TDS control with electrolytic hypochlorite generation

PWTAG Technical notes are updates or new material for the standards and guidance given in the PWTG book, Swimming Pool Water and the PWTAG Code of practice and should be read in association with these publications.

  • Subject: TDS control with electrolytic hypochlorite generation
  • Date: November 2014

Pools whose hypochlorite residual is generated in the plant room by electrolysing sodium chloride will tend to run with higher than normal levels of total dissolved solids (TDS). That should be allowed for, but also controlled. This note addresses the issue for the first time.

For conventional pools PWTAG’s guidelines recommend that TDS should not be allowed to rise more than 1,000mg/l above the source water (a figure that replaced a previously recommended absolute maximum of 3,000mg/l). This limit minimises corrosion, and also indicates that treatment chemicals are not being used in excess, to compensate for overloading.

This figure is unrealistic for pools using electrolysis. The figure now recommended is a maximum of 1,500mg/l above source water.


Pools in which hypochlorite is generated by electrolysis of sodium chloride in the pool water itself will necessarily operate with TDS levels well beyond anything dealt with in this note.

Both types of electrolysis demand attention to potential corrosion and to the safe venting of hydrogen. TDS is most reliably controlled by diluting with 30 litres of fresh water for each bather.