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Are Agricultural Soils Dumps for Microplastics of Urban Origin?
Luca Nizzetto,*
,†,‡
Martyn Futter,
§
and Sindre Langaas
†
†
Norwegian Institute for Water Research, NO-0349, Oslo, Norway
‡
Research Centre for Toxic Compounds in the Environment, Masaryk University, 62500, Brno, Czech Republic
§
Department of Aquatic Sciences and Assessment, Swedish University of Agricultural Sciences, Uppsala, Sweden
Due to their ubiquitous distribution and chemical
composition, microplastics (MPs) are increasingly being
recognized as a global concern. While it is widely acknowledged
that MPs in the ocean are a serious issue with potentially negative
effects on marine organisms, information about MPs in terrestrial
and freshwater environments is fragmentary.
Based on new MP emission estimates in industrialized
countries, we suggest that widespread application of sewage
sludge from municipal 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
1−3
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 runoffare eventually conveyed
to WWTPs. During wastewater treatment, over 90% of MPs are
retained in sewage sludge.
4
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 emis-
sions
1−3
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,
4
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 1−3produces 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.
5
Received: August 16, 2016
Viewpoint
pubs.acs.org/est
© XXXX American Chemical Society ADOI: 10.1021/acs.est.6b04140
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
Regulations generally prohibit use of sludge containing an
excess of harmful substances (including heavy metals and some
organic substances). Neither the relevant European (EU 86/
278/EEC) nor U.S. (Code 503) regulations mention MPs. We
expect that none of the common processing steps (e.g., drying,
pasteurization, composting, etc.) to produce biosolids from
WWTP sludge for agricultural use will remove the MP load.
Reports are emerging about the occurrence and impacts of MPs
in soil, both from breakdown of plastics and sludge additions.
Reproduction of worms was recently shown to be impacted at
MP exposure levels possibly representative of those in
agricultural soils receiving sewage sludge application.
6
Assessing potential impacts for agricultural sustainability and
human health is far from easy. MPs can potentially impact soil
ecosystems, crops and livestock either directly or through the
toxic and endocrine-disrupting substances added during plastics
manufacturing. These substances include short/medium-chain
chlorinated paraffins (candidates for inclusion in the Stockholm
Convention) and plasticizers, which can represent up to 70% of
the weight of plastics. Endocrinologically active alkylphenols,
such as bisphenols, and flame retardants including several banned
brominated compounds comprise up to 3% by weight of some
plastics. During use, plastic polymers efficiently accumulate other
harmful pollutants from the surrounding environment, including
a number of persistent, bioaccumulative and toxic substances, e.g.
PCBs, dioxins, DDTs and PAHs. Many studies on these topics
can be found in previous issues of ES&T.
It is striking that transfers of MPs (and the hazardous
substances bound to them) from urban wastewater to farmland
has not previously been considered by scientists and regulators.
Studies assessing the scale of contamination of agricultural soils
by MPs are notable for their absence while farm soils may
arguably represent one of their largest environmental reservoirs.
International monitoring programs, additional research into
ecological effects, and awareness raising among farmers,
wastewater treatment operators and the general public are all
urgently needed to frame the problem more accurately. Analysis
of risks and potential impacts of MPs on agricultural
sustainability and human/livestock health must be prioritized
to ensure a safe reuse of municipal waste. From a broader
perspective, these analyses are critical for safeguarding circular
economy objectives including those in which biological nutrients
in waste products can be safely and sustainably reintroduced into
natural and farm ecosystems.
■AUTHOR INFORMATION
Corresponding Author
*E-mail: Luca.nizzetto@niva.no.
Notes
The authors declare no competing financial interest.
■REFERENCES
(1) Lassen, C. et al. Microplastics. Occurrence, Effects and Sources of
Releases to the Environment in Denmark; The Danish Environmental
Protection Agency, 2015; Vol. 205.
(2) Magnusson, K. et al. Swedish Sources and Pathways for Microplastics
to the Marine Environment; IVL Swedish Environmental Research
Institute, 2016; Vol. 88.
(3) Sundt, P.; Schulze, P.-E.; Syversen, F. Sources of Microplastic-
Pollution to the Marine Environment; Norwegian Environment Agency
Miljødirektoaret, 2014; Vol. 86
(4) Carr, S. A.; Liu, J.; Tesoro, A. G. Transport and fate of microplastic
particles in wastewater treatment plants. Water Res. 2016,91, 174−182.
(5) van Sebille, E. A global inventory of small floating plastic debris.
Environ. Res. Lett. 2015,10, 124006.
Figure 1. Estimated rates and intensities of MP application to European agricultural lands. In countries with elevated sludge application rates per unit
area of farmland (such as, for example UK) the level of MPs accumulated in treated soils is expectedly potentially higher than in countries with similar use
of sludge but lower application intensity (e.g., France). National MP upper application rates were estimated as sludge production multiplied by
agricultural application rate multiplied by MP emission per inhabitant multiplied by total population. Areal application rates were estimated as sludge
production multiplied by agricultural application rate then divided by total farm area. Bubbles represent individual countries (Austria (AT), Belgium
(BE), Cyprus (CY), Czech Republic (CZ), Denmark (DK), Estonia (EE), Finland (FI), France (FR), Germany (DE), Greece (EL), Ireland (IE), Italy
(IT), Latvia (LV), Lithuania (LT), Luxembourg (LU), Netherlands (NL), Poland (PL), Portugal (PT), Romania (RO), Slovenia (SI), Spain (ES),
United Kingdom (UK)), and bubble sizes represent the fraction of sludge either applied directly to agricultural areas or in the form of processed
biosolids.
Environmental Science & Technology Viewpoint
DOI: 10.1021/acs.est.6b04140
Environ. Sci. Technol. XXXX, XXX, XXX−XXX
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