Variation in microplastic concentration, characteristics and distribution in sewage sludge & biosolids around the world
Daisy Harley-Nyang, Fayyaz Ali Memon, Andrea Osorio Baquero, Tamara S. Galloway
Abstract
Microplastics have been reported in wastewater treatment works across the world. The majority of microplastics are removed during the wastewater treatment process, with removal rates between 57 % and 99 %. What happens to the microplastics that are removed from the wastewater, and how they accumulate in sewage sludge and biosolids (by products of the wastewater treatment process), remains a topic of high interest. Here we systematically reviewed the current state of knowledge on the presence, concentration, and characteristics of microplastics in sewage sludge and biosolids globally to understand how biosolids may act as a pathway for microplastics pollution to soils. A systematic search was performed on the Web of Science and Science Direct data bases. Sixty-five studies reporting on microplastic pollution in sewage sludge and biosolid products were identified, spanning twenty-five countries. Reported microplastic concentrations varied considerably from 0.193 microplastics/g to 1.69 × 105 microplastics/g with a median microplastic concentration of 22.41 microplastics/g, illustrating how much microplastics are captured during the wastewater treatment process, and retained in the sewage sludge. The extent to which biosolid recycling pollutes the terrestrial environment was compared across countries. High numbers of microplastics were estimated to reach fields via biosolids with wide variation across countries (8.2 × 1010 to 1.29 × 1015 microplastics/year) although, there was no significant difference in microplastic concentration between fields with a history of biosolid applications and control fields. The comparative risk this delivery of approx. 0.4 to 6430 tonnes poses compared to the environmental benefits of nutrient and carbon recycling associated with biosolids reuse or compared to other sources of microplastics pollution remains a global research imperative. The next step in scientific research needs to focus on solutions to the biosolid and circular economy conundrum – biosolids are a valuable source of nutrients, but contain high concentrations of microplastics, which are ultimately entering the terrestrial environment.