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

The purpose of this research was to examine the impact of cation addition on an industrial activated sludge system and to determine the applicability of the findings of a previous VPI&SU doctoral student (Higgins, 1995) to this industrial sludge. From this research, the implications on the full-scale industrial wastewater treatment system could be postulated.

The addition of sodium and potassium to the feed stream of a laboratory-scale activated sludge system improved sludge volume index (SVI), but produced high supernatant solids. Sodium addition resulted in weak flocs and a deterioration in sludge dewatering. Potassium addition also resulted in poor sludge dewatering but did not significantly affect floc stability. The addition of magnesium improved SVI, supernatant solids, and floc stability, but did not significantly affect sludge dewatering.

Increasing the monovalent/divalent cation ratio in the reactor feed stream, reduced SVI but produced high supernatant solids. The soluble and bound extracellular protein content was observed to increase with as this ratio increased. No trend was observed with soluble or bound extracellular polysaccharide content. Batch addition of both calcium and magnesium improved sludge settling and dewatering, although this effect was not observed when sludge with low extracellular polymer content was examined.

This research confirmed that the effect of cations on activated sludge properties is highly dependent on the feed wastewater composition and starting and operating conditions. This implies the need for laboratory or pilot-scale testing before the effect of cation addition on a given sludge can be determined. For the industrial sludge studied here, the addition of 7 to 12 mM magnesium produced the most desirable sludge characteristics including improved SVI, supernatant solids and floc stability.