Plastic is a hot topic - its ubiquity in the natural environment, impact on aquatic ecosystems and the emerging evidence of its impact on human health seem to make headlines every day, writes Dr David Tompkins.
We know that there are microplastics in domestic wastewater, as they are shed in their thousands whenever we wash our clothes.
From there they partition into the sludge fractions and end up in biosolids – and ultimately onto agricultural land, which remains the single most important outlet for biosolids in the Republic and UK.
In (dry) tonnage terms, about 52kt of biosolids were applied to land in Ireland in 2014 (whether direct to agriculture, or via composting) while in 2012, 844kt were applied to agricultural land in the UK(1).
Biosolids supply organic matter to increasingly impoverished soils, and are very significant sources of phosphorus.
Indeed, a P-flow analysis from 2013 suggests that about eight per cent of P on UK agricultural land originates from biosolids, exceeded only by fertiliser (27 per cent) and livestock manures/slurries (62 per cent)(2). This equates to more than 22,000 tonnes of phosphorus from biosolids.
Source P application (1000s tonnes) % of P application
Manures 167 62%
Fertilisers 72 27%
Sewage sludges 22.5 8%
Composts etc 3 1%
Recycled slaughter waste 2 1%
Seeds & planting material 2 1%
When viewed in these terms, agricultural use of biosolids is both extremely important and at risk of being reduced or completely terminated as a result of negative perception associated with microplastics. But – how real is this risk?
Other European countries have already taken the plunge and restricted the use of biosolids on agricultural land. It was banned in Switzerland in 2006 following concerns around chemical contaminants and BSE prions, and partial bans are being implemented in Germany.
Interestingly, the German bans are not all-encompassing. Instead, from 2032, biosolids from works treating wastewater loads of 50,000PE or more will be prohibited from application to land – biosolids from smaller works will not.
Recognising the phosphorus contribution that biosolids make to farming, there is a parallel requirement to recover phosphorus from sludges where present at concentrations greater than two per cent (on a dry solids’ basis).
Mono-incinerated sludge ash
Phosphorus can either be recovered from the sludges directly or from (mono-incinerated) sludge ash. The change will cover about two-thirds of biosolids produced in Germany – where about half are already incinerated.
In the Netherlands, biosolids have been incinerated for a number of years – not because of any specific prohibition on their application to land, but because there simply isn’t land available due to volumes of livestock manures and slurries produced in the country.
On the other hand, many European countries take a similar approach to Ireland and the UK, relying predominantly on land application.
Of course, microplastics are not the only hazard present in biosolids (or indeed, in many other land-applied materials such as composts and digestates), and research on other hazards is extremely extensive.
Many chemical hazards selectively partition to biosolids during wastewater treatment and may persist in soil.
Where compounds break down in soils, their derivatives may also have an impact on soil biology and/or accumulate over time.
A key point is that, when investigated, the risks to humans from these hazards is almost always found to be very low.
This doesn’t mean that we should be complacent about emerging hazards such as microplastics – but that we should acknowledge that the pathway from biosolids to soil to crop to food product to humans is extremely long, and normally ensures that human exposures are minimal.
Impacts to soil fauna
Where attention should perhaps be focused is on impacts to soil fauna – both at a microscopic and macroscopic scale. These pathways tend to be less studied, as humans or grazing livestock are not the end points.
Instead, hazards can accumulate (for example) via soil bacteria to earthworms, and then to small mammals and predatory birds.
Focused research on the impacts of microplastics on human, livestock and wildlife pathways is at its very early stages – while we know the hazard (microplastic) is present, it would be premature to state whether this is actually harmful.
Nonetheless, the topic is of increasing public interest – and change may be forced by negative perception, rather than evidence. Regulators are just one in a large group of food chain stakeholders that have an interest in what is applied to land where crops are grown (or stock grazed).
The UK malting barley sector has long-standing prohibitions on the use of biosolids – as does the Irish Grain Assurance Scheme. Supermarkets may have their own farm assurance standards or rely on national standards.
Reliance on international assurance standards
Increasingly, supermarkets rely on international assurance standards such as GlobalGAP – which prohibits biosolids; not because they don’t recognise the benefit of the material, but because it is processed to different standards around the world. The simplest approach to such variability is to ban the use of the variable material.
Regulators, consumers, farm assurance schemes, supermarkets or others could shift their positions on biosolids at any time.
Rather than wait for any change to arrive, the current public awareness of plastics in the environment should instead be grasped as a golden opportunity for the water industry to engage with consumers, to:
1.) Promote the importance of biosolids in agriculture, and show how far the industry has moved from applying raw sewage to applying safe, sanitised biosolids;
2.) Increase understanding of the relationship between their own behaviours – in terms of inappropriate flushing, for example – on the quality of land-applied materials and the food chain; and
3.) Ensure that consumers become advocates for biosolids, rather than adversaries.
1. Data from: https://ec.europa.eu/eurostat/
2. Cooper, J. & Carliell-Marquet, C. (2013). A substance flow analysis of phosphorus in the UK food production and consumption system. Resources, Conservation and Recycling 74: 82–100
Author: Dr David Tompkins, head of knowledge exchange and innovation, Aqua Enviro.
Dr Tompkins will be speaking on this subject in more detail at the European Biosolids and Organic Resources Conference on Wednesday, November 14, in Leeds. www.european-biosolids.com.