Dewatering and settling of activated sludges: The case for using cation analysis

Seven mixed liquor samples from full-scale activated sludge systems were analyzed for their settling and dewatering properties, and the calcium, magnesium, sodium, and potassium concentrations were measured. The soluble cation concentrations were evaluated in terms of the monovalent to divalent cation ratio and the calcium to magnesium ratio. The monovalent to divalent ratio was positively correlated to activated sludge dewatering properties. In batch tests, the addition of calcium or magnesium to samples with a high monovalent to divalent cation ratio improved the dewatering rate of the suspensions by less than or equal to 30%. Conditioning and dewatering experiments were performed on thickened sludges from two of the industrial samples. Calcium was added to each sample, in one case, to lower the monovalent to divalent cation ratio and in the second case, to increase the calcium to magnesium ratio. In each case, addition of calcium to the sample reduced the optimum polymer dose for conditioning by 30% compared with control samples with no added calcium. Laboratory reactor studies were performed using mixed liquor and wastewater from three of the activated sludge plants. Two of the systems were deficient in magnesium, and one system was deficient in calcium. In each case, the deficient cation was added to the feed of the reactors, and settling and/or dewatering properties improved. Addition of cations to the feed of two full-scale activated sludge systems improved the settling dramatically, and in one system, the thickened solids content was doubled. These results indicate that cation imbalances are a common cause of sludge settling and dewatering problems in industrial activated sludge plants, and these imbalances can be corrected by addition of the cation deemed to be deficient by analysis of the monovalent to divalent ratio or the calcium to magnesium ratio.