The report summarizes the results on the sources, behaviour and ecological impacts of microplastics from sewage sludge application to agricultural soils.

IMPASSE (Impacts of MicroPlastic in Agrosystems and Stream Environments) was an initiative formed by six research institutes from Norway, Sweden, Spain, Canada, the Netherlands, and Slovenia under the EU ERA-NET scheme through WaterWorks2015. It was one of the first internationally funded projects to tackle the sources, behaviour, and ecological impacts of microplastics (MPs) from sewage sludge application to agricultural soils. IMPASSE has provided a new scientific basis towards developing effective policies and instruments for preventing plastics from contaminating wastewater resources.

This report summarizes the project results and provides recommendations to the ongoing policy debate on the safe use of biosolids from sewage sludge in the context of a circular economy. Stakeholders (farmers, the water industry sector, and governmental bodies) served both as recipients and providers of information throughout the project.

Wastewater Treatment Plants (WWTPs) receive large amounts of MPs – predominantly polyethylene and polypropylene fragments and polyester fibers – from households, industry, and surface/road run-off in urban areas. Most of these MPs are retained in the sewage sludge. In many countries WWTP sludge is converted into biosolids and applied to agricultural fields as a supplement to traditional fertilizers becoming one of the main sources of MPs in agricultural soils. 

MPs are increasingly seen as an environmental problem of global proportions. In the Spanish case study, it was estimated that around 10 billion MPs are discharged annually via wastewater effluents alone. However, there is a lack of knowledge regarding the consequences of MP pollution for agricultural landscapes and freshwaters.  

Impacts on biodiversity

While the risk posed by MPs to soil invertebrates at environmentally relevant levels are low, the occurrence of sublethal effects – even at environmentally realistic concentrations – imply that prolonged exposure can potentially induce negative effects on soil biota. The continued addition of MPs to agricultural soils will increase the pressure and risk to soil organisms, therefore regulations on the use of sewage sludge in agriculture should include legal thresholds for MPs.

In the context of a circular economy, it is expected that the use of biosolids in agriculture will increase due to the need of a cost-effective disposal of WWTP solid waste, increasing economic efficiency of agricultural production (e.g., by reducing the use of artificial fertilizers). Effective policies for protecting water ecosystems from MPs should initially focus on reducing sources other than biosolids, while measures to reduce releases of MPs at sources and their input to wastewater treatment plants will be necessary in the future to guarantee circularity in the use of sludge. Furthermore, climate change will result in an increased frequency of extreme rainfall events. Events such as extreme precipitation can release a large amounts of MPs to downstream environments. Management solutions designed around meteorological events can therefore serve to delay, but not reduce, MP export to the environment.

Decision support tool

Knowledge and models delivered by IMPASSE were used to frame and investigate economically viable, yet environmentally and socially sustainable management of biosolids from sewage sludge. A new mathematical model of physical transport of MPs in the soil and freshwater at the watershed scale (INCA-MP) was developed through the course of the project. The INCA-MP model, which is the first and only of its kind, is available as an executable computer program with a full graphical interface that will facilitate future scientific developments. In addition, a set of MP reference materials was developed and launched for commercialization. These products have been distributed internationally to support quality assurance and control, method validations and water treatment efficiency assessments.

IMPASSE has therefore provided a new scientific basis to continue the debate towards effective policies and instruments for preventing plastics from contaminating wastewater resources. In order to safeguard circularity in the use of sewage sludge, policies, management approaches and technologies that cost-effectively reduce or, better, remove completely MPs from sewage sludge are strongly endorsed. 

What´s next

The Project PAPILLONS (Plastics in Agricultural Production: Impacts, Life-cycles and Long-term Sustainability), built upon the IMPASSE consortium, includes 20 European and Chinese research partners and a list of 20 Stakeholders in the governance, industry, and farming sectors, and is expected to start in June 2021. The PAPILLONS project will contribute to answer some of the open questions regarding the sustainable use of plastic in agriculture, such as the impacts of microplastics on soil biota, and on crop efficiency.