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Development of a unified theory of oxygen transfer in activated sludge processes : the concept of net respiration rate flux Mahendraker, Venkatram

Abstract

The mass transfer of oxygen in clean water depends on many variables, which have been well researched and documented in the literature. For all practical purposes, the clean water mass transfer can be explained satisfactorily by the Lewis-Whitman two-film theory. However, oxygen transfer in wastewater process conditions is not well understood and the mechanism is unclear. The role of reaction rates and oxygen uptake rate (OUR) on the mass transfer phenomenon, under the process conditions, is in debate for a long time. Furthermore, due to the lack of complete understanding of the process impacts, no predictive technique exists to estimate the design mass transfer rates in the process environment. Currently, correction factors for temperature (θ & τ), pressure (Ω), total dissolved solids (β) and the process water quality (α) are used to arrive at the process mass transfer rates, based on the results obtained from clean water tests. However, often this approach seems to lead to erroneous factors and different oxygen transfer rates under actual process conditions. Out of these correction factors, α appears to be a complicated function of process variables. 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 floe 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). Among the processes studied, the UCT process was the most efficient, followed by the MLE and the CMAS process, at a single SRT of 10 days. An analysis of the reaction rates, carbon oxidation (CODoxi), nitrification (NIT), and total denitrification (TDN) (including simultaneous nitrification-denitrification (SND) showed that, a combination of high ratio of CODoxi to TDN and low ratio of NIT to TDN resulted in a higher oxygen transfer efficiency. On the other hand, a combination of high ratio of NIT to TDN and low ratio CODoxi to TDN resulted in lower oxygen transfer efficiency. Furthermore, a critical evaluation of the entire data set suggested that, 1) by converting the respiration rates to respiration rate fluxes and 2) by calculating the net respiration rate flux, oxygen transfer mechanism under process conditions could be comprehensively explained, regardless of the process variables. The net respiration rate flux is defined as the net oxygen demand per unit area of biomass due to the metabolic reactions of: 1) ordinary heterotrophic organisms (OHO) and phosphorus accumulating organisms (PAO), 2) nitrifying organisms, and 3) denitrifying organisms in the activated sludge process. The results of the analysis indicated that the presence of reaction conditions that enhanced the net respiration rate flux promoted efficient use of oxygen. It is also shown in this work that the EBPR process had the inherent advantage of generating the maximum net respiration rate flux, possibly because of their capacity to store carbon. This resulted in higher oxygen transfer rates and lower air requirements, in comparison to the MLE and CMAS processes, under similar operating conditions. In addition, it was found that the solution surrounding the floe (effluent) had a constant resistance to oxygen transfer within an experimental run, at a given air flow rate (AFR). The biological floe resistance (BFR) and the specific biological floe resistance (SBFR) were closely linked to the reaction rates. An increase in the oxygen uptake rate resulted in the reduction of BFR and SBFR, and vice-versa. In summary, based on the findings in this research, a conceptual model, "the unified theory of oxygen transfer in activated sludge processes," is proposed. This model attempts to explain the oxygen transfer on the basis of the net respiration rate flux occurring in the process. The proposed theory appears to be capable of successfully providing the qualitative knowledge regarding the mass transfer rates observed in activated sludge processes, regardless of the process variables involved.

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