Thermomechanical pulping whitewater was treated in an anaerobic followed by an aerobic Kaldnes moving bed biofilm reactor at 55°C. The anaerobic reactor was mixed by gas circulation and the aerobic reactor was mixed by aeration. The anaerobic reactor was started with mesophilic inoculum, while the aerobic reactor was started without inoculation. The reactors were operated on molasses water for one and a half years before the experiment was started. Total biomass concentrations (suspended and attached) were 3.3 g VSS/L in the anaerobic reactor and 1.6 g VSS/L in the aerobic reactor when starting feeding the reactors with TMP whitewater. After 7 months of operation the biomass concentrations had reached 5.5 and 6.5 g VSS/L in the anaerobic and aerobic reactors, respectively. The CODsol removals in the anaerobic reactor were around 30 % at loading rates up to 7 kg CODsol/m3d, and over-all CODsol removals of about 60 % were achieved. The results show that the anaerobic-aerobic moving bed biofilm process could be feasible for the thermophilic treatment of thermomechanical pulping whitewater.

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