Problem
Short circuiting and insufficient residence time are common problems in chlorine contact tanks. Water engineers have a duty of care to ensure that contact tank (CT) designs meet hydraulic efficiency requirements and that disinfectant dosing is effective in removing undesirable pathogens.

Solution
Computational fluid dynamics (CFD) simulation of contact tanks provides an invaluable tool in understanding chlorine contact kinetics and hydraulic performance. As chlorine is added, it reacts with organic and inorganic substances and begins to decay. This process can be modelled using a parallel decay model where reactions are included that account for both fast reacting substances and bulk decay of chlorine in the tank. Hydraulic assessments can also be performed to simulate experimental tracer tests and determine key criteria such as:

• Chlorine contact times

• Time for 10%, 90% and Mean Residence (t10, t90 and tg)

• Morrill index (Mo)

• Dispersion Index (s²)

Result
CFD modelling provides a cost-effective method to ensure contact tank designs are fit for purpose and has several advantages over scale tank tests including:

• Faster comparison between different tank designs

• Simulation of true scale and wall roughness (no scaling for equivalent Froude and Reynolds numbers)

• Predictions of chlorine consumption and bacteria populations