A new research study is warning that there may be greater levels of microplastics in the soil than in our oceans as a result of large amounts of microplastics emitted from households, industry and surface run-off in urban areas, which may accumulate in the sewage sludge being used as a fertilizer for agriculture and horticulture.

The research says that the world's oceans are not the only recipient of microplastics - the small plastic pieces less than five milliimeters long are also entering waste water treatment plants.

According to an article recently published in the journal Environmental Science & Technology by researchers from from one of Norway's water research institutes, the Norwegian Institute for Water Research (NIVA)and the Swedish University of Agricultural Sciences (SLU), sludge from municipal sewage treatment plants is applied to agricultural areas as a supplement to traditional fertilizers.

The problem, the research team said, is that while sewage sludge is treated to remove a variety of regulated hazardous substances, microplastics are not currently on the regulatory agenda, however.

Large fraction of microplastics end up in sludge in wastewater treatment plants

NIVA researcher Luca 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.

"Our estimates suggest that between 110.000 and 730.000 tons of microplastics are transferred every year to agricultural soils in Europe and North America, comprehensively. This level of microplastics exceeds the estimated total burden of microplastics currently present in ocean water."

David Noble, a spokesperson for Bluewater, a world-leading Swedish manufacturer of second-generation reverse osmosis residential water purifier solutions, commented:

“More research is clearly needed to get a clearer overview of a problem that may potentially impact human growth, reproduction and survival that appears to have been largely over-looked until now'.

“It is deeply worrying that chemical-based microplastics are not only entering the human food chain via marine life but also now possibly through food grown for human consumption.”

The Norwegian study said the amount of sewage sludge used as fertilizer varies substantially from country to country. In Europe and North America, approximately 50 percent of such sludge is reused as fertilizer on average.

The first simulation of microplastic fate on land and rivers

In an earlier study by the same authors and researchers at Oxford University, the first mathematical model describing the dynamics of microplastics’ fate in terrestrial environments and rivers was presented.

The simulations from the model - known as INCA Microplastics - showed a strong influence of meteorological conditions and river characteristics and flows in controlling the export of microplastics from agricultural soils and their transport to the ocean.

Application of sewage sludge to soils likely represent a considerable source of microplastics to the coastal and ocean environments. Similar predictions for the transport of microplastics in rivers were independently confirmed by a follow-up study by Besseling et al.

INCA Microplastics is an important tool for risk assessment and evaluating sludge management scenarios. It is the first model able to simulate microplastic applications to land, and the consequent fate of these materials in soils and surface waters.

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