Understanding the Alpha factor: Studies of the rheological properties of sludge and their effect on oxygen transfer efficiencies from Full Scale Systems

Aeration costs make up about 40-60% of the operating costs at wastewater treatment facilities for most conventional wastewater treatment systems. The oxygen transfer efficiency and the specific aeration efficiency associated with oxygen transfer are dependent on the alpha factor, which is defined as the ratio of the mass transfer coefficient under field conditions vs. mass transfer under clean water conditions. Previous studies suggest that the alpha factor is largely a function of the sludge concentration in the wastewater treatment process, with higher sludge concentrations leading to lower alpha factors. It has been determined that the reduction in alpha as mixed liquor suspended solids (MLSS) concentration increases is largely due to changes in the rheological properties of the sludge, specifically increases in the viscosity, as bacteria concentration increases. While such generalizations hold true for mixed liquor in municipal facilities, our findings suggest that the relationship between viscosity, sludge concentration and mass transfer (alpha factor) is impacted by site specific wastewater characteristics in industrial wastewater treatment systems. We find that viscosity is more reliably correlated with the α-factor than MLSS.