The impact of turbulence models on the simulation of residence time distribution in a multichamber ozone contactor

The flow and tracer transport in an ozone contactor is simulated by using computational fluid dynamics. The standard k- model, RNG k- model, Realizable k- model and SST k- model are used to describe turbulence. A step change method is used to simulate the residence time distribution. The residence time and cumulative residence time are compared with laboratory experiments. All turbulence models can capture the feature of the residence time distribution and cumulative residence time distribution. The residence time distribution in the initial period is sensitive to the used turbulence model, which can affect the value of MDI. The standard k- model behaves better among four turbulence models.

APPLICATION OF PRE-OZONATION FOR REMOVAL OF DISINFECTION BY- PRODUCTS PRE-CURSOR FROM SAIGON RIVER WATER

This study aimed to apply pre-ozonation for removal of disinfection by-products precursors to reduce the formation of trihalomethanes (THMs) in Saigon river water. The pre-ozonation process was conducted in a lab-scale ozone contactor. The raw water sample was collected from Saigon River at the Hoa Phu pump station in Cu Chi District, Ho Chi Minh City. The suitable condition for pre-ozonation was found. At ozone dose of 1.5 mgO3/mgDOC, pH of 8.5, and contact time of 15 minutes, the removal efficiency of turbidity, color, iron, manganese, dissolved organic carbon (DOC), trihalomethanes formation potential (THMFP) were 36 %, 25 %, 56 %, 81 %, 35 %, and 46 %, respectively. These results indicated that the pre-ozonation could reduce DOC concentration and thus resulted in significant mitigation of THMs formation. In comparison with pre-chlorination, THMFP of raw water pretreated by pre-ozonation was lower. Moreover, this study showed that the pre-ozonation was more effective than pre-chlorination in terms of turbidity, color, manganese, and DOC removals.

Optimization for Water Outlet of an Ozone Contactor by Computational Fluid Dynamics

Geometry optimization is an effective method to improve the hydraulic efficiency of an ozone contactor. The enhancement of hydraulic efficiency can lead to a smaller dose of ozone and thus minimizing the potential risk caused by disinfection by-products. Unlike adding some extra structures in geometries in previous studies, optimization for water outlet was applied to improve the hydraulic efficiency in this paper. The water outlet was modified to several parallel flumes arranged uniformly on the top of the contactor from overflow weir. In a typical contactor with diffusers, the effect of water outlet optimization on system performance was studied based on computational fluid dynamics employing a two dimensional Euler model coupled with species transport model and discrete particle model. The T10/HRT, the most common indicator of hydraulic efficiency, was improved by 10%-24% with less short-circuiting after optimization. Compared to the original geometry, optimization for water outlet brought about a 19.6% increase in log inactivation, which represents the disinfection efficiency. To maintain the same Cryptosporidium inactivation efficiency after the water outlet optimization, ozone dosage could be reduced by 18% from 2.0 mg/L to 1.64mg/L, which resulted in a minimization of bromate formation by 8.90%.