Oxygen Transfer Phenomena in Activated Sludge

This dissertation investigates the impact of activated sludge on oxygen transfer in activated sludge plants. The effect of surfactants on oxygen transfer is also studied and a method to determine the floc volume of activated sludge is developed. The α-factor, i.e. the relationship of wastewater to clean water oxygen transfer coefficient, is chosen to enable comparison of oxygen transfer coefficients. The main results are summarized as follows: • The floc volume significantly influences oxygen transfer and sedimentation of activated sludge. Oxygen transfer is reduced and sedimentation hindered with increasing floc volume. Floc volume influences oxygen transfer in coarse and fine bubble aeration systems in a similar fashion. This is in marked contrast to the effects of surfactants on oxygen transfer. • The mixed liquid suspended solids concentration is an incorrect parameter for comparing phenomena related to the floc volume. Better correlation can be achieved if the mixed liquid volatile suspended solids concentration is used. • Dissolved surfactants have no significant impact on the oxygen transfer in activated sludge. Instead, it seems that the impact on oxygen transfer by substances adsorbed to the floc surface increases, as sludge retention time decreases. • The oxygen transfer coefficient increases and the α-factor decreases with increasing airflow. Consequently, the oxygen transfer efficiency decreases. This phenomenon is especially pronounced at high oxygen transfer coefficients and high floc volumes. • The reactor designs (airlift, bubble column) tested in this study have no verifiable effect on the α-factor. • A combination of the experimental results shows that, for a constant floc volume, the α-factor improves with increasing sludge retention time, while it decreases with increasing floc volume for constant sludge retention times. Based upon currently available data, the consequences for practice are: • The increase in α-factor compensates the increase in oxygen uptake rate by the bacteria with increasing sludge retention time. Consequently, the required standard oxygen transfer rate decreases with increasing sludge retention time if the floc volume remains constant. • Adapting the sludge retention time during the summer to the minimum required (temperature adjusted) does not lead to a significant reduction in the required standard oxygen transfer rate