Prediction of flow field and oxygen utilization rate (our) in orbal biological system (obs) using cfd
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Date
2016-09-01
Authors
Noor Aida Saad
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Abstract
Orbal Biological System (OBS), which is one of the modified oxidation ditches, has been designed to provide an optimized approach of the biological treatment process. Due to the structural design of the OBS that has great potential to promote optimal treatment processes, it is very crucial to study its operating system. This intention is in line with the current demand on wastewater treatment system, which not only focusing on the achievement of specified effluent quality standards, but also taking into consideration other aspects such as economic and environmental. Because of this reason, the study was conducted to understand the complicated process of OBS. The preliminary process model and Computational Fluid Dynamics (CFD) model were used to obtain a better picture of the OBS. Three-dimensional, two-phase and an open channel CFD-based model was developed to study the mechanism of OBS. In the study, the CFD model was used to represent the OBS to overcome some of the limitations of the preliminary process model. The simulation results were used to study the flow pattern across the ditch, the velocity distribution at different depths, velocity distribution for different operating conditions, the volume fraction of air and water in the ditch, the pressure distribution across the channels and hydraulic residence time. The results of CFD model were used to calculate more specific volumetric flow rate, the Oxygen Utilization Rate (OUR) of the ditch and the power consumption associated with the values of OUR. Based on the study on the effect of different operating shafts on the performance of OBS, it was found that turning off the combination of shaft 2 (outer channel) and shaft 6 (middle and inner channel) was contributing to less power consumption but still having almost the same average OUR value (26,483 kg/d) like the real case (26,594 kg/d). The results show that although these shafts are turned off, but the biological reactions inside the ditch still occur as the biological reactions when all the shafts are operated. Forecast future power consumption associated with the OUR values, which may occur due to effluent quality standards that are more stringent, was also presented and discussed in this study. Based on the results, the outermost channels recorded the highest overall oxygen consumption, which is 79% compared with the middle channel with only 9%, followed by inner channel with 12%. In the future, if more stringent effluent quality standards imposed, the rate of oxygen consumption will definitely increase. If the rate of oxygen consumption at present, namely 21,109 kg/d increased to 30%, it will lead to increased electricity use by 3,241 MW/year (nearly RM 1,092,284/year). The research provides a better understanding on how this simulation tool will be able to be applied within wastewater process modelling. It can considerably contribute to the further expansion of wastewater treatment process models.