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The effect of cationic salt addition on the settling and dewatering properties of an industrial activated sludge

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Cations have been found to influence the settling and dewatering properties of activated sludge, especially for industrial wastewaters in which very high concentrations of monovalent cations are often found. Because the cation content of wastewaters is often influenced by upstream processes, an understanding of the influence of various cations can be an important consideration in pinpointing operational problems in wastewater treatment plants. An industrial activated-sludge treatment plant was studied to determine whether variations in settling and dewatering properties of the mixed liquor and waste solids could be caused by changes in the cation content of the wastewater. Laboratory reactors received both return activated sludge and wastewater from the industrial treatment plant, and the feed was supplemented with either sodium (Na+), potassium (K+), or magnesium (Mg+). It was found that when the monovalent-to-divalent (M:D) cation ratio on a milliequivalent basis was increased to greater than approximately 2:1 by either Na+ or K+ addition, dewatering properties became poorer and polymer conditioning requirements increased. The soluble protein content also increased as the mixed liquor M:D ratio increased, indicating release of biopolymer from the flocs. Magnesium addition at low doses caused a decrease in the dewatering rate, but at higher doses both the settling and dewatering properties of waste solids improved substantially. Conversely, when Na+ and K+ concentrations in the raw wastewater decreased significantly to less than 10 and 0.1 milliequivalents (meq), respectively, settling and dewatering properties improved substantially and addition of Mg2+ did not improve conditions beyond those of the unamended control. Each of the cations studied caused unique changes in the properties of activated-sludge solids that could not be correlated with the M:D ratio, suggesting that some of the effects are not simply physical/chemical but may be physiological as well.


Document Type: Research Article


Publication date: July 1, 1998

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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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