In the past few years, the paper industry has been able significantly to reduce the specific amount of effluent it produces. It is considered that the standard now achieved is not susceptible of improvements without a loss of product quality. The problem substances imported with the raw materials will tend rather to increase the specific amount of effluent produced by paper mills that process waste paper, and it will only be possible to operate completely closed circuits in exceptional cases. Integrating a biological purification plant into the Whitewater circuit can help to solve this problem, which is specific to waste paper. Operating a purification plant in the half-stuff production sector makes it possible to achieve substantially lower quantities of specific effluent even in integrated paper mills if the water circuits for half-stuff production and for the papermachine are kept largely separate, and if the freshwater intake is effected via the papermachine's circuit. Possibilities of reducing the specific quantity of effluent produced in wood-free production processes are not discussed, since the use of biological plants in this sector appears problematic for a variety of reasons. The use of anaerobic technology - with appropriate subsequent treatment if necessary - appears suitable because of its favourable energy balance. The possibilities and limits of using anaerobic technology for treating highly concentrated effluents from paper mills are largely known. The use of this technology in the Whitewater circuits of paper mills is still in the planning stage. Simple model calculations can be used to estimate the effects of operating a purification plant on the equilibrium concentrations in the white-water circuit and to detect possible problems.

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