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Hydraulic Residence Time Effects on Performance of an Activated Sludge Unit Treating Wastewater Containing Dichlorophenol

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Wastewaters containing chlorophenol compounds are difficult to treat by biological means because of the toxic effects of those compounds on microorganisms. To investigate the adverse effects of chlorophenols on microorganisms, synthetic wastewater containing 2,4 dichlorophenol (DCP) was biologically treated in an activated sludge unit at different hydraulic residence times (HRTs) between 5 and 40 hours, whereas the feed chemical oxygen demand (COD), DCP concentrations, and sludge age were kept constant at 2500 ± 50 mg/L, 150 mg/L, and 20 days, respectively. The resazurin method based on dehydrogenase activity was used for assessment of the feed and effluent wastewater toxicity. Percent COD, DCP, and toxicity removals increased, and the effluent COD, DCP, and toxicity levels decreased with increasing HRT. Biomass concentration in the aeration tank increased with increasing HRT because of low levels of DCP at high HRT levels, resulting in high COD, DCP, and toxicity removals. The sludge volume index decreased with increasing HRT, yielding well-settling organisms as a result of low levels of toxicity and high concentrations of active cells. Percent DCP and COD removals decreased with increasing specific DCP loading rate. The rates of DCP and COD removals showed a maximum at a low DCP concentration of 6 mg/L in the aeration tank, corresponding to a 25-hour HRT.
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Keywords: 2,4 dichlorophenol; activated sludge; hydraulic residence time; toxicity

Document Type: Research Article

Publication date: 2006-07-01

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    Water Environment Research (WER) publishes peer-reviewed research papers, research notes, state-of-the-art and critical reviews on original, fundamental and applied research in all scientific and technical areas related to water quality, pollution control, and management. An annual Literature Review provides a review of published books and articles on water quality topics from the previous year.

    Published as: Sewage Works Journal, 1928 - 1949; Sewage and Industrial Wastes, 1950 - 1959; Journal Water Pollution Control Federation, 1959 - Oct 1989; Research Journal Water Pollution Control Federation, Nov 1989 - 1991; Water Environment Research, 1992 - present.
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