The effect of liquid-level height (H) on floc growth during flocculation has been investigated in unbaffled square stirred tanks with a fixed bottom width (D). Firstly, flocculation tests were performed by using an in situ recognition system for floc morphology to evaluate flocculation performance within each tank at three typical shear rates of G(ave) = 10, 30 and 70 s(-1). Then, turbulent flow fields generated under all flocculation-test conditions were predicted by computational fluid dynamics (CFD) simulations, followed by a detailed discussion based on experimental and numerical data. It was found that stirred tanks with different H values (characterized by H-to-D ratio) caused distinct turbulent flow fields at the same shear rates examined, reflected by different turbulent intensities in the impeller zone and different non-uniform nature of turbulence in the overall flocculating system. Accordingly, the evolution of floc size and structure was greatly affected due to various aggregation and breakage kinetics. Moreover, the effect of H-to-D ratio on floc growth appeared to be related to the predominant growth mechanism(s), i.e., for the shear rates of G(ave) = 10 and 30 s(-1), little or no significant breakage occurred and more significant non-uniformity of turbulence generated in the stirred tank with a higher H-to-D ratio led to increasingly pronounced behavior of restructuring, forming aggregates with more compact structure caused by primary-particle rearrangement, whereas at G(ave) = 70 s(-1), breakage dominated over aggregation and resultant aggregates became smaller and more compact with increasing H-to-D ratios, possibly due to the irreversibility of floc breakage. Therefore, an appropriate liquid-level height of tank should be designed to enhance floc formation and subsequent particle removal efficiency. The combined means of flocculation tests and CFD simulations may provide useful insights for optimizing the design and operation of stirred-tank reactors for flocculation. (C) 2018 Institution of Chemical Engineers. Published by Elsevier B.V. All rights reserved.