The effect of cations on the settling and dewatering of activated sludges: Laboratory results

The effect of cations on the settling and dewatering of activated sludge was investigated using laboratory scale activated sludge reactors. Three parameters were investigated: the effect of the calcium to magnesium ratio; the effect of increasing divalent cation concentrations; and the effect of increasing monovalent cation concentrations. The study showed that the settling and dewatering properties of the activated sludge were dependent on both the concentrations and ratios of cations in the feed. Some activated sludge systems required both calcium and magnesium in the feed while others only required one of these cations for good settling and dewatering properties. A minimum of 0.7-2.0 meq/L each of calcium and magnesium was necessary for acceptable settling and dewatering. Increasing the feed concentrations of calcium and magnesium above this level improved flee strength, settling and dewatering properties and increased the bound protein concentration. The poor settling and dewatering of suspensions receiving low divalent cation concentrations in the feed could also be improved by batch addition of calcium and/or magnesium. However, improvements were greater when an equal concentration of the cation was added to the feed. Addition of the monovalent cation, sodium, to the feed resulted in a deterioration in settling and dewatering properties when the monovalent to divalent cation ratio exceeded approximately 2 to 1, expressed on an equivalent basis. However, the deterioration could be reversed by increasing the calcium and magnesium concentration in the feed which reduced the monovalent to divalent cation ratio below 2 to 1.