Improvement of lime softening treatment of secondary effluent by the addition of fly ash

Abstract

The sludge generated in the lime softening of recirculating cooling water has a large bulk volume and is difficult to dehydrate. The current study tested the effect of fly ash addition on the pollutant removal efficiency and the sludge properties improvement in the softening process. The hardness and alkalinity removal efficiency before and after the addition of fly ash were evaluated. The effects of fly ash addition on chemical sludge properties, such as the sludge water content, particle size, and microstructure, were also investigated. With a fly ash dosage of 270 mg/L, the removal rates of calcium hardness, total hardness, and total dissolved solids increased by 20.7, 11.5, and 6.1% compared with those obtained in the absence of fly ash. The addition of fly ash can improve the stability of treated water by reducing the scaling potential. Fly ash can improve the sludge properties. At a fly ash dosage of 200–250 mg/L, the median diameter of the flocs is four times bigger than the flocs produced in the absence of fly ash, thereby accelerating the sludge settling speed. After fly ash addition, the sludge water content decreased, facilitating sludge dehydration and resulting in a more compact sludge.