DAFT Optimization: How Much Air is Enough?

Because of issues with plant efficiency and the inconsistency of year round performance, the Philadelphia Water Department (PWD) identified areas where they would like to see the performance of their DAFT systems improve. The objective was to investigate factors that would improve the DAFT's performance, particularly by increasing float solids concentrations and reducing the suspended solids loads returned to the treatment plants via the underflow. An evaluation of existing plant operational data 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 from the volume of flow. The mass of the precipitated air volume was compared to the theoretical mass of air dissolved under the same ambient conditions to compute efficiency. A well designed saturation system should provide an efficiency of approximately 88-percent. The measured efficiencies ranged from approximately 48 to 78 percent. There are a number of actions that could be taken to improve efficiencies including nitrogen purging from the saturator headspace, verification that impingement plates are adequately designed and fully intact, and adequately sized nozzles within the saturators. Improving the A/S ratios would reduce underflow solids thereby reducing the return solids loads and overall operating costs at the plants.