The physical-chemical treatment of waste consists of transforming waste by means of methods using chemical reactions or physical separation processes. The choice of treatment process should be adapted to the waste characteristics, content and elements contained; the main selection parameters are as follows:
- state of waste: liquid, sludge;
- type of pollutant contained (Chromium 6, cyanide etc.)
- nature of organic or mineral pollution;
- pH of the solution.
The main processes are as follows:
- chemical cracking/ultrafiltration: applies to organic waste and makes it possible to treat oil emulsions and water-oil mixtures. The principle is to separate the oil phase from the water phase by using an emulsion breaker or semi-permeable membranes.
- centrifugation: mostly applies to organic waste and makes it possible to separate water from hydrocarbons in certain oil emulsions.
- neutralization: neutralises an acid or basic mineral solution, a pre-requisite for the precipitation of the metals contained.
- precipitation: in mineral solutions, precipitates the metals contained in a solution in the form of hydroxide sludge, by adding lime wash or caustic soda. This sludge is then dewatered (press or dryer) to be treated or stored.
- dechromatation: a chemical reduction reaction enabling the transformation of highly toxic hexavalent chromium into low-toxicity trivalent chromium, which is then precipitated in the form of hydroxide sludge.
- cyanide removal: a chemical reaction oxidising toxic cyanides into cyanates; the solution obtained is then treated by traditional neutralisation/precipitation processes.
- mechanical dewatering: concentrates sludge by extracting part of the aqueous phase. It generally includes an initial settling phase, which reduces the liquid volume, and is followed by pressing (filter presses, belt presses, vacuum filters) or centrifugation.