Characteristics, quantity and thickening of sludge from a partial lime softening plant

Water softening using the lime process leads to the production of large quantities of sludge. Determining the quantity of sludge produced and its dewatering properties is very important in the design, operation and cost of the subsequent sludge handling facilities. Presented are the results of a study on the partial softening process of a 200 MLD water treatment plant in Edmonton, Alberta, Canada. The plant currently softens surface water from a hardness of 180 mg l^-1 as CaCO₃ to 110 mg l^-1 as CaCO₃ in a solid contact clarifier. The sludge is gravity thickened prior to centrifugation after which the sludge is hauled off-site for disposal. Results from the study were used to assess the current operation as well as to provide required information for the facilities needs in view of a proposed expansion to double the current production. Results from measured data indicate that the sludge production can be established by a mass balance, chemical (Ca⁺⁺, Mg⁺⁺) balance or stoichiometric calculations. Bulk density of the lime sludge was correlated to the sludge solids content by a quadratic expression. Similarly, the settling velocities at various sludge densities were well correlated. The total organic carbon (TOC) content of the sludge was found to be unusually high. Analysis of operating records found that by maintaining a proper sludge blowdown interval and duration, the sludge withdrawal can be optimized for solid contact clarifiers with intermittent sludge withdrawal.