Luca Nizzetto and Sindre Langaas, from the Norwegian Institute for Water Research (NIVA) and Martyn Futter, from the Swedish University of Agricultural Sciences (SLU) in Uppsala, say microplastics in soils have largely been overlooked.
In an article recently published in Environmental Science & Technology, the researchers said sewage sludge is in principle waste, but it can also represent a resource in agriculture and horticulture. Fertiliser based on sludge contains valuable nutrients, but sustainable use requires that the levels of undesirable substances in the sludge is kept under control.
However, they said, wastewater treatment plants receive large amounts of microplastics emitted from households, industry and surface run-off in urban areas. Most of these microplastics accumulate in the sewage sludge.
Today, sludge from municipal sewage treatment plants is applied to agricultural areas as a supplement to traditional fertilisers. These applications are generally well regulated as sludge might contain hazardous substances of different sorts. Microplastics are, however, not currently on the regulatory agenda for the use of sludge in agriculture.
The potential consequences for sustainability and food security have not been adequately analysed, said the researchers.
Nizzetto said: “We have found figures from the Nordic countries suggesting that a large fraction of all the microplastics generated in Western societies tend to end up in the sludge in wastewater treatment plants. Via the sludge the particles are transferred to agricultural soils.”
The amount of sewage sludge used as fertiliser varies greatly from country to country. In Europe and North America approximately 50 per cent of this sludge is reused as fertiliser on average. According to Statistics Norway, about two thirds of the sludge is reused in this manner.
The researchers’ estimates suggest that between 110.000 and 730.000 tons of microplastics are transferred every year to agricultural soils in Europe and North America. This is more than the estimated total burden of microplastics currently present in ocean water.
These figures are of concern since the effects of microplastics accumulating in agricultural soils are unknown.
There is very little knowledge on the effect of microplastics on soil organisms, and their impact on farm productivity and food safety is unknown.
“Clearly further research is needed to get an overview of the problem – and to find solutions – so that the growing need in the community for recycling and so-called circular economy can be safeguarded,” said Nizzetto.