Gas Dispersion in Biological Wastewater Treatment – Smaller Bubbles for More Efficiency

In Germany wastewater is treated in 10000 plants which use 4.400 GWh of energy per year. Up to 80% of the whole energy in these plants is consumed for the aeration of microorganisms in the so called activated sludge tank. Aeration is the essential part of the process since the microorganisms need sufficient amount of oxygen to degrade ammonia. Aerators with flexible membranes located at the bottom of the aeration tank are currently state of the art for this process. However the process suffers from some limitation such as pressure drop, insufficient mixing and underutilization of oxygen. These are mainly due to scarce knowledge about gas dispersion inside the tank. The type of aerator defines the initial size of bubbles dispersed into the tank. The typical bubble size generated by flexible membrane aerators has been determined to be between 2-4 mm by Hasanen et. al. However, this is considerably higher than the optimal bubble size. Motarjemi and Jameson have calculated the optimal value for the 95% oxygen transfer to water in 3-6 m tank depth to be in the range 0.7…1.0 mm.
A novel approach uses solid perforated stainless steel aerators for gas dispersion. In this work bubble formation of stainless steel aerators has been experimentally studied for comparison to membrane aerators. Preliminary results of the stainless steel membrane sparger showed a significant reduction in the bubble size by 47 % and consequently an increase in bubble residence time compared to flexible membranes. Moreover, uniform bubble size has been generated across the sparger which is not the case for membrane aerators. The pressure drop of these novel aerators is compared with flexible membranes. Mass transfer measurements were done in a large scale bubble column setup under realistic process conditions.