Investigation into combined ozone and biological treatment of pulp bleaching effluent : a thesis submitted in partial fulfillment of the requirements for the degree of Master of Technology in Environmental Engineering at Massey University
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Date
1999
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Massey University
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Abstract
An investigation into combined ozone and biological treatment of pulp bleaching plant effluent was conducted. Treatment efficiencies were evaluated in terms of color, COD and BOD
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removal. The effectiveness of ozone oxidation and subsequent biological treatment of pulp bleaching effluent were examined separately and the overall color, COD and BOD
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removal through the two-stage combined treatment were determined. Ozone pretreatment was carried out in a vertical column batch reactor under a constant ozone flowrate 5 L/min condition. Changes of color. COD and BOD
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in pulp bleaching effluent during ozonation process were recorded. The subsequent biological treatment was investigated in two lagoon systems. One was an anaerobic-aerobic lagoon system and the other was an aerated lagoon system. The separate contribution made by each zone of the anaerobic-aerobic lagoon to the overall effluent treatment was evaluated. To assess the effect of the ozone pretreatment on the followed biological treatment, the ozonated bleaching effluent and the non-ozonated raw bleaching effluent were parallelly operated in identical biological systems. Comparison of results obtained from treatment of the ozonated and non-ozonated effluent identified the improvement of a two-stage combined treatment over a biological treatment alone. Results obtained from ozone treatment of two batches of Eo and mixed (Eo and DC) bleaching effluent indicate that ozone was most effective in color removal (up to 74% measured at pH 7), followed by BOD
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increase (up to 39%) and lesser effective in COD removal (up to 19% only). A color removal formula was developed to model color removal kinetics. The mathematical formula succinctly describes the color removal performance and offers an alternative option to study color removal kinetics during ozone treatment of pulp bleaching effluent. Because of the ozone pretreatment, the effectiveness of the subsequent biological treatment for COD and BOD
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removal was improved. However, when the followed biological system included an anaerobic zone, a considerable color increase (98%) in the ozonated effluent was observed during the treatment. If the followed biological treatment was carried out under an aerobic condition only, the color increase in the ozonated effluent was very small (21%). This observation suggests that biological treatment of ozonated effluent should avoid involving an anaerobic condition, otherwise the color removal achieved during the ozone treatment would be lost in the subsequent biological stage. It would obviously be economically infeasible. The combined ozone oxidation and biological treatment regime improved the overall color removal (34-68%), COD removal (45-51%) and BOD
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removal (82-95%) over a single stage biological treatment which only achieved up to 17% color removal, 30-35% COD removal and 64-92% BOD
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removal. For removal of COD and BOD
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, the combined ozone with anaerobic-aerobic lagoon treatment outperformed marginally the combined ozone with aerated lagoon system. However, for color removal, the efficiency of the combined ozone with aerated lagoon treatment was much higher (68%) than that of the combined ozone with anaerobic-aerobic lagoon treatment (43%). The anaerobic zone of the anaerobic-aerobic lagoon was identified as the main sources of color increase and limited the overall color removal for such a combined treatment. In summary, the combined ozone with aerated lagoon system was the better option for treatment of pulp bleaching plant effluent.
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Keywords
Factory and trade waste, Purification, Sewage, Ozonization, Biological treatment, Wood-pulp industry -- By-products