Séparation gravitaire de l'eau et des suspensions solides

Gravity separation of water and solids in suspension

Ancien Elève de l'Ecole Polytechnique Docteur ès-Sciences, Chargé de la Recherche Soeiété Degrémont

Abstract

Water recycling in industry is not basically different from water purification for human consumption or industrial purposes. The first phase, if not the only phase of the purification process, consists in the removal of suspended solids. Average particle size varies considerably even for a given type of pollution (see Table 1) and natural elimination by gravity takes too long below a certain particle size. Pollution by suspended solids may be characterized as follows : - a) by content, i.e. suspended solids concentration ; b) by turbidity. Units have been determined for each case, as well as measurement methods whereby process efficiency can be established. Solid particles are extracted by two different methods : - By gravity : flotation or sedimentation ; - By filtration or straining (distinction between macro, micro and ultra-filtration). These methods are unsuitable for direct elimination of colloids, for the following reasons : -Retention times are inconsistent with equipment operating conditions ; - Prohibitive cost. The elimination or colloids requires preliminary action to eliminate stability factors and allow collection of much coarser colloid particles for subsequent separation from the water by the above methods. Tests have been devised for establishing how to best deal with colloids. Two main types of test are electrophorelic measurement and flocculation tests. As pollution is often at least partly colloidal, most purification plants must include a flocculation-coagulation stage followed by one or more separation systems. As regards sedimentation, technological developments have resulted in the design of such widely-used compact units as sludge recirculation and sludge-bed equipment whose efficiency is such that filtration has often become the final process completing clari-flocculator action.