Exemples d'élimination industrielle des suspensions solides en vue du recyclage

Examples of industrial separation of suspended solids for recycling

Ingénieur Civil des Mines, Chef de Département Société Degrémont

Abstract

Once he is familiar with the polluants introduced into the water by the process the water treatment expert is able to eliminate them and return water of good quality to the process. Pollution is subdivided into four major categories (Table 1) in an attempt to schematize the principal methods and instruments required to eliminate pollutant. As a general rule, recycling is limited by economical rather than technical factors. This is illustrated by the three following examples :

TREATMENT OF STEAM GENERATOR CONDENSATE

For this application, recirculation of very pure boiler circuit (turbine) water costs much less than surface water treatment. The most suitable methods for various input products corrosion products and/or dissolved salts) are discussed, e.g. pre-coat fillers, magnetic fillers, mixed beds.

FIBRE AND WATER RECOVERY IN THE PAPERMAKING INDUSTRY

The amount of water the paper mills draw from the natural medium has to be limited and the maximum possible quantity of valuable effluent constituents recycled (fibres alone or with fillers depending on manufactured paper properties). Two specific experiments show how "purification circuits" can be adapted to the problem. Diagram 3 shows the two processing stations used, both of which allowed recovery of the fibre and all or part of the fillers with approximately 75 per cent of the water. Methods employed were flocculation followed by sedimentation, flotation or filtration according to the products to be eliminated, with special consideration of the fibre/fillers ratio. Table 4 shows how concentration in the circuit varies with gradual closure of the latter. An increase in the concentration of dissolved organic and mineral matter is seen to limit possible circuit closure.

RE-USE OF WATER AFTER BIOLOGICAL TREATMENT

Re-use of water can be a particularly attractive proposition for such "routine" proposes as cooling and soil washing. This provides users with substantial local water supplies at low cost after conventional domestic sewage treatment, adequate filtration and sterilization. More "noble" recycling applications on the other hand require sophisticated equipment for such chemical and physical processes as phosphate precipitation, elimination of ammonia, efficient, thorough filtration, also adsorption on activated carbon followed by sterilization. The water thus obtained has similar properties to those of drinking water and is undoubtedly better than most of the surface water available in industrialized countries.