This study proposes a secondary flow mixer for neutralizing reagent in a de-ballast pipe. Because the proposed mixer has a U-bent pipe configuration, it can be fabricated using commercially available pipe parts at low cost. Numerical simulations were conducted with four computational domains: one straight pipe (SP) and three U-bent pipes. The normalized coefficient of variation (CoV/CoV0) values of the U-bent pipe outlet for UP1, UP2 and UP3 were 0.096, 0.036 and 0.015 (90.4%, 96.4% and 98.5% degree of mixing, respectively). Instability was observed in the secondary flow motion because of the continuous curvature change of the U-bent pipe series and the density difference between mixing fluids. This instability enhances the mixing performance. Despite the high mixing quality, the U-bent pipe mixer caused a pressure drop of 0.5–2.1 % compared to the backpressure of a ballast water treatment system.