In-mill thermophilic treatment of individual wastewater streams to achieve water system closure has received much attention in pulp and paper mills. Aerobic biological treatment of thermomechanical pulping (TMP) condensate was conducted using thermophilic (55°C) and mesophilic (35°C) sequencing batch reactors (SBRs) for a period of 143 days at a cyclic time of 6, 8 and 12 h. A soluble chemical oxygen demand (SCOD) removal efficiency of 77 to 91% was achieved, given an organic loading rate of 0.7–1.3 kg/m3 d. The COD removal efficiency of the thermophilic SBR was slightly lower than that of the mesophilic SBR. Majority of the soluble COD was removed by biodegradation with a small portion (9–13%) of soluble COD stripped by aeration. The settleability (sludge volume index) and the flocculating ability (effluent suspended solids) of thermophilic sludge were comparable to or slightly poorer than that of the mesophilic sludge. The level of filaments in thermophilic sludge was usually higher than that in mesophilic sludge. The results of the study indicate that both thermophilic and mesophilic SBRs can be successfully operated for in-mill treatment of TMP condensate. The treated effluent has the potential for subsequent reuse in the mill.

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