The Influence of Sludge Characteristics on DAFT Performance

Dissolved air flotation thickeners (DAFTs) have been employed successfully for many years to thicken sludges generated at wastewater treatment facilities. However, for a number of reasons, DAFTs sometimes have not performed optimally and results have been disappointing. The objective of the work described in this paper was to investigate the factors that would improve overall DAFT performance, particularly to increase float solids and reduce the suspended solids loads returned to treatment plants via the DAFT underflow. This paper describes the systematic approach used to identify the factors causing the inefficiencies at two wastewater treatment plants, and the methodologies used to quantify component efficiencies.An evaluation of operational data at the two plants showed that in most cases the parameter that most influences float solids concentration was the SVI and the parameter that most influences the underflow concentration was the air/solids ratio.Because of the importance of the air/solids ratio in maintaining low underflow solids concentrations, an on-site evaluation of the efficiency of the air saturation systems was performed. Representative tanks were tested using a saturator monitoring unit which isolates a small portion of the pressurized flow from the saturation tank and measures the mass of air precipitated per unit flow. A well designed saturation system should provide an efficiency of approximately 88-percent. The measured efficiencies ranged from approximately 48 to 78 percent. A number of actions to improve efficiencies were identified, including nitrogen purging from the saturator headspace, adequately designed impingement plates and adequately sized nozzles within the saturators. Improving the air/solids ratios would reduce underflow solids thereby reducing the return solids loads and overall operating costs at the plants.