We propose a systematic procedure that combines computational fluid dynamics (CFD) modeling and experimental work to answer two research questions that are usually posed by researchers and managers of water treatment plants: ‘Is the effect of wind on settling tanks important?’ and ‘How can we determine this effect in our settling tanks?’ We apply this procedure in the water treatment plant of Aharnes, Athens to derive the following conclusions. (1) The effect of wind increases with increasing co-current wind velocity, increasing settling velocity and decreasing flow rate. (2) In windy steady-state flow conditions, the degree of complexity and three-dimensionality of the flow field that is observed in calm conditions is reduced and the removal efficiency decreases from 85.1 in calm conditions to 82.0%. Predicted efficiencies for constant and variable inlet solids' concentrations compare favorably with measurements. (3) In windy, transient flow conditions, field data show that the effect of wind on the tank's efficiency can be very pronounced and within the first half hour of the windy period the efficiency decreases to approximately 55%; the present model does not capture this effect, because it cannot simulate the sludge layer and the subsequent re-suspension of the settled solids.