Removing particulates from centrate of dredged sediments at Suitanghe river, Shanghai

The centrifugal dewatering of dredged slurry from a contaminated river would produce centrate that contained high levels of pollutants and particulates. The direct discharge of the centrate back to the river has predominant negative effects on the river water quality. In this study, particulates in the centrate were proposed to be removed by the use of a combined clarification + sand filtration + ultra filtration process. Jar tests revealed that dual conditioning by 100 mg/L polyaluminum chloride (PAC1) followed by 30 mg/L polyacrylamide (PAM) obtained a more satisfactory effluent quality than single coagulant conditioning. Process parameters for clarification stage were then optimized using orthogonal tests at bench-scale tester. The helical-flow contact clarification could effectively remove particulate matters from the centrate with a hydraulic retention time (HRT) of 25 min. Since most pollutants were strongly associated with the particulates, the removal of suspended solids could effectively decontaminate the discharge as well. The effluent quality from sand filter could meet with the Class II criteria of Integrated Wastewater Discharge Standard in China (GB8978-1996, Integrated Wastewater Discharge Standard (in Chinese); National Environmental Protection Agency of China: Beijing, China, 1996), whereas that from ultrafilter met with the Water Quality Standard for Non-potable Use of China (CJ25.1-89, Water Quality for Non-protable Use (in Chinese); Construction Ministry of China: Beijing, China, 1989).