Why is sewage sludge applied to land?

Sewage sludge is a source of nutrients such as phosphorous and nitrogen, as well as organic matter than can benefit soil health. Fidra recognise that, given the growing need to move away from the application of chemical (or synthetic) fertilisers, sewage sludge can be viewed as a renewable circular alternative. However, due to the presence of chemical and microplastic contaminants, sewage sludge as it stands is not a safe clean resource and so Fidra support the precautionary principle.

What contaminants are in sewage sludge?

There are several contaminants that cannot be removed during sewage sludge treatment and are unaccounted for within compulsory monitoring procedures under the current UK sludge in agriculture regulations. These include PFAS, microplastics, pharmaceuticals, bisphenols, phthalates, parabens, pesticides, and organophosphorus and brominated flame retardants. Read more about these contaminants in sewage sludge in our blog.

Is sewage sludge treated before land application?

Yes. Treated sewage sludge (or biosolids) under the UK biosolids assurance scheme should undergo treatment via processes such as thermal treatment, anaerobic digestion, and lime pasteurization. Advanced sludge treatment involves anaerobic digestion and thermal drying.

Current treatment processes are not able to remove microplastics (large plastic debris is filtered out). Treatment processes are also unable to remove chemicals such PFAS, bisphenols or pharmaceuticals, and they become concentrated in the sludge.

If we can’t recycle treated sewage sludge, what are the alternatives? Are there unintended consequences?

Unfortunately, the current alternatives to land application include incineration and thermal conversion of sewage sludge. These outlets, coupled with the extraction of nutrients, metals and energy are currently the only safer alternative disposal routes that enable destruction of contaminants such as PFAS (requires temperatures of at least 850 C). Unintended consequences include air pollution, ash disposal via land or landfill, greenhouse gas emissions, and the associated health risks. Thus, source control is the preferred option to avoid contamination of waste in the first place and to avoid the need for interim incineration or thermal conversion of biosolids. Research into sustainable short-term solutions is encouraged, for instance, to tackle the issue of incinerator capacity, utilisation of contaminated waste as an alternative source of fuel and incorporation of resultant ash in cement products by the cement industry.

What happens in other countries?

The use of sludge on agricultural land varies by country. Some European countries have banned the application of sewage sludge to land including the Netherlands in 1995 and Switzerland in 2005.

Germany, a country with similar population density to the UK, has limited application of treated sewage sludge to land due to the presence of contaminants with unknown risks. The sludge they do spread on agricultural land (around 23% of total sludge produced), has upper limits on glass and plastics.

Northern Ireland mono-incinerates most of their sludge which diverts it away from land.

In the US, the state of Maine has banned the application of sewage sludge to land after PFAS levels contaminated crops and water.

Which organic fertilisers are cleaner alternatives to sewage sludge?

Non-industrial organic fertilisers such as animal manures and slurries could be considered to be cleaner alternatives as, unlike biosolids, they are not a sink for contaminants released from human activities such as use of pharmaceuticals, microfibres from washing machines and PFAS from food and personal care products. Animal wastes may be sources of contaminants such as veterinary pharmaceuticals and microplastics, especially if the grass/feed the animals consume is contaminated but typical contamination levels for complex industrial, non-industrial and mixed organic fertiliser products need thorough investigation.

What about increased reliance on chemical fertilisers in order to avoid contaminated organic fertilisers?

Sewage sludge should be diverted away from land until it is a clean safe renewable and circular resource and we acknowledge the growing need to move away from the application of chemical (or synthetic) fertilisers that are either finite mineral resources or derived from petrochemical feedstocks. Processes have been developed to extract nutrients from sludge, including recovering phosphorus for use as a fertiliser.

What can regulators do to protect the environment?

Read Fidra’s asks in our latest joint NGO Position Paper.

Regulators should uphold the precautionary principle to protect soils and the environment from the risks from contaminants not currently regulated. This includes halting the use of treated sewage sludge on agricultural land until it is proven to be a safe and clean resource.

• Source control is the preferred option to avoid contamination of waste in the first instance, and to avoid the need for interim incineration or thermal conversion of biosolids. Research into sustainable alternative interim uses is encouraged, for instance, here have been studies in the incorporation of biosolids into construction materials.
• Farmers should be provided with financial support for nature-friendly farming initiatives and operational guidance to seek less contaminated alternatives.
• Regulatory plans to incorporate the Safe Sludge Matrix UK level voluntary agreement into new UK Agricultural Use of Sewage Sludge Regulations should be robust, carefully monitored and strong enough to protect soils and the wider environment from pathogens, as well as the already regulated potentially toxic elements (PTEs).

What can we all do to prevent contamination of sewage sludge?

Be conscious of what goes down your drains and follow Scottish Water’s kitchen and bathroom checklists!

What pathogens have been found in sewage?

Based on work done by James Hutton Institute for Scottish Gov, pathogenic bacteria such as Escherichia coli (or E. coli) and Salmonella are commonly monitored in treated sewage sludge. Other pathogens found in sewage include zoonotic bacteria such as the bacterium Coxiella burnetiid. There are also viruses pathogenic to humans released in human faeces like Hepatitis A and E, rotavirus and norovirus. Sewage sludge contains antibiotics, antibiotic-resistant bacteria (ARBs) and antibiotic resistance genes (ARGs), which can be released into the environment via application to land.

Other contaminated organic waste materials of concern?

Composts and digestates derived from food and garden waste have been found to be contaminated with microplastics, PFAS and bisphenol-A.  As part of our PFAS project, Fidra looked at PFAS contaminated compostable packaging products. Sewage sludge is sometimes mixed and co-digested with other wastes (e.g. distillery/brewery waste) for improved biogas (methane) production. Co-digestion will dilute contaminants rather than eliminate them.

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