A high-efficiency mini-hydrocyclone for microplastic separation from water via air flotation

A mini-hydrocyclone was designed to remove microplastics from water through air flotation. The hydrocyclone geometry parameters were optimized through the combination of the orthogonal design method and numerical simulation. The performance of the optimized mini-hydrocyclone was experimentally evaluated. The experimental and numerical underflow pressure drop results showed a consistent trend in the considered flow range. The separation efficiency increased with increasing inlet flow rate, but increased, and then decreased, with the flow split ratio. The separation performance of microplastic in the entire size range could be improved through the injection of air bubbles into the water. The separation efficiency enhancement ranged from 5 % to 15 % within a split ratio range of 0.04 to 0.23. The separation efficiency increased rapidly with a low flow rate of air injection and remained high (> 92 %) with a further increase in the air flow rate. The concentration ratios of the hydrocyclone were all > 7.8 owing to low flow split ratios. The maximum concentration ratio reached 9.7 at a feed flow rate of 0.45 m(3)/h, a split ratio of 0.1 and an air holdup of 5.4 %. The results demonstrated that the designed hydrocyclone with injected microbubbles can achieve high separation efficiency and concentration rate at the same time. The air flotation-induced increase in grade efficiency was mainly due to the density reduction effect of MP-bubble agglomerates, and the volume expansion effect was relatively small. The cyclonic air flotation technology can be applied to small-scale water treatment, such as separating microplastics in a washing machine.