Physical and biological factors affecting oxygen transfer in the activated sludge wastewater treatment process

“The activated sludge (AS) process is widely utilized for wastewater treatment due to its versatility and general resiliency, but is energy intensive, requiring aeration and mixing energy inputs to maintain the biochemical reaction and optimize treatment performance. This study is focused on determining different factors that influence oxygen transfer and consequently its energy demand. AS morphology, as described by the d10, d20, d32, specific filament length (SFL), 30 min settleability (SV30), etc., does influence the OTE. Filamentous organisms increase the hydrodynamic radius of suspended particles, which dilates mixed liquor apparent viscosity (µapp) and impedes mass transfer. For example, an increase in the SFL from 3.8 x 1010 to 2.7 x 1011 µm g-1 resulted in an increase of µapp in excess of 100% and decrease of OTE of 29%. The volumetric mass transfer coefficient (kLa) was most significantly affected by the mixing intensity (P VR-1).085, superficial gas velocity (Usg) and the µapp-0.75 where the µapp principally influenced by the SFL and ultimate settleability (SVULT), a new parameter based on regression analysis of times series settling data. The addition of surfactants to the influent wastewater increased OTE; however, greater dispersed growth resulted which mediated increases in the OTE, especially at higher feed concentrations. Complicating the issue of oxygen mass transfer in the AS process is the presence of floc and filamentous organism at the gas-liquid interface, which block the surface, restricting mass transfer. This effect tended to be more significant for short (10 d) SRT processes as compared to long (40 d) processes, yielding modified enhancement factors, E’A, of 0.59 to 0.78, respectively”--Abstract, page iv