Microplastics Separation Using Stainless Steel Mini-Hydrocyclones Fabricated With Additive Manufacturing (3D Printing)

Introduction
Microplastics (MPs) have been widely detected in natural and engineered water systems. Removing MPs from water resources is becoming a major challenge for water and wastewater treatment facilities1,2. The recalcitrant chemical properties of MPs allows them to easily transport, accumulate, and persist in the environment3,4. To address the challenges of water resource security under the context of climate change, an increasing amount of effort from governments and water agencies has been devoted to water reclamation. Removal of these contaminants from water and wastewater is a major problem in water reclamation largely due to the lack of verified treatment technologies that eliminate MPs5. Hydrocyclones, a hydraulic device widely used in industries such as oil refinement, food processing, and pharmaceutical manufacturing, have been adopted to process plastic waste and could be potentially used for microplastic separation6–8. Mini-hydrocyclones (MHCs) are designed with smaller nominal diameter and cut size and are proven to be more efficient for fine particle separation9,10. This study is the first to incorporate MHCs that were designed and manufactured by 3D printing with stainless steel to separate microplastic particles from water. The objective of this study is to investigate the separation efficiency of fine MP particles using MHCs both single-stage and in series.
Material and Methods
Mini-hydrocyclones of two different configurations were designed for separating MPs with densities lower (MHCL) and higher (MHCH) than water density, respectively (Fig.1). The design parameters (Fig.2, Table 1) (i.e. geometrical proportion) for MHCH were modified from Bradley11 since its parameters were more suitable for lower cut sizes. The geometrical proportions from Yang et al.12 were selected as the reference for MHCL because the density and particle size ranges of the medium to be separated in the reference paper are similar to those in this study. Three metal MHCs (MHCH1, MHCH2, and MHCL) (Fig.1) were manufactured using 3D-printing technology from Shanghai Yuerui 3D Technology Co., Ltd. EOS Stainless Steel 316L was used as the material for the three MHCs. The surface roughness of the hydrocyclones' inner wall is 5 ± 2 µm, which can reduce the effect of inner wall friction on MHC flow field during separation. Polyamide (PA) (ρ = 1150 kg/m3) and low-density polyethylene (LDPE) (ρ = 924kg/m3) are abundantly found in various aquatic environment2,3,6,15,16. Thus, PA and LDPE particles were selected to represent MPs with high and low densities, respectively. The separation efficiency test was conducted using single-stage MHCs (Fig.3a) and MHCs in series (Fig. 3b). Operational parameters including flow rate, split ratio, particle concentration, and feed pressure were also varied to assess MHCs' separation performance under different conditions. Overflow, underflow, and feed samples were collected and processed to calculate total mass separation efficiency (Et) and grade separation efficiency (Eg). Contamination control was implemented throughout the study, including thoroughly cleaning the hydraulic circuit after each trial and covering MHC inlets and outlets with aluminum foil to prevent rusting. Negative control samples were also taken from the system, and an insignificant amount of plastic particle contamination was determined.
Results and Discussion
The optimal operational parameters of the MHCs were determined from the single-stage arrangements. Both MHCL and MHCH1 were operated with flow rate (Q) varying from 0.3 GPM to 0.6 GPM and split ratio (R) varying from 15% to 45%. A separation efficiency peak was observed in MHCL and MHCHat a larger flow rate (0.6 GPM) (Fig.4) because the MP particles experienced larger centrifugal forces due to increases in tangential velocities. In addition, (R) of 35% was found to be optimal for both MHC configurations (Fig.5). Split ratios less than 35% resulted in unstable and insufficient MP removal of all sizes, while split ratios higher than 35% amplified turbulence and diminished removal by breaking up MP aggregates. A performance comparison test was conducted between MHCH1 and MHCH2 to determine the effects of different inner diameters. It was found that the head loss in MHCH1 was greater than that in MHCH2 (Fig. 6). This was because the shear force had stronger effects on the fluid mixture in MHCH1 due to its smaller internal chamber.
Moreover, MHCs were arranged in series (MHCH1 to MHCH2 and MHCL and MHCH1) (Fig.3b). An improvement of 10-15% was observed in the separation of PA particles when the two MHCHs were used, demonstrating how MHCs in series can enhance the separation of MPs with similar densities. The separation efficiencies of each individual MHC reduced when MHCH and MHCL were arranged in series. This can be explained by the fact PA and LDPE particles were moving in opposite directions and the increased particle collision subsequently reduced particle velocity.
Conclusions and Future Studies
This study demonstrated the use of additively manufactured MHCs as an alternative for MP removal from water. Under optimized operational parameters, the separation efficiency can reach up to 95% and 87% for high and low density microplastics, respectively. In general, larger particles or MP aggregates are more efficiently separated. Mini-hydrocyclones in series were observed to separate microplastics of different densities, however separation efficiency can be improved via operational parameters or chemical enhancement. The study shall continue with further tests of microplastics in various aquatic environment and with chemical enhancement, as well as secondary microplastics.
Acknowledgements
The research project is funded by Santa Margarita Water District and Brown & Caldwell.