[Pool Care and Maintenance][Trevor Croll Home Page]
Producing Chlorine by Electrolysis
When direct current is passed through Common salt (Sodium chloride (chemical formula NaCl)) in water ("Hydrogen oxide" (chemical formula H2O))
Salt + Water (converts to =) Chlorine + Caustic Soda + Hydrogen
The total Chemistry is: 2NaCl + H2O = CL2 + 2NaOH + H2
This system uses a two chamber holding tank containing (NaCl) in solution. The chambers are separated by a porous diaphragm or membrane. The positive electrode is in one chamber and the negative in the other. The diaphragm allows electricity and sodium ions to pass through but doesn't allow the chloride ions or water to pass. The chemicals produced at each electrode can not come into contact with each other. The chlorine gas that bubbles from the positive electrode chamber is drawn off and introduced to the pool or spa water. Once in the water the chlorine produces hypochlorous acid (HOCl) and hydrochloric acid (HCl). The hydrogen gas is often vented off into the air and is very explosive. A small portion of the caustic soda from the cathode chamber can be added to the pool to neutralize the acidic effects of the chlorine gas. In all "brine systems" the anode chamber must be periodically refilled with water and salt, and the cathode chamber must be periodically drained of caustic soda and refilled with fresh water. Some systems require distilled water to be used. Some systems use a sophisticated electronic indicator system that advises the user that maintenance is required while others rely on mechanical alarms.
During the summer season the cathode chamber needs to be drained off about every 4-6 weeks. Some models require that you drain the chamber to a jug. Others allow you to hook up a hose flushing system for disposal.
The caustic soda solution can be used for pH adjustments in the pool or spa or for other household uses such as cleaning drains or it can simply be disposed of. A typical brine chlorine generator designed for a 100,000 liter pool uses 25 kg of salt in the anode chamber. This must be replenished 2-4 times per year. Estimated salt consumption is about 40-100 kg per year.
This generator produces chlorine directly in to the pool or spa
from a low concentration of salt added to the pool water. (The
pool has salt added to it). Electrolysis takes place in an electrolytic
cell installed "in-line" in the recirculation system.
Thus the name. Inside the cell are layers of plates that are electrically
charged by a separate power supply. Depending on the generator
you need to maintain a salt concentration of about 2500-6000 parts
per million in the pool or spa for the unit to operate effectively.
This means adding 100-250 kg of salt per 40,000 liters of water.
As the salt water passes through the cell it is super chlorinated preventing the build up of chloramines and because the caustic soda is produced at the same time and is not removed or separated, it neutralizes the acidic condition produced from the chlorine gases. This means the pH of the water is not affected. To prevent scale build up on the plates the control unit may periodically reverse the charge on the plates to repel any build up that the opposite charge attracted.
Once the chlorine generated from the water does its job, it reverts back to salt and water to be used over and over again. So it is only necessary to add more salt to replace loss due to bather drag off, splash out, over flow and filter backwash.
Chlorine generators are easily automated. Measurement of the REDOX (ORP) potential is an indicator of the need for more chlorine. Electrolysis of Sodium Bromide works as well.
The costs of a chlorine generator are:-
Total dissolved solids (TDS) in water create a corrosive situation
even when the pH is in its ideal range. Pools should be partially
or completely drained when the TDS level reaches 1500 ppm above
the salt water level recommended by the chlorine generator manufacturer.
The pH has to be properly maintained as suggested by the manufacturer.
Prepared by Trevor Croll,
42 Pearse Street, Keperra, Queensland Australia, 4054.
Phone 61 7 3855 1115