A survey of the feasibility of aluminum recovery from coagulation sludges

Coagulation - flocculation processes are at present widely applied for both surface water (in colloidal particles and heavy metals removal) and wastewater treatment (in tertiary processes such as phosphorus precipitation). The most common coagulants are aluminum (Al3+) and iron (Fe2+, Fe3+) salts. Precipitated metal hydroxides, collected in the subsequent sedimentation or filtration units, may result in the production of bulky gelatinous sludge suspensions that are difficult to thicken and dewater. The high metal content in the resulting sludge allows the coagulant to be recovered and reutilized within the water treatment plant itself. The main advantages and the subsequent economic benefits obtainable by applying the coagulant recovery and reuse process lie in the following aspects: 1) the reduction in the quantities of commercial coagulant to be used (roughly around 80% under the same turbidity removal conditions); 2) the dramatic reduction in metal content of the sludge to be disposed of; 3) the reduction in sludge volume and weight due to the lower total solids content and to the improvement in sludge dewaterability. The paper presents an overview of the basic principles of aluminum recovery and shows a number of data as obtained by some experimental lab-scale investigations.