Characterization and treatment of lime sludge dewatering effluents

Abstract

With ever increasing regulatory constraints on the disposal of residuals produced during water treatment, the cost of handling, treatment, transport, and disposal of these residuals has become a significant portion of overall water treatment costs. In plants practicing lime softening, a major source of residuals is lime sludge. Of the many options proposed for the management of lime sludges most include sludge dewatering. The dewatering process produces effluents that also must be disposed of. This paper presents the results of the characterization of dewatering effluents from a large water treatment plant. Also presented are treatment and management options which were evaluated through designed bench-scale experiements.The quality and quantity of supernatant effluents from the thickener and centrate from the centrifuge were determined. It was found that the supernatant is composed predominantly of calcium ions and caustic alkalinity which can be treated through pH adjustment. The centrate was found to be of much poorer quality with poor settling and filterability properties. These qualities were found to be mostly attributed to the high organic content of the centrate. Bench-scale tests found that polymer pretreatment could substantially improve both the settleability and filterability of the centrate. Key words: water treatment, sludge dewatering, softening, polymers, sludge conditioning.