Peatland drainage and peat extraction result in runoff water rich in humus, sediments, and nutrients that requires simple purification methods to prevent pollution of surface waters. Chemical treatment of drainage water has been suggested as best management practice. However, good chemical purification results require flocculators to achieve efficient particle aggregation. This study evaluated gravity-driven hydraulic flocculators using full-scale three-dimensional (3D) computational fluid dynamic (CFD) turbulence models to simulate hydraulics, combined with data obtained in jar tests to estimate optimal mixing conditions for coagulation [velocity gradient (G-values)]. The CFD model was first run for several barrier configurations. The optimal structure was then tested for different ratios of distance between barriers [or opening slot width (B)] and flocculator width (W). The relationship between distribution of G-values and the target value of 40–60 s− 1 was determined for different designs. The best B= W ratio was found to be 1= 4. For this ratio, the flow depth was the only variable parameter that needed to be optimized by the CFD model to achieve target G-values. DOI: 10. 1061/(ASCE) IR