It includes to those processes that place at the entry of the treatment plant to eliminate solid residues, sands and fats, which of not being separated would damage mechanically the equipments of the following procedural steps and to settle in the pipelines and conduits of the installation, obstructing them or they would produce loss of efficiency (p.e. fats in the biological reactor). In all the processes there are eliminated the pollutants of the residual water by physical means.
REFINE
It is an operation in which the solid ones of major size of the residual water are eliminated. The water makes pass for gratings or sieves and it has as aim separate all those materials of excessively thick size that beside representing for yes a form of pollution (solid in suspension), they can damage or prevent the successive phases of treatment. 
The process of sand elimination is in use for separating the sand, gravel, etc., dragged in suspension for the unfluent one. This sand originates deposits in channels and pipelines, graze and wear on the mechanical components in movement and impedes the elimination and digestion of the muds separated in the tanks of sedimentation, on having increased its density.
FATS ELIMINATION
The fats elimination consists of the separation of the fats and oils dragged by the residual water. The fats in the waste water make numerous problems in the process of purification.
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Adhesion to devices, conduits or deposits, impeding the purification.
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Obstruction of the thin grids.
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Formation of a cap, in the surface of the decanters, which impedes the sedimentation on having attracted up small particles of organic matter.
- Impedes the correct aeration in the purification of active sludges.
HOMOGENEIZATION
The most common situation is that the entry of residual water to the treatment plant is changeable in the time, so much regarding wealth as to pollutant load. These variations, specially the unloads, impede the correct development of the treatments, already be of type physicist - chemist or biological.
