Biosolids are the solid by-product of wastewater treatment plants (WWTPs), comprising organic matter derived from municipal human waste and other materials entering household and industrial drains. Australian WWTPs process not only domestic sewage but also liquid trade waste – non-domestic wastewater from industrial, commercial, and community facilities. This includes effluent from food processing, healthcare, manufacturing, and even landfill leachate. In practice, material from the earlier primary stages of sewage treatment that may include high levels of plastic, cotton debris and grease are often also referred to as biosolids.
Consequently, biosolids represent a complex matrix containing thousands of chemicals, including persistent organic pollutants such as PFAS (per- and polyfluoroalkyl substances) and a range of complex matrices. Contaminants with a propensity to bind to solids will typically end up in the biosolids rather than the treated liquid effluent of WWTPs.
About 370,000 tonnes of dry weight biosolids or 1.6 million tonnes of biosolids in dewatered form (also called wet biosolids) are produced in Australia annually1. Approximately 85% of the biosolids is land applied to improve the organic content of soils however recently there is a trend away from land application as regulations are tightened – particularly regarding per- and poly-fluoroalkyl substances (PFAS) – and a greater recognition that biosolids contain a diverse range of contaminants, some of which we may not yet have discovered.
The recently introduced NEMP 3.0 guidelines provide a nationally consistent framework for managing PFAS in biosolids to protect human health and the environment while supporting resource recovery. It introduced risk-based criteria for PFAS compounds such as PFOS, PFHxS, and PFOA, setting guideline values for both restricted and unrestricted land application. Rather than imposing blanket bans, the guidelines recommend site-specific risk assessments, with exceedances prompting further investigation rather than automatic prohibition. This approach reflects growing concern over PFAS persistence and potential pathways to soil, groundwater, and the food chain. However, the changes have significant implications for biosolids reuse programs, as stricter thresholds may render large volumes of biosolids not fit for land application, potentially shifting management strategies toward thermal destruction or landfilling.
Thermal desorption
Thermal desorption is a form of thermal destruction and is being used at several treatment facilities in Victoria, including EarthSure (https://www.ventia.com/campaign/earthsure-veolia-and-ventia-joint-venture, Figure 1), a joint venture facility between Ventia and Veolia.
Figure 1. The EarthSure thermal facility at Dandenong South, Victoria
Thermal desorption – unlike incineration which is used extensively in North America, Europe and Asia to treat solid waste – is 2-stage process that desorbs and destroys organic contaminants. In the 1st stage contaminants are desorbed at lower temperatures, for example 450-550 degrees Celsius, from a solid treatment matrix, such as soil, into the gas phase. In the 2nd stage the contaminants are destroyed in the gas phase at higher temperatures, for example 900-1100 degrees Celsius. Hazardous Waste Incinerators are a 2-stage process as well, the primary differences being the higher temperatures used in the 1st stage of the process resulting in the solids being destroyed as well as the gas phase contaminants. Thermal desorption has the benefit of destroying contaminants in a homogenous gas stream rather than a heterogeneous matrix of various solids and gas.
Treating biosolids within a thermal desorption process has several important synergies. Biosolids contain a high calorific value (the amount of heat energy released when a substance is completely burned in the presence of oxygen) from the high organic content and high content of anthropogenic material, such as plastic and cotton. This means the energy required to thermally treat biosolids can be significantly reduced compared to soil meaning less natural gas (the typical energy source for thermal desorption plants) per tonne of biosolids may be required for treatment.
Adopting a scientific approach to treatment
All treatment processes have a level of uncertainty and it’s our job as practitioners to reduce that uncertainty to an acceptable level. There are 3 key areas of interest with regards to thermal treatment of biosolids. Firstly, ensuring that the process adequately desorbs contaminants from the matrix. Secondly, ensuring that the contaminants now in the gas phase are destroyed to an appropriate level so that contaminants are not simply desorbed from the solids into the gas phase and discharged into the atmosphere untreated. Lastly, ensuring that the process does not generate products of incomplete destruction (PIDs).
Each point will be discussed separately. The 1st area of interest is the easiest to show, simply test the biosolids before and after they are treated – stockpile to stockpile. All organic contaminants will be removed to non-detect when the dryer, the part of the process where the desorption occurs, is at the correct temperature and residence time. Experienced operators will be able to determine the correct operating conditions very quickly.
The 2nd and 3rd areas of interest are relatively simple to show for single compounds such as PFOS or PFOA but become much more complex when considering that PFAS is a family of 1000’s of compounds and when these compounds undergo thermal degradation, new compounds can be formed. Reducing uncertainty when considering this complexity is not something that can achieved quickly and requires many years of collaborative research with academia and industry. Ventia began supporting and collaborating in fundamental research with university partners on the thermal degradation of PFAS back in 2016. That early research paved the way for a 3-year Australian Research Council2 project in collaboration with Associate Professor John Lucas, Professor Eric Kennedy, Professor Michael Stockenhuber and Dr Nathan Weber among others.
The research contributed to several important discoveries including an understanding that complete mineralisation of PFAS to hydrofluoric acid – the target of successful thermal treatment of PFAS – requires an abundance of hydrogen at the correct operating conditions. Without this hydrogen or the correct operating conditions, the process can generate unwanted shorter-chain PFAS compounds (such as perfluoroalkanes).
The exciting new chapter of the research is taking the laboratory research to the field. Ventia and EarthSure, are party to 2 SERDP projects3 valued at $US3.5M being undertaken over the next 3 years. These projects undertaken in Australia and Germany will utilise quantum chemical modelling and numerous, typically laboratory-based, cutting-edge analytical techniques and apply them to the full-scale thermal desorption process at EarthSure. It’s through this continued collaborative research that we are gaining the knowledge to ensure world’s leading best practice is being applied to the treatment of biosolids and other PFAS contaminated materials.
Conclusion
Biosolids are the solid by-product of WWTPs, consisting mainly of organic matter from human waste and industrial sources, including trade waste from sectors like food processing and healthcare. They form a complex matrix containing thousands of chemicals, notably persistent pollutants such as PFAS. Australia produces about 370,000 tonnes of dry biosolids annually, with most historically applied to land to improve soil, though tightening regulations – especially regarding PFAS – are shifting practices toward alternatives like thermal treatment. However, ensuring complete contaminant destruction, avoiding harmful by-products, and addressing PFAS complexity require rigorous testing and ongoing research, such as Ventia’s ongoing collaborations with international experts to advance best practices for PFAS treatment.
About Ventia
Ventia is a leading essential infrastructure services provider that makes infrastructure work for communities across Australia and New Zealand. Our service capability spans the full asset lifecycle – from design, minor capital works and operations and maintenance, asset management and facilities management, through to decommissioning – complemented by technology-enabled solutions and deep technical expertise. With Ventia, clients can trust that their environmental challenges will be addressed with sustainable, effective, and cutting-edge solutions, leading to a cleaner, safer, and more sustainable future. Ventia is proud of its history of supporting collaborative research and furthering best-practice treatment of contamination.
References
- Australian and New Zealand Biosolids Partnership Australia Biosolids Statistics – https://www.biosolids.com.au/guidelines/australian-biosolids-statistics
- Australian Research Council, Special Research Initiative Thermal decomposition of per- and polyfluoroalkyl substances (Project SR180100023)
- SERDP Projects:
a. Investigating the Thermal Decomposition of PFAS in a Full-Scale Commercial Hazardous Waste Incinerator (Project ER24-C6-4073)
b. Combining Experimental Measurements with Theoretical Insights for a Mechanistic and Predictive Understanding of PFAS Thermal Destruction (Project ER24-C6-4068)
This submission from Ventia was provided to ALGA as a Corporate Partner.