Effects of baffles on bubble rising behaviour – extending bubble residence time and mass transfer

Up to 80 % of the total energy budget of wastewater treatment plants is consumed by the activated sludge process. To improve the energy efficiency the specific standard oxygen transfer rate (SSOTE) must be increased. This can be achieved by increasing the overall oxygen mass transfer inside the aeration tank. Oxygen mass transfer happens from the gaseous phase of the bubble to the liquid phase of the activated sludge. The amount of transferred oxygen depends on various parameters like interfacial area, surface active substances, concentration gradients, turbulences and bubble residence time. However, parameters such as the interfacial area cannot be changed without additional energy usage.

Our approach is to increase the length of the bubble rising path and to decelerate the bubbles by inserting baffles in the activated sludge tank. The overall oxygen mass transfer is then extended due to the longer bubble residence time in the aeration tank. Furthermore, the bubbles can be guided in their rise inside the aeration tank. By directing bubbles into less oxygenated sludge higher concentration differences occur which lead to higher local oxygen mass transfer.

Hydrophobic and hydrophilic materials were selected for the laboratory experiments to investigate their effects on the bubble rising behaviour. The target parameters were bubble size distribution, equivalent Sauter mean diameter of the bubbles, bubble rise velocity, bubble residence time, local and global mass transfer coefficient under the variation of wetting behaviour of the material (hydrophilic, hydrophobic), roughness profile of the material, inclination angle and length of plate. The wetting characteristics of these plates were also tested within activated sludge to analyse the influence of a developed biofilm to the surface characteristics of the plates. A variety of materials with different surface properties where investigated: glass, PVC, PTFE, stainless steel and various coated and treated stainless steel plates.