The following processes in the metal industry produce wastewater:

- electroplating                                                   - printed circuit manufacture

- picking                                                             - battery manufacture

- anodizing                                                          - enamelling

- burnishing                                                        - mechanical workshops

- galvanization                                                    - grinding

- annealing                                                          - painting

Several production areas need acids, alkalis and various salt solutions. Toxic material that some of these may contain must be removed. Such materials are:

Cadmium, mercury (in battery manufacture), ammonia, compounds, chlorine, chromium, cyanide (in the manufacture of printed circuit boards), fluorine, lead, copper, nickel, nitrites, sulfides, zinc, carbohydrates, cobalt (in enamelling).

Cyanide is perhaps the most toxic of the inorganic materials that may be found in wastewater. Oxidation to practically non-toxic, soluble cyanates removes the danger. Sodium hypochlorite and iron-II-sulfite are the oxidizers used.

Six-valent chromium must be reduced to the three-valent form, as only this can be precipitated as the insoluble hydroxide in the next, neutralizing stage.

Continuous supervision and regulation of the pH and the redox value are important in neutralization and sedimentation just as in decontamination, whether by reduction or by oxidation. Combination pH/redox electrodes should be protected from poisons and other contaminants such as bridge electrolytes, Orbsint, Sensopac. Recycling part of the wastewater supplements conventional treatment. Other techniques used are micro filtration and ultrafiltration, reverse osmosis and ion exchangers. Conductivity measurements control the latter two.