Alum recovery has recently gained more attention because many water utilities need to improve their sludge handling and disposal practices. As part of an overall sludge management program, alum recovery can reduce the amount of solids and allow for reuse of the recovered alum as a coagulant. It also has other potential uses such as phosphorus control at wastewater treatment plants.The city of Durham, N.C., performed full-scale testing of alum recovery utilizing the existing sand drying beds for sludge dewatering. The existing sand beds were undersized, but site constraints prevented construction of the necessary drying bed area to properly dewater the settled solids. Full-scale tests of alum recovery by acidification of the settled solids were conducted to (1) develop design criteria for full scale implementation, (2) test the full-scale dewatering of the solids remaining after alum recovery, (3) test the effectiveness of the recovered alum for reuse at the water treatment plant on a full scale, and (4) evaluate the use of recovered alum for phosphorus removal at the wastewater plant. BackgroundThe full-scale tests were conducted at the 22.mgd (83.ML/d) Williams Water Treatment Plant in Durham. Treatment processes at the plant include the use of alum (aluminum sulfate) for coagulation of turbidity and for color removal.

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.The city of Durham, N.C., performed the water treatment plant as well as for full-scale testing of alum recovery using alum sludge generated in a 22-mgd water phosphorus removal in the city-owned treatment plant. Earlier testing"' had wastewater plant.The results of the earlier tests showed evaluated the effectiveness of sand beds for dewatering of solids and the effective-excellent dewatering performance of acidified sludge as compared with alum ness of recovered alum as a coagulant in sludge. They also showed very good per-formance of recovered alum as a water plant coagulant and as a phosphorus-removal chemical in the wastewater plant. As a followup to these tests, a mechanical dewatering evaluation has been conducted with acidified solids from an alum recovery process. The full-scale tests described in this article were conducted to determine the technical feasibility of mechanical dewatering of acidified alum solids and to develop design data. BackgroundThe full-scale tests were conducted at the 22-mgd Williams Water Treatment Plant in Durham, N.C. Treatment processes at the plant include the use of alum (aluminum sulfate) for coagulation of turbidity and for color removal.Existing sludge facilities at the plant are designed to store and thicken sludge