Evaluation of flow and scalar transport characteristics of small public drinking water disinfection systems using computational fluid dynamics

Abstract

This study focuses on the evaluation of flow and scalar transport characteristics of small disinfection systems, primarily through computational fluid dynamics (CFD) as well as physical conservative tracer studies. Original research was performed on a pipe loop, series of pressurized tanks, and two separate open surface tank contact systems and a case study was performed on a baffled tank system. The flow dynamics for each of these respective disinfection systems were evaluated using CFD. The flow dynamics govern the transport of any quantity (e.g., a passive scalar, conservative tracer, or chlorine-containing species) through the system visualized through plotting the effluent concentration (e.g., passive scalar for computational models and conservative tracer for physical experiments) through time forming what is commonly referred to as a residence time distribution (RTD), or flow-through, curve. Physical experiments provided validation for the CFD models that give a more complete view of hydraulic efficiency thus overcoming the common "black-box" approach to contact tank design using only the theoretical detention time (TDT) (defined as the system volume V divided by the volumetric flow rate Q). The differing geometries of contact tank systems yield significantly different flow paths with varying degrees of separation, recirculation, inlet and outlet effects, and wall effects prompting the need for the evaluation of hydraulic efficiency to be unique to the system. Yet current practice evaluates the hydraulic efficiency of disinfection contact tank systems based on the TDT and the rising limb of the RTD curve, designated by the United States Environmental Protection Agency (USEPA) as baffle factor (BF). Research presented in this study using CFD models and physical tracer studies shows that evaluation methods based upon TDT tend to overestimate, severely in some instances, the actual hydraulic efficiency as obtained from the systems' flow and scalar transport dynamics and subsequent RTD curves. The main objectives of this study were to determine the systems' respective hydraulic efficiencies and to analyze an alternative measure of hydraulic efficiency, the ratio t10/t90, where t10 and t90 are the time taken for 10 and 90 percent of the input concentration to be observed at the outlet of a system. The pipe loop system was dominated by advection and thus showed little variance in the values of BF and t10/t90. Analysis of the series of pressurized tank systems showed significant regions of turbulent mixing and recirculation corresponding to a system that was much less efficient than the pipe loop system. BF values for the pressurized tank systems were nearly 100 percent greater than t10/t90 values as a result of a system behavior further from plug flow. The open surface tank systems exhibited the most uneven flow paths and lowest efficiencies seen in this study with BF and t10/t90 values differing by at least 100 percent. These systems exhibited significant degrees of short-circuiting and recirculation largely due to their inlet and outlet configurations. Finally, the baffled tank system showed an increase in system efficiency with the number of baffles (e.g., increase in advective forces) and a corresponding decrease in the variance between BF and t10/t90 values. Overall, the research presented in this thesis provides an extensive evaluation for the flow and scalar characteristics of the described small public drinking water disinfection systems allowing for the development of t10/t90 as a more representative evaluation of hydraulic efficiency.