Indicators for technological, environmental and economic sustainability of ozone contactors.

Various studies have attempted to improve disinfection efficiency as a way to improve the sustainability of ozone disinfection which is a critical unit process for water treatment. Baffling factor, CT10, and log-inactivation are commonly used indicators for quantifying disinfection credits. However the applicability of these indicators and the relationship between these indicators have not been investigated in depth. This study simulated flow, tracer transport, and chemical species transport in a full-scale ozone contactor operated by the City of Tampa Water Department and six other modified designs using computational fluid dynamics (CFD). Through analysis of the simulation results, we found that baffling factor and CT10 are not optimal indicators of disinfection performance. We also found that the relationship between effluent CT obtained from CT transport simulation and baffling factor depends on the location of ozone release. In addition, we analyzed the environmental and economic impacts of ozone contactor designs and upgrades and developed a composite indicator to quantify the sustainability in technological, environmental and economic dimensions.

Impact of sludge layer geometry on the hydraulic performance of a waste stabilization pond.

Improving the hydraulic performance of waste stabilization ponds (WSPs) is an important management strategy to not only ensure protection of public health and the environment, but also to maximize the potential reuse of valuable resources found in the treated effluent. To reuse effluent from WSPs, a better understanding of the factors that impact the hydraulic performance of the system is needed. One major factor determining the hydraulic performance of a WSP is sludge accumulation, which alters the volume of the pond. In this study, computational fluid dynamics (CFD) analysis was applied to investigate the impact of sludge layer geometry on hydraulic performance of a facultative pond, typically used in many small communities throughout the developing world. Four waste stabilization pond cases with different sludge volumes and distributions were investigated. Results indicate that sludge distribution and volume have a significant impact on wastewater treatment efficiency and capacity. Although treatment capacity is reduced with accumulation of sludge, the latter may induce a baffling effect which causes the flow to behave closer to that of plug flow reactor and thus increase treatment efficiency. In addition to sludge accumulation and distribution, the impact of water surface level is also investigated through two additional cases. Findings show that an increase in water level while keeping a constant flow rate can result in a significant decrease in the hydraulic performance by reducing the sludge baffling effect, suggesting a careful monitoring of sludge accumulation and water surface level in WSP systems.

Evaluating hydraulic and disinfection efficiencies of a full-scale ozone contactor using a RANS-based modeling framework.

The capability of predicting hydraulic and disinfection efficiencies of ozone disinfection contactors is essential for evaluating existing contactors and improving future designs. Previous attempts based on ideal and non-ideal models for the hydraulics and simplified mechanisms for chemical reaction modeling have resulted in low accuracy and are restricted to contactors with simple geometries. This manuscript develops a modeling framework for the ozonation process by combining computational fluid dynamics (CFD) with a kinetics-based reaction modeling for the first time. This computational framework has been applied to the full-scale ozone contactor operated by the City of Tampa Water Department. Flow fields, residence time distribution, ozone concentration distribution, and concentration-contact time (CT) distribution within the contactor have been predicted via the computational framework. The predictions of ozone and bromate concentrations at sample points agree well with physical experimental data measured in the contactor. The predicted CT values at the contactor outlet demonstrate that the disinfection performance of the ozone contactor operated by the City of Tampa Water Department is sufficient to meet regulation requirements. The impact of seasonal flow rate change on disinfection performance is found to be significant and deserves attention during the management and operation of a water treatment plant.

Taenia eggs in a stabilization pond system with poor hydraulics: concern for human cysticercosis?

The objective of this study is to compare the removal of Taenia eggs to the removal of Ascaris eggs in a wastewater stabilization pond system consisting of three ponds in series, where the hydraulic residence time distribution has been characterized via a tracer study supported by computational fluid dynamics modeling. Despite a theoretical hydraulic retention time of 30 days, the peak dye concentration was measured in the effluent of the first pond after only 26 hours. The smaller-sized Taenia eggs were detected in higher concentrations than Ascaris eggs in the raw wastewater. Ascaris eggs were not detected in the pond system effluent, but 45 Taenia eggs/L were detected in the system effluent. If some of these eggs were of the species Taenia solium, and if the treated wastewater were used for the irrigation of crops for human consumption, farmers and consumers could potentially be at risk for neurocysticercosis. Thus, limits for Taenia eggs in irrigation water should be established, and precautions should be taken in regions where pig taeniasis is endemic. The results of this study indicate that the theoretical hydraulic retention time (volume/flow) of a pond is not always a good surrogate for helminth egg removal.

Computational fluid dynamic analysis of flow velocity waveform notching in umbilical arteries.

Umbilical artery Doppler velocimetry waveform notching has long been associated with umbilical cord abnormalities, such as distortion, torsion, and/or compression (i.e., constriction). The physical mechanism by which the notching occurs has not been elucidated. Flow velocity waveforms (FVWs) from two-dimensional pulsatile flows in a constricted channel approximating a compressed umbilical cord are analyzed, leading to a clear relationship between the notching and the constriction. Two flows with an asymmetric, semi-elliptical constriction are computed using a stabilized finite-element method. In one case, the constriction blocks 75% of the flow passage, and in the other the constriction blocks 85%. Channel width and prescribed flow rates at the channel inflow are consistent with typical cord diameters and flow rates reported in the literature. Computational results indicate that waveform notching is caused by flow separation induced by the constriction, giving rise to a vortex (core) wave and associated eddies. Notching in FVWs based on centerline velocity (centerline FVW) is directly related to the passage of an eddy over the point of measurement on the centerline. Notching in FVWs based on maximum cross-sectional velocity (envelope FVW) is directly related to acceleration and deceleration of the fluid along the vortex wave. Results show that notching in envelope FVW is not present in flows with less than a 75% constriction. Furthermore, notching disappears as the vortex wave is attenuated at distances downstream of the constriction. In the flows with 75 and 85% constriction, notching of the envelope FVW disappears at ∼3.8 and ∼4.3 cm (respectively) downstream of the constriction. These results are of significant medical importance, given that envelope FVW is typically measured by commercial Doppler systems.