Are Agricultural Soils Dumps for Microplastics of Urban Origin?

D to their ubiquitous distribution and chemical composition, microplastics (MPs) are increasingly being recognized as a global concern. While it is widely acknowledged thatMPs in the ocean are a serious issue with potentially negative effects onmarine organisms, information aboutMPs in terrestrial and freshwater environments is fragmentary. Based on new MP emission estimates in industrialized countries, we suggest that widespread application of sewage sludge frommunicipal wastewater treatment plants (WWTPs) to farmlands is likely to represent a major input of MPs to agricultural soils, with unknown consequences for sustainability and food security. Terrestrial emissions are the dominant source of MPs, including those conveyed to receiving waters by WWTP outfalls. Recent reports based on product life cycle data offer the first quantitative insights into national MP emission inventories. MPs originate predominantly from automobile tire wear, household and laundry dust, industrial processes (e.g., blasting and deflashing of plastics), and through deterioration of surfaces made of or coated with plastic, for example, artificial turf and polymeric paint. Most of these emissions occur in urban and residential areas. In developed regions, municipal/industrial effluents and even diffuse urban runoff are eventually conveyed to WWTPs. During wastewater treatment, over 90% of MPs are retained in sewage sludge. Effectiveness of MP retention is dependent on particle density and size. MPs with a density greater than water are almost completely retained in sewage sludge during primary and secondary treatment. Tertiary filtration treatment effectively removes larger floating particles, while smaller and lighter particles, expectedly, are released with wastewater effluents. The use of sewage sludge as fertilizer for agricultural applications is often economically advantageous and is common in many developed regions. In Europe and North America about 50% of sewage sludge is processed for agricultural use. Using national data on farm areas, population and sewage sludge fate (http://ec.europa.eu/eurostat), with estimates of MP emissions and applying broad but conservative uncertainty ranges, we estimate that between 125 and 850 tons MP/million inhabitants are added annually to European agricultural soils either through direct application of sewage sludge or as processed biosolids. This is at least equal to, and probably much higher than our estimate of 110 to 180 tons MP/million inhabitants emitted annually to surface waters based on refs 1−3. In Europe, in fact, between 1270 and 2130 tons MPs/million inhabitants are released to urban environments, annually. Conservatively assuming that 10−90% of MPs produced from road wear and debris from building coating are collected by sewers, between 360 and 1980 tons MPs are expected to reach municipal WWTPs. Here, an uncertain fraction of MPs from car tire debris (conservatively, 20−80%) and >90% of MPs from personal care products are likely to be retained in sludge, giving a total input of between 250 and 1700 tons/million inhabitants each year. Sludge application to agricultural land was calculated as the sum of direct application and application of processed biosolids, excluding the fraction of wastewater sludge incinerated, disposed in landfills or subject to other nonagricultural uses. These figures are highly conservative as sludge is only ever applied to a small percentage of agricultural land. There is a broad range of sludge application rates and intensities to European agricultural land (Figure 1). Application rates (estimated as compost plus direct application) range from 0 to 91%, with an average of 43%. This equates to average and maximum areal per-capita loadings of 0.2 and 8 mg MP/ha/yr. MPs inputs estimated here possibly reflect the situation in other countries with similar socioeconomic conditions and/or similar use of plastics (e.g., in Asia and the Americas). A rough extrapolation from data in refs 1−3 produces a total yearly input of 63 000−430 000 and 44 000−300 000 tons MPs to European and North American farmlands, respectively. This would be an alarmingly high input. Comprehensively, this exceeds the total accumulated burden of 93 000−236 000 tons MPs currently estimated to be present in surface water in the global oceans.