The UK research exploring how to make sewage sludge safer and more sustainable

Management & Regulation, Waste & Resources

31 January 2025

Technologies like pyrolysis and hydrothermal oxidation help address the challenges of PFAS and microplastics in sewage sludge recycled onto agricultural land

There’s an urgent need for new models of sewage sludge treatment — a more sustainable cycle that’s based around its value and benefits. The good news is that technologies under development have the potential to transform the UK’s wastewater industry processes into a flagship cycle of safer disposal and new revenue streams.

A total of 3.6 million tonnes (between around 60 to 80 per cent of that produced in the UK) is recycled each year onto agricultural land. The concerns associated with this practice are well known, with countries like the Netherlands and Switzerland having banned sludge recycling onto land because of the levels of microplastics and other contaminants, where the wider impact on soils and health is still little known or understood. The UK, through the Environment Agency, is facing ongoing pressure to follow their example and introduce similar legislation.

A ban would have unwanted consequences in terms of shutting off a major supply of low-cost fertiliser to agriculture, necessitating greater use of chemicals on land, loss of valuable materials such as a phosphorous and ammonia, and the serious issue of how else to dispose of sludge. New research by Cranfield University and a number of UK water companies is demonstrating emerging thermochemical alternatives, including pyrolysis and hydrothermal oxidation (HTO).

The potential of pyrolysis

Pyrolysis involves high temperature treatment in the absence of oxygen, a process that works by breaking down organic matter, such as sludge. Common contaminants within sludge can be managed using pyrolysis, reducing the concentrations of emerging pollutants. These include microplastics, pharmaceuticals, hormones, antibiotics, antimicrobials and the per and polyfluoroalkyl substances (PFAS) also referred to as ‘forever chemicals’. PFAS are synthetic compounds that have entered water systems as a result of the manufacture of products such as ‘non-stick’ cookware, waterproof clothing, paints and food packaging (CIWEM members can read more about PFAS in ‘At the source’, in the Winter 2025 issue of The Environment).

The outputs from the pyrolysis process include bio-char, bio-oil and a syngas (energy-rich gas including hydrogen, carbon monoxide, ethylene and short-chain organic compounds). In principle, the bio-char will still be usable as a soil conditioner for agriculture, once there is regulatory approval. In other words, the sludge is rendered safe and leads to the production of energy and useful materials. The new testing of the viability of pyrolysis — supported by £6 million funding from Ofwat — is led by Thames Water working alongside Cranfield and partners such as Southern Water and Yorkshire Water.

The pyrolysis process, though, has its drawbacks. Firstly, the sludge needs to be dried at the start of the process, requiring large amounts of energy and pressure. Secondly, useful materials are destroyed at the same time as the contaminants. Furthermore, the value of bio-char in itself may prove to be limited because the pyrolysis process can destroy nutrients, making the sludge more of a soil conditioner than fertiliser.

Hydrothermal oxidation – the no waste alternative

Other work is exploring the alternative of hydrothermal oxidation (HTO), a form of ‘pressure cooker’ approach which oxidises the sludge into a liquid that is rich in phosphorus and ammonia. HTO requires energy to generate the necessary pressured environment, but then provides its own source of heat. Importantly, there’s no liquid return to deal with and no need for an initial drying treatment – the bio-solids can be taken directly from an anaerobic digester into the HTO plant.

In terms of circularity, the principle of HTO means a way of maximising the recovery of resources from the sludge — there’s no waste and it can still be used as a cleaner, richer source of phosphorus and ammonia. The project, led by Anglian Water, will look to demonstrate the extent of the effectiveness of HTO, in which situations, and will also lead to a business case for its introduction in terms of the materials recovery involved and potential markets.

The need for a whole-system approach

A related piece of research between Cranfield, Severn Trent and a number of other water companies is aimed at developing whole-system approaches to managing PFAS in drinking water. As many water companies are having to install new treatment options for control of PFAS in drinking water, it is imperative that new approaches are developed to ensure effective removal of PFAS, while minimising operational costs.

There are a number of ways by which we can effectively remove PFAS from water, including using membrane processes, such as reverse osmosis and nanofiltration, and sorbent materials such as activated carbon and ion-exchange media. However, all of these processes produce waste streams that contain a high concentration of PFAS.

This waste is difficult and expensive to dispose of and may ultimately pollute the environment. This project is therefore investigating ways of concentrating this waste, followed by an assessment of the various potential destruction methods for breaking down the PFAS into harmless products. This includes electrochemical and ultrasonic processes, advanced oxidation/reduction systems and foam fractionation. This project aims to make recommendations to the UK water sector by February 2026 on the most appropriate whole-system treatment process for removal of PFAS and management of the resultant waste streams.

There’s no single answer to producing cleaner cycles of water, wastewater and biosolids. Which is why this new phase of testing and demonstrations, assessing the role of each option in different circumstances, will be critical for making sure there is a platform of evidence for long-term, sustainable transformation in the water sector.


Authors:

Bruce Jefferson is professor of water engineering

Dr Stuart Wagland is a reader in energy and environmental chemistry

Peter Jarvis is a professor of water science and technology

All are based at the Cranfield Water Science Institute, Cranfield University.

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