Laboratory and field tests were conducted on activated sludge from an industrial wastewater treatment plant in order to monitor the settling and dewatering properties and to assess the impact cations may have on these properties. The influent to the wastewater treatment plant contained a high concentration of sodium ions and a low concentration of divalent cations. The sludge exhibited poor settling and dewatering properties. Initial laboratory results indicated an improvement in settling and dewatering properties through the addition of calcium and magnesium. After addition of magnesium during field trials, floc density and settling properties improved considerably. In addition, residual ammonium ions in the mixed liquor appeared to interact with the activated sludge flocs to influence their dewatering properties. It was observed that an increase in ammonium ion in the soluble sludge fraction was related to deterioration in the dewatering properties. During these trials, the ammonium ions demonstrated a greater influence on dewatering properties than did the magnesium ions. The tests conducted at the treatment plant revealed that complex interactions between cations and sludge influenced the settling and dewatering properties in a manner that depended on ratios and concentrations of monovalent and divalent cations in the activated sludge feed and solution.

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