Phosphorus fertilisation is an essential part of today's agriculture, but phosphorus resources are finite and the substance is on the EU's list of critical raw materials. 90 per cent of the phosphorus used in the union today is imported (most of it from a single country - Morocco), but there is an EU policy aimed at recycling more of the substance from different resources, including sewage sludge, says Arno Rosemarin, who is a senior research fellow at the Stockholm Environment Institute.

Sewage treatment plants were designed primarily to clean water and not optimised for recycling. Originally, phosphorus was removed from wastewater using aluminium or iron compounds. This resulted in phosphorus compounds in the sludge that were unavailable to plants, making it difficult to reuse in agriculture. In recent decades, biological methods or calcium or magnesium have been used instead, resulting in a plant-available product.

“So there has been some improvement in the treatment plants in terms of producing a recyclable product", says Arno Rosemarin.

Sewage sludge contains not only phosphorus, but also a wide range of chemicals and microplastics from the use of medicines and cosmetics or from washing clothes, for example. In combined sewer systems, rainwater from urban runoff also contributes with other compounds and heavy metals.

"We should be aware of and proud of Svenskt vatten, which has spent decades assessing the levels and fate of all these substances in water, sludge and soil," says Arno Rosemarin. "The whole thing is a discussion about risk assessment".

Arno Rosemarin, Stockholm Environment Institute. Photo: Michaela Lundell

Several sludge recycling methods

There are several options for sludge reuse, explains Arno Rosemarin. Direct application of treated sludge to fields could be done at relatively low cost, but only 60 per cent of the phosphorus is recovered and contaminants may be present.  Extracting nitrogen and phosphorus from the wastewater, for example by struvite precipitation, increases phosphorus recovery and results in a purer product, but at a higher cost.

Another method is to burn the sludge to ash by mono-incineration. This produces a sterile product and even higher phosphorus recovery (more than 90 per cent). 

"But it is expensive," says Arno Rosemarin, adding: "It burns all the carbon and nitrogen in the atmosphere and leaves behind heavy metals.”

A fourth method is pyrolysis, which produces biochar. This recovers 70-90 per cent of the phosphorus and also sequesters carbon, but it is not yet known what happens to the persistent pollutants that are present.

"It's not in the church yet, so to speak", says Arno Rosemarin.

Large variation between EU countries

In general, sludge disposal in the EU is a big problem for countries, says Arno Rosemarin, and there are big differences between them in how they deal with it. In Ireland and Lithuania, for example, a large proportion of sludge is used in agriculture, while in Malta and Bosnia-Herzegovina it is landfilled.

Some countries are pushing the development towards more mono-incineration, such as Germany where direct reuse of sludge is to be banned. However, such incineration demands for new facilities. 

“It’s a very expensive potential lock-in technology”, warns Arno Rosemarin.

Sweden, on the other hand, is taking a different route from basically all other countries.

“That is to work in a very constructive fashion with all the potential pollutants and try to eliminate them at source”, says Arno Rosemarin. “The program called REVAQ, run by Svenskt vatten, is taking the lead on this and is a certification system, that prevents the entry of toxins into the wastewater system and allows municipalities to decide what to do with their sludge.” 

Currently about 50 per cent of the treated sludge in Sweden is used on farmland.

The bottom line, means Arno Rosemarin, is that less phosphorus needs to be used, and used as efficiently as possible. More needs to be recycled, and source separation is essential and cost effective, as well as avoiding lock-in technologies such as mono-incineration.

"Cleaning up sludge means cleaning up society, it's a mirror of society.”

Microplastics from soil – a problem or not?

Geert Cornelis, a researcher at the Swedish University of Agricultural Sciences, has studied the effects of microplastics - one of the unwanted substances in sewage sludge - in soil. Microplastics are particles less than five millimetres in size that arise from macroplastics through physical or chemical processes and are currently found in many places, including remote ones. Particles smaller than 1 micron are called nanoplastics.

Adding sewage sludge to the soil increases microplastics, he states, but do they stay there and is there a problem? The answer to both questions, he says, is no.

“There is no clear indication that the concentrations exceed ecotoxic effects or enter the food chain.”

Geert Cornelis, Swedish University of Agricultural Sciences. Photo: Michaela Lundell

In some places, plastics are used extensively in agriculture, for example to conserve water and prevent the growth of pests. After use, the plastics need to be removed, but they can be difficult to get rid of, and are sometimes simply ploughed into the ground.

Swedish agriculture doesn't use as much plastic as in many other countries, but sludge is an exposure route for microplastics. This has raised the question of whether sludge application increases microplastic concentrations in the soil, and to what extent.

Microplastics moved by worms

Geert Cornelis and his colleagues went to a field where sludge had been applied since 1996 and found that microplastic concentrations were significant down to 40 cm, i.e. below the ploughed layer.

“Microplastics will be mixed in by plowing, but they appear to be moving further down. Our hypothesis is that it is mostly because of earth worms digging.”

The researchers could detect larger concentrations of microplastics along the holes digged by the worms, the so-called drilosphere.

“The plastics are milled down by earth worms and eventually may end up in rivers or in the sea.”

However, the effect levels of microplastics are currently in the high mg per kilogram range, whereas what has been measured in the soil is a few mg per kilogram.

Whether the threshold will ever be reached remains to be seen. Different events point in different directions. The use of plastics in agriculture is likely to increase, although the direct use of microplastics may be banned. The possible promotion of biodegradable plastics could also reduce levels, as could the likely reduction in plastic concentrations in sewage sludge.

"If you had the policy makers here in the room, what would you want them to do tomorrow?" asks moderator Gun Rudquist.

"To really investigate biodegradable plastics, I think that is the way to go," says Geert Cornelis. "We can't live without plastics in agriculture. But we can't live with a lot of plastic in the soil either.

"I think to put the excreta from humans on the phsycological map, and the transformation of society that changes waste into a resource," says Ano Rosemarin.

Text: Lisa Bergqvist

See a recording of the seminar