A partnership between Cranfield University and UK water companies is looking at exciting alternatives to capture and destroy contaminants in sludge – including new ‘pressure cooker’ technology.
There’s an urgent need for new models of sewage sludge treatment – a more sustainable cycle that’s based around its value and benefits. 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.
The concerns with biosolids – 3.6 million tonnes of which are recycled each year onto UK agricultural land (between around 60 and 80% of the total – are well known.
Countries like the Netherlands and Switzerland have banned sludge recycling onto land because of concerns over the levels of microplastics and other contaminants where the wider impact on soils and health is still little known or understood.
The UK is, through the Environment Agency, facing ongoing pressure to follow the example of introducing legislation.
A ban would have unwanted consequences in terms of shutting off a major supply of low-cost fertiliser to agriculture, the need for 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 between Cranfield and a number of UK water companies is demonstrating emerging thermochemical options, including pyrolysis and hydrothermal oxidation (HTO).
Pyrolysis involves high temperature treatment in the absence of oxygen, which breaks down organic feedstocks. 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.
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. The sludge needs to be dried initially, requiring large amounts of energy and pressure. Useful materials are destroyed at the same time as the contaminants; and the value of bio-char in itself may prove to be limited.
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 phosphorus and ammonia rich.
HO requires energy to generate the necessary pressured environment but 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 HO plant.
In terms of circularity, the principle of HO means a way of maximising the recovery of resources from the sludge – there’s no waste, it can still be used as a cleaner, richer source of phosphorus and ammonia. The project, led by Anglian Water, will both look to demonstrate the extent of the effectiveness of HO, in what situations, and lead to a business case for its introduction in terms of the materials recovery involved and potential markets.
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 sorbents such as activated carbon and ion-exchange media.
However, all 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 assessment of destruction methods for breaking down the PFAS into harmless products. This includes electrochemical and ultrasonic processes, advanced oxidation/reduction systems and foam fractionation.
The project aims to make recommendations to the UK water sector 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 (Professor of Water Engineering), Dr Stuart Wagland (Reader in Energy and Environmental Chemistry) and Peter Jarvis (Professor of Water Science and Technology), Cranfield Water Science Institute, Cranfield University.
