The final processing and coating of metals lead to the production of undesirable by-products or waste, where emissions of gases, liquid and solid waste are included. The waste are produced mainly from organic solvents, ketones, aromatic hydrocarbons and acids, which are used in the preprocessing/cleaning stage, as well as from metals (in the form of dissolved salts) that participate in the coating stage.

Due to the toxicity of the contained components (heavy metals, cyanides, acids, bases), the liquid and solid waste from the coating industry are considered to be the most dangerous .

The liquid waste produced during the coating processes, comes from the rinsings that take place in the preprocessing stage and in the coating stage and from the used baths. Depending on the kind of the baths, acid or alkaline, it may contain high concentrations of metal ions and cyanides, oils, greases, solvents, various organic components and dispersed solids.

The solid waste that is met in metal processing industries, is mainly produced by the processes of coating and from the liquid waste treatment. The produced mud contains metals in the form of hydroxides and oxides, cyanides, greases, cleaning agents, etc. It should be noted that, due to the toxicity of many of the coating salts (eg cyanides) and the high concentration of heavy metals that the produced mud contains, the main problem of the solid waste is its treatment and its disposal .

Moreover, it must be added that in a metal processing industry there are also many sources of gases emissions . The most important emitted gases are those created during the chromium plating and the ones produced from the organic solvents that are used during the metal cleaning in the preprocessing stage. In addition, the use of nitric and nitrous acid and of hydrochloric acid steam while cleaning the metal surface, produces extremely dangerous wastewater.