Effect of HRT, SRT and temperature on the performance of activated sludge reactors treating bleached kraft mill effluent

Laboratory scale research on the effects of hydraulic retention time (HRT), solids residence time (SRT), high operating temperatures and temperature shocks on activated sludge (AS) treatment of kraft pulping effluent was performed using two 51 continuously fed bioreactors. Baseline performance of the reactors was established at 35 degrees C by operating the reactors at steady state (HRT 10-12 h; SRT 12-15 d) for a period of two months. During this period percent removal of BOD, GOD, and toxicity averaged 87.9 +/- 4.3, 32.4 +/- 9.0, 97.7 +/- 0.4, respectively. Reactor MLVSS was 1675 +/- 191 mg/l, effluent VSS was 45.5 +/- 11.2 mg/l and specific oxygen uptake rate was 16.5 +/- 3.3 mg O-2/g MLVSS . h. Varying HRT between 12 and 4 h and SRT between 5 and 15 d indicated that HRT had more of an effect on treatment performance than SRT. Longer HRTs led to improved BOD, GOD, toxicity and AOX removal, while longer SRTs were not shown to significantly affect performance. Shorter HRTs and longer SRTs led to significant increases in specific oxygen uptake rates (SOURs). For reactors operated at temperatures between 41 and 50 degrees C, removal of BOD and acute toxicity was comparable to that observed at mesophilic temperatures. COD removal was improved over that observed at mesophilic temperatures, possibly as a result of improved dissolution of organic compounds at the higher temperatures. The effect of temperature shocks (decreases of 7 degrees, 16.5 degrees, 32 degrees and 40.5 degrees C) on reactor performance was proportional to the size of the disturbance. Reactor performance returned to pre-shocking levels within 12-24 h for the two smaller temperature shocks. Approximately 72 h was needed for the system to recover from the two larger temperature shocks (32 degrees and 40.5 degrees C).