OXYGEN-TRANSFER IN ACTIVATED-SLUDGE SURFACE-AERATED PROCESS

Aeration of activated sludge tank represents the main part of energy consumption in wastewater treatment plants. Hence an evaluation of aeration system ef efficiency and a good knowledge of the global oxygen transfer phenomena is necessary. First of all, this study proposes a correlation between the oxygen transfer and the working parameters in clean water. The oxygenation capacity is linearly correlated to the specific power with a specific oxygen input of 2.56 kg O2.kWh-1. Then a factor alpha is introduced to correct the value of the gas-liquid transfer coefficient obtained with clean water. In this study, alpha ranges between 0.51 and 0.91. Its value increases only with Oxygen Uptake Rate. The alpha values inferior to 1 seem to indicate a physical blocking effect at the gas-liquid interface due to the cells presence. The agitation speed, the biomass concentration (between 1.8 and 4.6 kg.m-3) and the microorganisms ''activity'' to consume oxygen, do not affect alpha. All those results can be explained and characterised by a two-film oxygen transfer theory in presence of a slow reaction.