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Abstract

A wastewater treatment process is provided that entails both a biological or activated sludge process and a ballasted flocculation process. A first stream of wastewater is directed to a biological system for biologically treating the wastewater. The biological treatment process produces a first effluent and a first sludge. In cases where the biological system is an activated sludge process, the first sludge is recycled. A second stream of wastewater is directed to a ballasted flocculation system. The second stream of wastewater can be a stream independent of the first stream of wastewater or can be derived from the wastewater passing through the biological system. Sludge in the form of activated sludge or mixed liquor from the biological system is mixed with a second stream of wastewater in the ballasted flocculation system for the purpose of biologically treating the second stream of wastewater. Forming a part of the ballasted flocculation system is a settling tank that produces a second effluent and a second sludge. The second sludge is routed to a separator where granular material that forms a ballast in the process is separated and recycled. To effectively remove suspended solids from the wastewater passing through the ballasted flocculation system, the second sludge recycle rate is controlled or maintained at a flow rate of about 15% and higher of the influent flow rate to the ballasted flocculation system. Furthermore, as the mixed liquor suspended solids (MLSS) concentration increases from, for example, 1000 mg/l to about 4000 mg/l, the recycle rate of the second sludge may be varied from about 15% to about 40% of the influent flow rate to the ballasted flocculation system.

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