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Effects of high operating temperatures, hydraulic retention time and solids residence time on activated sludge treatment of kraft pulping effluent Barr, Tanya Alexandra


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. This research was conducted as four separate experiments using two 5 litre continuously fed bioreactors. Each unit consisted of a bioreactor and a clarifier which were fully automated, using pumps and timers, to control feed, waste and recycle rates. Feed consisted of weekly shipments of primary clarified effluent (PCE) from Western Pulp's (WP) Squamish pulp mill. Six standard assays were routinely performed in order to assess treatment performance and efficiencies. These assays included biochemical oxygen demand (BOD), chemical oxygen demand (COD), volatile suspended solids (VSS), specific oxygen uptake rate (SOUR), Microtox - toxicity test, and adsorbable organic halides (AOX). The first experiment was start up and steady state operation of a bioreactor. The unit was run at 35°C for a period of two months in order to attain results which could be used as a set of baseline data for comparison purposes for the three following studies. The HRT and SRT were 10-12 hours and 12-15 days respectively. The assay results indicated treatment performance was very comparable to full scale pulp mill AS treatment facilities. The second experiment studied the effects of varying HRT and SRT on treatment efficiencies. Nine different operating conditions were examined based on a 2x3 factorial design method. HRT was varied between 12, 8 and 4 hours, while SRT was varied from 15, 10, 5 days. The results from this study indicated that HRT had more of an effect on treatment performance than SRT. Longer HRTs led to improved BOD, COD, toxicity and AOX removal, while longer SRTs were not shown to significantly affect the removal of these. Shorter HRTs and longer SRTs led to significant increases in SOURs. The third experiment studied the effects of increased operating temperatures on bioreactor performance. Operating temperatures were increased 1°C every 1-2 weeks from 41°C up to 50°C over a period of four months. BOD, COD, toxicity and AOX removal were within normal operating parameters as determined by the steady state study. SOURs were somewhat lower than the rates at 35°C, however microbial activity was well within an acceptable range. The fourth and final experiment analysed the effects of induced temperature shocks on the AS system. Four separate temperature decreases occurred from a baseline temperature of 50°C over a period of five weeks. These decreases were 7°C, 16.5°C, 32°C and 40.5°C. Each temperature decrease lasted 8-10 hours, after which the temperature of the bioreactor was returned to 50°C. Immediately after each temperature shock the same six assays, used in the previous experiments, were performed on the unit. Analysis was performed prior to the temperature shock and then just after the shock at periods of 1, 6, 12, 24 and 72 hours. Results indicated that smaller temperature shocks had no detrimental effects on treatment performance, while the larger shocks did. For the two smaller temperature shocks (7°C and 16.5° C), recovery from the effects occurred within 12-24 hours. Approximately 72 hours was needed for the system

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