Closed Water Circuit Paper Mills
A closed water circuit is achieved when no wastewater leaves the paper mill. The process water is then almost 100 % utilized. Only evaporation and vaporization losses and the water content in the paper and in the residual matters must be replaced by fresh water (1–2 L (kg paper)–1). A closed water circuit is shown sche¬matically in Fig. 10.4.
The essential requirement for reusing circulating water in the tertiary circuit is adequate save-all clarification. The quality of the clarified water must be suitable for the operation of sensitive systems (e. g. showers, trim showers). There must be enough storage capacity to compensate for variations in quality caused by disturbances and interruptions in production. Until now, closed circuits have been realized only in paper mills processing recovered paper. In Germany about ten recovered paper processing mills have closed the process water circuit totally.
Most are small paper mills producing corrugating medium and test-liner
10.2 Solid Waste
.(a) Paper machine, (b) stock preparation plant,
.(c) storage, (d) save-all, (e) fresh water,
(f ) high density stock, (g) clarified water.
In a new development the aerobic activated sludge plant was replaced by a com¬pact aeration reactor, in order to reduce the odor of the anaerobic treated process water and to eliminate CaCO3, which might cause problems when the biologically treated water is led back into the papermaking process