Toxicity of chemi-thermomechanical pulping (CTMP) wastewaters was tested in batch assays and in anaerobic sludge blanket reactors. In batch serum bottle tests particulate constituents were responsible for 80-90% of the inhibition of acetoclastic activity, and the soluble (fines-free) fraction accounted for 10-20%. Performance of two 2-stage anaerobic sludge blanket reactor systems was compared, one receiving fines-free (NF) and the other unaltered (F) CTMP waste for 140 days. Both reactors became acclimatized to CTMP and developed a tolerance to resin acid concentrations as high as 300-1500 mg·1−1 in the sludge bed. The fines-free reactor was superior throughout, but the differences in performance did not reflect the batch test results. At the organic loading rate of 17-22 g soluble COD.g VSS−1·d−1 NF treated 100% CTMP waste with 42% and F with 38% removal efficiency. Fines accumulation in the sludge bed of F reactor caused a 70% increase in bed volume, resulted in deterioration of sludge settleability, and led to an increased likelihood of biomass loss.

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