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The potential environmental harm of nanoparticles contained in sewage sludge

Lysimeter: a piece of outdoor equipment used by Rothamsted to examine the fate and movement of nanomaterials 

An international study, involving Rothamsted Research scientists, reveals the potential harm of tiny metal particles contained in treated sewage sludge.

A team of international scientists, including scientists from Rothamsted Research, have found that treated sewage sludge containing tiny man-made metal particles, called nanoparticles, may be toxic to plants and soil microorganisms. The build-up of what are man-made metal particles in sewage sludge has the potential to impact the use of this recycled material as an agricultural fertiliser. The research, which was funded as part of the Environmental Nanoscience Initiative (ENI), is published in the journal of Environmental Science & Technology.

Sewage treatment is the process of removing contaminants from wastewater, by physical, chemical and biological processes, to produce clean wastewater. A by-product of sewage treatment is a semi-solid slurry, called sewage sludge which undergoes further treatment before being spread on agricultural land as treated sewage sludge.

Nanoparticles of silver, titanium dioxide and zinc oxide are increasingly used in common industrial and consumer products like sunscreens, cosmetics and textiles and thus could end up in sewage treatment plants, where they are removed from wastewater and reside within sewage sludge as nanomaterials.

For many decades treated sewage sludge has been used successfully as a fertiliser on agricultural land, as it can provide much needed nutrients and organic matter to the soil. However, until recently manufactured nanomaterials have not been present in sewage sludge. The research, led by American scientist, Dr Jason Unrine, questioned whether current U.S. Environmental Protection Agency regulations for metals would be protective when treated sewage sludge contained nanomaterials.

The experimental research studied the effects of treated sewage sludge containing the nanomaterials on legumes and their nitrogen-fixing bacteria, and the community of other soil microorganisms. The experiment was set up to examine the worst-case scenario for soil exposed to treated sewage sludge containing the likely U.S. maximums of silver, titanium dioxide and zinc oxide nanoparticles.

Scientists at Cranfield University first constructed three pilot wastewater treatment plants to produce sewage sludge containing nanoparticles, dissolved metals at an equivalent concentration and a control sludge representing what is currently spread to land. Rothamsted Research scientists then mixed each treated sewage sludge with soil and allowed the mixtures to age outdoors. The aged treated sewage sludges were sent to the University of Kentucky for analysis. A research team at Carnegie Mellon University examined the chemical form of metals in the three different soils. 

It was found that the treated sewage sludge with nanomaterials prevented colonisation of plant roots by nitrogen-fixing bacteria.  As a result the plants had stunted growth and took up far more zinc compared to sewage sludge containing typical forms of the metals. The make-up of microbial communities in the soil was also changed.

Professor Steve McGrath, Head of Rothamsted Research’s Department of Sustainable Soil & Grassland Systems said: "Manufactured nanoparticles containing metals are being increasingly released into the environment and this is the first time that anyone has looked at the effects on an important a legume-bacterium symbiosis after nanoparticles have been processed through a full treatment works and then mixed with soil.

After the changes that take place during these processes, it is surprising that manufactured nanomaterials  prevented the formation of nitrogen-fixing root nodules and that more zinc was taken up, than with a treatment containing no manufactured nanomaterials. However, this was a worst case scenario and we now need to look at the effects at lower application rates". 

Dr Jason Unrine said:  “This study is the most realistic simulation of the potential effects of nanomaterials in treated sewage sludge on agricultural lands to date. 

It demonstrates that current U.S. regulations for treated sewage sludge may not be protective for nanomaterials, although current predicted concentrations of nanomaterials in treated sewage sludge are far below regulatory limits, and more work needs to be done to confirm these findings.”



Notes to Editors

About NERC

NERC is the largest funder of environmental science in the UK. We invest £330m in cutting-edge research, training and knowledge transfer in the environmental sciences. Our scientists study and monitor the whole planet, from pole to pole, and from the deep Earth and oceans to the edge of space. We address and respond to critical issues such as environmental hazards, resource security and environmental change. Through collaboration with other science disciplines, with UK business and with policy-makers, we make sure our knowledge and skills support sustainable economic growth and public wellbeing - reducing risks to health, infrastructure, supply chains and our changing environment.

The research was co-funded by the NERC’s Environmental Nanoscience Initiative NE/H013679/1.

About Environmental Nanoparticles Initiative (ENI)

This work was funded as part of phase II of the Environmental Nanoscience Initiative (ENI). This programme is funded by the Natural Environment Research Council (NERC), Defra, Environment Agency, Engineering & Physical Sciences Research Council (EPSRC), Biotechnology & Biological Sciences Research Council (BBSRC), Medical Research Council (MRC) and Department of Health in the UK and the US Environmental Protection Agency (USEPA). This phase of the initiative has funded three consortium grants, each with research based in the UK and in the US, covering topics such as nanoparticles in sewage sludge and the impacts on plants, nanoparticles in the aquatic environment and impacts on aquatic species, and nanoparticles from consumer products and impacts on human health.

About the U.S. Environmental Protection Agency

The research was funded by the U.S. Environmental Protection Agency’s Science to Achieve Results (STAR) program (RD834574).

About University of Kentucky

The University of Kentucky is the Commonwealth of Kentucky’s flagship and land-grant research university, and is one of only eight universities in the U.S. with colleges of Agriculture, Engineering, Medicine, and Pharmacy on a single campus. UK has a total of 30,000 students and offers bachelor’s, master’s, doctoral, and professional degrees.  UK is located in Lexington, Ky., known as ‘the heart of the Bluegrass region.’  Many of the best-known thoroughbred horse farms in the world are located in Lexington and the surrounding Central Kentucky area.

About Cranfield University

Cranfield is an exclusively postgraduate university that is a global leader for education and transformational research in technology and management.

In environmental technology, Cranfield combines its scientific expertise in the management of water, soil and biodiversity with world-class innovation in environmental engineering and business management to provide truly sustainable solutions. Across policy, risk and regulatory advice, customised training programme development and leading Masters programmes, Cranfield works across the full environment spectrum to deliver a wide range of sustainability options.

About Carnegie Mellon University

Carnegie Mellon is a private, internationally ranked research university with programs in areas ranging from science, technology and business, to public policy, the humanities and the arts. More than 13,000 students in the university’s seven schools and colleges benefit from a small student-to-faculty ratio and an education characterized by its focus on creating and implementing solutions for real problems, interdisciplinary collaboration and innovation.

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About Rothamsted Research

We are the longest running agricultural research station in the world, providing cutting-edge science and innovation for over 170 years. Our mission is to deliver the knowledge and new practices to increase crop productivity and quality and to develop environmentally sustainable solutions for food and energy production.

Our strength lies in the integrated, multidisciplinary approach to research in plant, insect and soil science.
Rothamsted Research is strategically funded by the Biotechnology and Biological Sciences Research Council (BBSRC)


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