Why is sewage sludge applied to land?

Sewage sludge provides nutrients like phosphorus and nitrogen, along with organic matter than can enhance soil health. Fidra acknowledges that, with the increasing need to transition away from chemical (or synthetic) fertilisers, sewage sludge has the potential to be seen as a renewable, circular alternative. However, due to the presence of chemical contaminants and microplastics, sewage sludge in its current state is neither safe nor clean. As a result, Fidra supports applying 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 and their impacts in our blog and in the James Hutton Institute’s new Sewage Sludge Risks Research.

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 proportion of sewage sludge applied to agricultural land in the UK (87%) is far higher than the European average (40%). Some countries have effectively banned the application of sewage sludge altogether, including the Netherlands in 1995 and Switzerland in 2005. Germany applies a very limited amount of sewage sludge to agricultural land (around 23% of total sludge produced) and has much tighter limits on the amounts applied per year and restricted use on food for human consumption. Sweden is considering a nation ban on the agricultural application of sewage sludge and Scandinavia already has tighter heavy metal restrictions than the UK. France, Spain and Italy apply less  sewage sludge to agricultural land than the UK and have recently adopted new regulation requiring the monitoring of ‘forever chemicals‘ and microplastics coming from urban wastewater.

The U.S. has broadly similar regulation on biosolids to the EU, but state-level restrictions are now increasing due to ‘forever chemical’ concerns. Maine was the first to completely ban the land application of biosolids after widespread PFAS contamination, with Connecticut recently doing the same. Michigan, New York, Wisconsin, and Colorado have imposed PFAS limits or monitoring requirements for sewage sludge, while Massachusetts and Oklahoma are considering further regulations. As awareness grows, more US states are expected to tighten biosolids policies.

• • Take a more precautionary approach to unregulated contaminants, potentially limiting the application of sewage sludge until it is proven to be a clean and safe resource.
  • Support calls for greater source control of contaminants from manufacturers or industry, helping to avoid contaminated waste in the first place.
  • Increase monitoring and testing of emerging contaminants (e.g. PFAS).
  • Develop alternative treatment and nutrient recovery technologies to explore different options for removing contaminants from sewage sludge while still being able to use it as a resource.
  • Provide farmers with financial support for nature-friendly farming initiatives and operational guidance to seek less contaminated alternatives.

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.

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. However, animal wastes may still carry contaminants like veterinary pharmaceuticals and microplastics, particularly if the feed or grass consumed by the animals is contaminated. Comprehensive investigation is needed to fully understand typical contamination levels across complex industrial, non-industrial, and mixed organic fertiliser products.

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

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. There should be focus on utilising these technologies to maximise the use of clean and safe fertilisers on our soils.

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.

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!

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