Process variables influencing clarification and thickening by dissolved air flotation are summarized. Operational adjustments to full-scale thickeners are usually limited to the speed and periodicity of the float scrapers which, in turn, directly influence the effective drainage time of liquid from the floating sludge mass. Expressions are derived which relate float solids concentration and total float depth to the effective drainage time. Design and operation charts are derived from the equations. Results obtained from the City of Philadelphia Northeast WWTP are used to demonstrate the validity of the derived expressions. The enhanced thickening characteristics obtained when water treatment plant sludge was discharged to the WWTP is demonstrated. Polymer addition had only a slight beneficial effect. Use of the charts for operational control is demonstrated by means of an example.