Contact Tank Design Impact on Process Performance

Athanasios Angeloudis,Carlo Gualtieri,Thorsten Stoesser,Roger A Falconer

doi:10.1007/s10666-016-9502-x

Athanasios Angeloudis, Carlo Gualtieri + Show 2 more

Open Access

https://doi.org/10.1007/s10666-016-9502-x

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Abstract

In this study three-dimensional numerical models were refined to predict reactive processes in disinfection contact tanks (CTs). The methodology departs from the traditional performance assessment of contact tanks via hydraulic efficiency indicators, as it simulates directly transport and decay of the disinfectant, inactivation of pathogens and accumulation of by-products. The method is applied to study the effects of inlet and compartment design on contact tank performance, with special emphasis on turbulent mixing and minimisation of internal recirculation and short-circuiting. In contrast to the conventional approach of maximising the length-to-width ratio, the proposed design changes are aimed at addressing and mitigating adverse hydrodynamic structures, which have historically led to poor hydraulic efficiency in many existing contact tanks. The results suggest that water treatment facilities can benefit from in-depth analyses of the flow and kinetic processes through computational fluid dynamics, resulting in up to 38 % more efficient pathogen inactivation and 14 % less disinfection by-product formation.

Highlights

Disinfection contact tanks (CTs) are designed to prevent waterborne pathogen transmission and are integral components of water treatment works
With the exception of CT-7 and CT-8, all of the modified designs exhibited an residence time distribution (RTD) peak occurring earlier compared with the original set-up at the outlet monitor point (Fig. 5d). This was Operational and hydraulic efficiency parameters for each design are summarised in Table 3, highlighting how the theoretical retention time (Tm) was altered according to the internal volume occupied by the geometry modifications
The validity of the numerical model data was assessed through comparisons of (a) predicted time-averaged velocities and (b) transient tracer transport breakthrough curves against available experimental data

Summary

Introduction

Disinfection contact tanks (CTs) are designed to prevent waterborne pathogen transmission and are integral components of water treatment works. While the design of CTs somewhat depends on the disinfection method adopted, they are frequently divided into compartments using internal baffles, facilitating a meandering flow structure. For plug flow and a uniform crosssectional distribution of the disinfectant (e.g. chlorine or ozone), the optimal contact time for the desired quality standards would be consistently achieved. The hydrodynamics in CTs deviates further from plug flow if the flow recirculates due to the tank geometry. RTD curves are able to detect short-circuiting, leading to insufficient exposure of pathogens to disinfectants, or internal recirculation, resulting in overexposure and the formation of by-products, which are of concern in the light of their association with health implications [27]

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