New insights into the effect of surfactants on oxygen mass transfer in activated sludge process

Abstract

Surfactants have been thought to significantly reduce oxygen transfer in activated sludge processes through reductions to oxygen diffusion at the air-liquid interface. However, this mechanistic understanding may not completely describe interactions between surfactants and biomass which can affect oxygen transfer. In this study lab-scale complete-mix activated sludge reactors with SRTs of 10 d, 20 d and 40 d were used to reevaluate the impacts of a commonly used surfactant, sodium dodecyl sulphate (SDS), on oxygen transfer. A 10 mg L-1 SDS feed concentration substantially increased oxygen transfer efficiency (OTE) for 10 day and 20 day SRT reactors, respectively, and slightly increased OTE for the 40 d SRT reactor. A 50 mg L-1 SDS feed concentration slightly increased OTE for the 10 d SRT reactor, significantly increased OTE for the 20 d SRT reactors, but significantly decreased OTE for the 40 d SRT reactor, as compared to the influent feed without SDS. These results were not consistent with the conventional explanations. Reactor effluent testing, dynamic oxygen uptake and oxygen transfer rate testing of the mixed liquor, and system response analysis also demonstrated that the influent SDS did not directly impact the oxygen transfer performance. Results from a principal component analysis approach confirmed that the activated sludge morphological parameters, not the SDS in the influent, were most significant factors that impact OTE. Understanding the mechanisms through which surfactants affect oxygen transfer is critical to development of effective strategies for sustainable operation of wastewater treatment plants.