Effect of mineral materials and cations on activated and alum sludge settling

The ballasting of activated and alum sludge with bentonite, clinoptilolite, montmorillonite, acid modified magnesium silicate, and a blend of talc and chlorite was investigated in batch settling tests. Mineral materials were added as such, and saturated by Ca or Na ions. The sludge settleability was also investigated when soluble cations were added as salts. Single cation solutions of Ca or Na, or combined mixtures of Ca, Na, K, and Mg were used. The impact of mineral materials on alum sludge filterability was tested by capillary suction time (CST) measurement. Mineral materials, except bentonite and test sand, improved activated and alum sludge settling by 21–47% based on sludge volume when a dosage of 3 g/l was used. Cation saturation of mineral material, except bentonite, did not change the settling improvement. Sludge settleability was unaffected by soluble cation addition as salts. Clinoptilolite, the blend of talc and chlorite, and montmorillonite improved most the activated sludge settling. Montmorillonite, the blend of talc and chlorite, and the acid modified magnesium silicate improved most the alum sludge settling. Differences in density, particle size, or the amount of soluble cations did not explain alone the ranking of mineral materials in settleability improvement. Alum sludge filterability was affected only by bentonite as such and by Na-saturated bentonite to negative direction.