions all separated in solution):
2NaCl + 2H2O â†’ electrolysis â†’ 2NaOH + H2 + Cl2
2NaOH + 2HCl â†’ chemical reaction â†’ 2NaCl + 2H2O
Net reaction : 2HCl â†’ H2 + Cl2 (as gases bubbling off) .
Use of another acid, such as sulphuric acid to neutralise pool alkalinity will reduce the pH but will produce sulphate ions rather than chloride ions. Sulphate ions conduct electricity equally well as chloride at the electrode, but will not produce any chlorine. A salt water pool "poisoned" with sulphate may have to be drained and refilled to remove the offending sulphate ions.