Procédés employés pour le recyclage de l'eau en papeterie

Water recycling in the papermaking industry

Ingénieurs à la Société Alsthom, Division Neyrpic Groupe " Techniques des Fluides ", Grenoble

Abstract

Paper mills discharge two distinct types of effluent : water containing cellulose waste and water from the machines. It is for the latter, which contains relatively little oxidizable matter, that recycling problems arise. A further distinction must also be made between mills using fresh (frequently imported) pulp and mills using waste paper stock. Water quality requirements vary considerably according to use, e.g. in stuff chests, for wire-cleaning, in boilers, and between production and auxiliary circuits. Water economy is primarily associated with minimal loss of solids and should therefore be sought first of all in the actual manufacturing process. Substantial economy can be achieved by use of circuits operating "in cascade" at gradually decreasing rates of flow. Internal recycling with local partial treatment is already in use, but should be more generally applied (see Fig. 1). Thus, the effluent is either processed as a whole, or each of its components by a different method using different equipment. Flocculation can be resorted to in various processes, e.g. multi-plate, simple or floc-bed sediment trap. Pre-coat filters, flotation equipment and rapid-action sand filters are also suitable. The following examples of high-performance equipment for present or future use are discussed ; - Waco pre-coat filters (Fig. 3); -Seelar floc-bed separators (Fig. 2). Recycling by a single station at which all the effluent is processed require high-performance equipment as regards total suspended matter, COD, BOD and colour. Optimalization studies of circuits and processes are almost invariably necessary for efficient recycling system design. Such studies are carried out stagewise in close collaboration with the factory management and independent water processing and papermaking specialists. Contact must also be maintained with the regional finance authorities for all matters relating to cost and pollution. The following are typical design factors : - a) Individual equipment : Reduction of oxidizable and suspended matter and colour, buffer effect, discharge, local arrangements, cost, reagent and power requirements, maintenance and supervision, down-times, etc. b) The overall plant : Modification of sewers, circuits, head losses and water-hammer, COD and BOD control in closed circuit operation, fermentation, smells, slimes, corrosion, temperature rise limits, combination with higher production rates, etc. c) Non-technical factors : Modification of end products, marketing variations. Final conclusion : Efficient recycling depends very closely on efficient clarifier operation combined with sound overall design.