Sodium Hypochlorite Generators are the heart of each Electro-chlorination Package. Each Generator matches with a dedicated Transformer/Rectifier unit. The modules of each Generator are connected in electrical series to the Rectifier. The modules are also hydraulically connected in series. In fact, feed seawater passes through the modules, where it is electrolyzed.

Each generator work with a constant feed seawater flow-rate. This flow-rate is kept constant by the flow loop controller (magnetic flow transmitter + flow control valve) installed on the inlet line of each generator. Electrolytic cells are bipolar in design. This means that the cathodes of every cell are directly connected to the anodes of the next cell.

The electrolyzers operate under constant seawater flow rate while the D.C. current is adjusted so that the generation of chlorine instantaneously matches the water treatment demand. Finally, sodium hypochlorite solution and by-product hydrogen, in mixed phase, are sent through Generators outlet header to the sodium hypochlorite storage and hydrogen dilution system. 

Chlorine is generated at the anode:

2 Na+ + 2 Cl- → Cl2 + 2 Na+ + 2e-

Hydrogen is evolved at the cathode with the corresponding formation of hydroxyl ion:

2 H2O + 2e- → 2 OH- + H2

The overall electrochemical reaction is:

2 Na+ + 2 Cl- + 2 H2O →Cl2 + H2 + 2 Na+ + 2 OH-

Then chlorine and hydroxyl ions react chemically producing sodium hypochlorite and sodium chloride, as follows:

2 Na+ + 2 OH- + Cl2→ Na+ + ClO- + Na+ + Cl- + H2O

The overall electrochemical reaction can be expressed as follows:

NaCl + H2O → NaClO + H2

Side reactions, both chemical and electrochemical, take place simultaneously with the basic reactions, such as the decomposition of hypochlorite to chloride, the anodic oxidation of hypochlorite to chlorate (traces only) the cathodic reduction of hypochlorite to chloride and the anodic evolution of oxygen. All these side reactions affect the current efficiency so that D.C. power required to produce hypochlorite is actually higher than the theoretical one.

Moreover, some cations which are present in seawater (e.g. Calcium, Magnesium) form hydroxides and carbonates resulting in suspended solids, which are mostly carried out of the electrolyzers by the chlorinated seawater stream. However, due to deposit formation on the electrode surface, the electrolyzers must undergo a periodic cleaning with diluted hydrochloric acid (6 % w/w). Best frequency of acid washing, for a given plant site, can be validated only on the basis of the field experience.