The potential of electron beams for the removal of microplastics from wastewater and sewage sludge

Wastewater and sewage sludge treatment plants are known to be important entrance routes for microplastics (MPs) into the aquatic environment. Although traditional wastewater treatment plants with primary and secondary purification processes can remove over 99 % of MPs, the majority of these are believed to be deposited into the sewage sludge. This provides a route into the environment, especially if the treated sludge is used as a fertilizer on agricultural land. Although methods do exist for the removal of MPs from water, there are limited studies with sludge. The possibility of using electron beam treatment for the removal of microplastics from wastewater and sewage sludge has been investigated in the laboratory using both tap water and actual sewage sludge. Microplastics were created from six types of plastics in daily use, <1.5 mm in size, and treated with a range of doses in tap water and in sewage sludge, taken from a sewage sludge treatment plant, both before and after anaerobic digestion, with a 10MeV accelerated electron beam. In this treatment, the electrons were found to act as a coagulant by modifying the surface charge of the microplastics. This significantly increases the sedimentation of four out of six of the types of MP in both water and sludge, while the other two largely remain floating. Density separation techniques of the treated samples have then demonstrated removal efficiencies in the range 85–95 % for five of the MP types and of around 70 % for the sixth at reasonable EB doses. This potentially provides a route to high efficiency removal from sludge.