Enhancement of activated sludge wastewater treatment with hydraulic selection

Abstract

The slow settling rate of activated sludge (AS) in secondary clarifiers and batch reactors results in increased hydraulic retention time, AS with a higher water content pumped to the digesters, and higher suspended solids concentration in the clarified supernatant. Settling rates of AS can be improved through the removal of poor settling floc with a diameter less than 100 µm. Currently, the main technology used for the enhancement of AS settling is aerobic granular sludge reactors—very few of these reactors have been adopted into the United States market due to their cost and difficulty to integrate the technology into current wastewater treatment infrastructure. The objective of this study was to develop a technology that could enhance the settling properties of AS while maintaining nitrification/denitrification and that could easily be integrated into current wastewater treatment infrastructure. We have developed a hydraulic selection process that removes poor settling floc directly from the secondary clarifiers or batch reactors. The hydraulic selection process is a vacuum system that is suspended in the AS, the system then develops a unique pressure difference based on poor settling sludge density, floc diameter, and terminal settling velocity. The selection process captures and removes poor settling floc allowing the remaining floc to grow and increase the settling rate of AS. The hydraulic selection process was tested for 120 days in a 120 L batch reactor. Results from the experiment showed that the hydraulic selection process was successful, in 63 days the settling velocity increased from 0.2 to 20 m/hr, mean floc diameter increased from 62 to 430 µm, and sludge volume index decreased from 280 to 45 mL/g. Nitrification was maintained for nearly the entire duration of the experiments with the exception of a few days due to drastic changes in influent wastewater concentrations. Denitrification was only maintained for a limited period during the experiment due to an unfavorable carbon to nitrogen ratio.