Mechanical Dewatering of Alum Solids and Acidified Solids: An Evaluation

The city of Durham, N.C., performed full‐scale testing of alum recovery from alum sludge generated in a 22‐mgd water treatment plant. Purposes of the tests were to determine the technical feasibility of mechanical dewatering of acidified alum solids and to develop design data. Three mechanical dewatering devices—a belt press, a centrifuge, and a diaphragm filter press—were screened in bench‐scale tests. The belt press did not warrant further testing. Manufacturers of the centrifuge and the filter press then conducted dewatering tests at the treatment plant. Tests showed that with proper chemical conditioning, both devices could be successfully used to dewater acidified solids to 30 percent or more. Recovered alum is proposed for two uses in Durham: as a coagulant in the water plant and as a phosphorus‐removal chemical in one of the city's wastewater treatment plants. Recovering and recycling alum can significantly lower final sludge disposal costs, which are increasing as more restrictive solid waste regulations take effect.