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 tem perature shocks on activated sludge (AS) treatment of kraft pulping ef fluent was performed using two 51 continuously fed bioreactors. Baseli ne performance of the reactors was established at 35 degrees C by oper ating the reactors at steady state (HRT 10-12 h; SRT 12-15 d) for a pe riod of two months. During this period percent removal of BOD, GOD, an d toxicity averaged 87.9 +/- 4.3, 32.4 +/- 9.0, 97.7 +/- 0.4, respecti vely. Reactor MLVSS was 1675 +/- 191 mg/l, effluent VSS was 45.5 +/- 1 1.2 mg/l and specific oxygen uptake rate was 16.5 +/- 3.3 mg O-2/g MLV SS . h. Varying HRT between 12 and 4 h and SRT between 5 and 15 d indi cated that HRT had more of an effect on treatment performance than SRT . Longer HRTs led to improved BOD, GOD, toxicity and AOX removal, whil e longer SRTs were not shown to significantly affect performance. Shor ter HRTs and longer SRTs led to significant increases in specific oxyg en 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 o f improved dissolution of organic compounds at the higher temperatures . The effect of temperature shocks (decreases of 7 degrees, 16.5 degre es, 32 degrees and 40.5 degrees C) on reactor performance was proporti onal to the size of the disturbance. Reactor performance returned to p re-shocking levels within 12-24 h for the two smaller temperature shoc ks. Approximately 72 h was needed for the system to recover from the t wo larger temperature shocks (32 degrees and 40.5 degrees C).