The European paper industry produces 70 million tons of paper annually, being one of the most water consuming industrial processes. The reduction in water use leads to partially recirculated water into industrial processes. This results in quality deterioration and a drop in productivity. Because of this, the socalled zero effluent operation has only been applied in a few papermills around Europe. Existing technologies usually operate within the mesophilic range. However, most of the mills work at a 50-70°C water circuit temperature range due to the positive effects on productivity and energy consumption. In this paper, a thermophilic aerobic treatment (55°C) combined with ultrafiltration is tested, this combination reaches mesophilic COD removal rates. Moreover, problems with water settleability under high temperatures are now avoided and the suspended solids free effluent gives water more possibilities of being reused.

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