Comparison of mesophilic and thermophilic aerobic biological treatment in sequencing batch reactors treating bleached kraft pulp mill effluent

With system closure, mills are discharging warmer and more concentrated effluent which would be more economically reused if treated thermophilically. The influence of high temperature biological treatment was studied in four parallel sequencing batch reactors over 40 weeks in mesophilic (35°C) and thermophilic (45, 55 and 60°C) operation. All SBRs were inoculated with mesophilic sludge obtained from mill scale activated sludge reactors and the temperature of the reactors were raised slowly to give enough time for acclimatization of the sludge. The COD removal percentage varied from 40–76% with steady-state averages of 75±9.7%, 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 45°C in comparison with 55 and 60°C. AOX removal (20–75%) decreases with increasing temperatures, although the differences are not statistically significant between mesophilic and thermophilic operation. Chemical compound analysis of feed and various treated effluent showed a better removal of long chain fatty acids at thermophilic (60°C) conditions in comparison to the mesophilic (35°C) conditions. Smaller pinpoint flocs were observed in the 55 and 60°C SBRs, which are responsible for an increase in suspended solids concentrations in the treated effluents from 20 to 80 mg/l. The sludge volume index (SVI) results varied from 50–150 ml/g at different temperatures with a minimum at 45°C, thereby indicating that, in terms of settling characteristics, operation at 45°C was optimal.