Thermophilic aerobic biological treatment of bleached kraft pulp mill effluent and its effect on floc formation and settleability

With system closure, pulp mills are producing warmer and more concentrated effluents which would be more economically reused if treated thermophilically. The objective of this study was to investigate the feasibility of thermophilic aerobic biological treatment of bleached kraft pulp mill effluent (BKME) with particular attention to the effect of temperature on settling. These objectives were achieved by operating four parallel sequencing batch reactors over 60 weeks in mesophilic (35°C) and thermophilic (45, 55 and 60°C) conditions in three separate experimental runs. All SBRs were inoculated with mesophilic sludge obtained from mill scale activated sludge reactors and the temperatures of the reactors were raised slowly to give enough time for acclimation of the sludge.;Aerobic thermophilic biological treatment of BKME is feasible and optimal treatment occurred near 45°C. Chemical oxygen demand (COD) removal varied from 40--76% with averages of 75 +/- 10%, 73 +/- 10%, 62 +/- 9% and 63 +/- 10% for SBRs operating at 35, 45, 55 and 60°C, respectively. The COD removal was significantly higher at 35 and 45T in comparison with 55 and 60°C. All SBRs reached their respective recalcitrant portion of COD within the first 12 hours and followed a first order removal rate with a higher recalcitrant portion at higher temperatures. Chemical analysis of feed and effluents showed higher removal of long chain fatty acids at thermophilic conditions in comparison to the mesophilic conditions. There were no statistically significant differences in adsorbable organic halogen, biochemical oxygen demand, lignin, colour and toxicity removal at all operating temperatures.;Microbial flocs developed at temperatures ≥55°C were pinpoint, and were responsible for an increase in suspended solids concentrations in the treated effluents from 20 to 80 mg/L. The sludge volume index results varied from 50--200 mL/g at different temperatures with a at 45°C. The yield of extracellular polymeric substances varied from 55 to 70 mg/g of MLSS for mesophilic and thermophilic operations. Carbon substrate utilization profile results indicated different microbial communities at different temperatures. Higher life forms completely disappeared at 55°C and above. Prolonged anoxic filling, pure oxygen and phosphorus limiting conditions did not improve settling at 55°C.;Five filamentous bacteria: Type 021 N, Type 0803, Type 0092, Type 1863 and Nostocoida limicola were identified at 35°C; their abundance varied with temperature. With the exception of Type 021 N, all other types were also present during thermophilic operation.