Coagulants removal and recovery from water clarifier sludge

Several million tons per year of water clarifier sludge are produced in Europe, with forecasts of the figure doubling by the next decade. End disposal of reference sludge is mainly based on controlled landfilling, after conditioning to minimise the volume of solids. The conditioning operation is carried-out in acidic or alkaline media, thus also allowing for coagulants recovery (AI, Fe species). The quality of the chemicals recovered may not be sufficient to justify their reuse, e.g., to water clarification operations. With the aim of improving the purity of coagulants recovered, a new ion exchange process for selective removal, separation and recovery of Al(III) and Fe(III) species from the clarifier sludge is presented. The IERAL (Ion Exchange Recovery of Aluminium) process is based on the use of a commercial weak electrolyte carboxylate resin (Purolite C106, from Purolite Co., UK), allowing for the removal of metals from the clarifier sludge acidic leachate (pH 3.5), followed by selective separation and recovery of the aluminium and ferric species during the resin regeneration step. Together with the performance of a fully automated 50 L/d pilot plant, this paper reports the basic principles of the process.