EVALUATION OF DYNAMICS AND TOTAL EMISSION OF MICROPLASTICS AT WASTEWATER TREATMENT PLANTS WITH OXIDATION DITCHES

Abstract

 Dynamics and total emission of microplastics (MPs) were evaluated at 5 wastewater treatment plants that applied an oxidation ditch (OD) process. Number density of MPs with a major diameter of 0.5 to 5 mm in effluent was 2.0 to 4.5×10 pieces/m³, and the mass density was 1.87×10-2 to 7.72 mg/m³. In the OD process, MPs were subdivided and increased in number, but most of them were discharged with sludge. The average MPs removal from the OD and secondary sedimentation tank was 92% in number and 90.6% in mass. The 87% number of MPs and the 96.7 % mass of MPs were removed from the sand filtration on average. Based on the investigation and statistics, the annual MPs emissions from all sewage treatment plants with ODs in Japan are estimated: 1.4×10¹⁰ pieces/year and 1.17×10³kg/year for a major diameter of 0.5 to 5mm, and 7.9×10⁹pieces/year and 1.11×10³kg/year for a major diameter of 1 to 5mm, respectively.