Reuse of water treatment residuals from lime softening, Part I: Applications for the reuse of lime sludge from water softening

Lime sludge, an inert material mostly composed of calcium carbonate, is the result of softening hard water before distribution as drinking water. A large city such as Des Moines, Iowa, produces about 32 000 tons of lime sludge (dry-weight basis) annually. This is about half of the lime sludge produced in Iowa per year in eight different cities, and these cities currently have 371 800 tons (dry-weight basis) stockpiled. The Iowa Department of Natural Resources directed those cities using lime softening in drinking-water treatment to stop digging new lagoons to dispose of lime sludge. The situation in surrounding Midwestern states is similar, and there will be millions of tons of lime sludge in stockpiles. Five Iowa water-treatment plants, all producers of lime sludge, funded the research. The research goal was to find useful and economical alternatives for the disposal of lime sludge. Feasibility studies tested the efficacy of using lime sludge in cement production, power-plant SOx control, dust control on gravel roads, wastewater neutralization, and infill materials for road construction. All the potential applications were demonstrated to be at least feasible, except for dust control. Fill material from admixture with fly ash showed the most promise as a bulk, if variable, demand application.