| The objective of this study was to examine the impact of process variables (solids retention time, influent wastewater quality and process configuration) on oxygen transfer under laboratory process conditions and under the influence of a small number of variables. The variables that were kept constant included, influent flow rate, recirculation flow rates, diffuser design, influent concentration of minerals (micro and macro nutrients), aerobic HRT, reactor design, temperature, pressure, and operating DO in the aerobic reactor. The other goal of this research was to improve the current understanding of the oxygen transfer mechanism in the activated sludge processes, by developing suitable theoretical model. Furthermore, the role of biological floc and the reactor solution (in an aerobic reactor) in oxygen transfer was also evaluated as part of this research.; To achieve the set goals, three activated sludge processes, namely, the completely mixed activated sludge process (CMAS), modified Ludzack-Ettinger (MLE) process and the University of Cape Town (UCT) enhanced biological phosphorus removal (EBPR) process were studied under controlled laboratory conditions. These systems were fed with a defined composition of substrates (carbon, nitrogen and phosphorus) and other nutrients with synthetic wastewater and the processes were rigorously monitored for a number of key parameters. The respiration rate (OUR) was measured in an ex situ batch respirometer and data obtained from these batch-tests were used in the determination of the oxygen transfer parameters, using the ASCE steady-state method. The process oxygen transfer parameters were also measured by other techniques (non steady-state HPA and CPL methods, and off-gas method). Further, the clean water oxygen transfer parameters of the diffusers were obtained in batch tap water tests, in a reactor that was a replicate of the aerobic reactor. To arrive at the oxygen transfer parameters in reactor solution (effluent), batch oxygen transfer tests were also performed with the filtered effluent water, collected on the test days. In all, 15 runs were conducted, and collectively the systems were operated for 620 days.; Based on the data obtained and the analysis conducted, it is demonstrated that, for a given process configuration and influent wastewater quality, as the SRT increased, oxygen transfer tended to improve within the range of SRTs studied (2.5 to 15 days). (Abstract shortened by UMI.)... |
