Effects of humus soil on the settling and dewatering characteristics of activated sludge

Effects of humus soil on the settling and dewatering characteristics of the activated sludge were investigated. Addition of the humus soil improved the sedimentation characteristics of the activated sludge remarkably. The initial lag phase was diminished and the zone settling velocity increased as the humus soil dosage increased. For estimation of the chemical effect of the humus soil on sludge settling, cation mixtures equivalent to the cations leaching from the humus soil were added and only the chemical effect was examined. The physical effect was then isolated by subtracting the chemical effect. The chemical effect on sludge settling was about30∼50% of the equivalent humus soil powder dosage and the physical effect was more important in improvement of the sludge settling properties. Dewatering characteristics of the activated sludge were dependent on both the concentration and distribution of the cations leaching from the humus soil. Divalent cations, calcium and magnesium from the humus soil decreased the bound water content of the sludge, which in turn increased floc density and cake solids content. However, excess sodium ion increased the SRF (specific resistance to filtration) slightly. Addition of humus soil to thickened activated sludge improved the dewatering properties, which could be well explained by the effect of the cations leaching. The SRF showed a good trend with the monovalent to divalent cation (M/D) ratio, which could be utilized as a useful tool for evaluation of the sludge dewatering characteristics.