Use of CFD for wastewater disinfection process analysis: E. Coli inactivation with peroxyacetic acid (PAA)

Historically, the priority in disinfection reactor design has been microbial inactivation with a high factor of safety, neglecting other processes such as disinfection byproduct (DBP) formation and chemical use minimization. With heightened awareness and regulation of DBPs, advanced chemical engineering principles and practices are being employed in disinfection process analysis and design in order to minimize DBP production while ensuring sufficient microbial inactivation. Computational fluid dynamics (CFD) is increasingly popular as a tool to evaluate reactor performance and assess design concepts. This study presents the results of CFD analyses of two disinfection reactors with very different aspect ratios and design, but being characterized by the same theoretical hydraulic residence time (i.e., 30 minutes). Both reactors were used to inactivate E. coli with the novel disinfectant peroxyacetic acid (PAA). CFD calculations were performed for inactivation of E. coli (10000 CFU/100 mL) with PAA concentrations of 4 and 8 mg/L. Calculations were carried out for two serpentine reactors with aspect ratios (channel width to water depth) of 2:1 and 1:3. In the second reactor, perforated baffle plates were placed at the end of each channel in an attempt to promote uniform flow. Both reactors achieved adequate removal of organisms, but displayed non-ideal hydrodynamics, with significant recirculation regions that reduced the volume available for reaction and promoted short-circuiting. CFD simulations emphasized that reactor hydrodynamics impacts on the specific disinfection performances inducing different ratio of reactor volume active during the disinfection process.