AWWA MTC61175

This paper begins by discussing how the low-pressure membrane market has grown with the applications of these systems expanding significantly. Early low-pressure membrane applications were used historically for turbidity removal of relatively high quality source waters, but now they are used to provide complete treatment on difficult-to-treat source waters in basically one step. As such, the solids characteristics of the reject wastewater from these systems have changed significantly also. Based on some actual testing of membrane reject waters from a case study by the City of Lancaster, Pennsylvania, the settling and thickening characteristics of direct feed membrane reject waters are significantly different than conventional water treatment sludges. Conventional gravity thickening or flotation thickening appear to realize sludge concentrations of no greater than 1.5% (and polymer addition does not help significantly), while mechanical (i.e. belts or centrifuges) or enhanced thickening (i.e. DensaDeg) will be required to reliably achieve a thickness of 2 to 3 percent. Includes tables.