Thermophilic (55°C) anaerobic removal of chlorinated phenolic compounds (CPC) from softwood bleaching effluents was studied using four different types of anaerobic processes: an upflow anaerobic sludge blanket (UASB) reactor (R1); a UASB reactor enriched with sulfate (R2); a UASB reactor with recirculation (R3); and a fixed bed reactor with recirculation (R4). The concentrations of 16 CPCs were analyzed by gas Chromatography. In all processes most of the chlorinated phenols, catechols, guaiacols, and hydroquinons detected in the bleached kraft mill effluent (BKME) were either eliminated or reduced by as much as 80-95%. However, 2,4-DCP, 2,6-DCP, 4,5-DCG, 3,4,5-TCC, and TeCC were accumulated or only partially removed in all reactors, except in R4 where at high concentrations they were significantly reduced. All the reactors removed 30-70% of the COD and 25-67% of the adsorbable organic halides (AOX). The reactors with recirculation had slightly higher removal efficiencies than the other reactors, while the addition of sulfate had no apparent effect. High concentrations of BKME reduced the COD removal efficiency but not the CPC removal.

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