Biosolids: PFAS Stabilisation using RemBind®
A Viable Management Option
What are Biosolids?
Biosolids are nutrient-rich organic materials that result from the treatment of raw sewage sludge during wastewater treatment processes. They are often beneficially reused on agricultural land as they not only add valuable nutrients to the soil, but can also improve the moisture-holding capacity and other soil parameters.
However, wastewater treatment processes tend to also concentrate certain heavy metals such as copper and zinc, so the application of biosolids to agricultural land needs to be carefully managed to ensure that environmental regulations are not exceeded.
PFAS in Biosolids
More recently, Per- and Polyfluoroalkyl Substances (PFAS) have been discovered in various biosolids. PFAS, dubbed the ‘Forever Chemicals’, are a group of synthetic chemicals that accumulate in the environment and there is some evidence to show that some of these compounds can cause toxicity to human health and the environment.
These chemicals were used in a wide range of products including firefighting foams and waterproofing agents used in clothing, shoes, and upholstery sprays, and so can make their way from industrial and household wastewater into sewage sludge and ultimately into biosolids.
The discovery of PFAS contaminants in biosolids has received significant media attention recently because biosolids have been applied to some land and pastures for decades, and although the heavy metals levels have been carefully managed, the PFAS levels have remained largely uncontrolled.
Thermal treatment of biosolids (e.g. pyrolysis or incineration) is being touted as a management solution as it destroys the PFAS molecules. However, this approach may not be commercially viable in all situations, as it requires high energy inputs and has relatively limited volume capacity. It is not practical for treating agricultural soils that have already been contaminated with PFAS via broadacre spreading of contaminated biosolids.
Stablilisation of PFAS in Biosolids — A Viable Approach
Stabilisation (also called ‘immobilisation’) refers to the addition of inert minerals to biosolids to ‘lock up’ the PFAS contaminants and prevent them from leaching and causing harm to human health and the environment. The immobilisation process is relatively cost-effective and practical to apply and the treated biosolids can still be applied to agricultural land for beneficial reuse. The technology can also be applied in-situ to reduce PFAS uptake into plants.
PFAS Stabilisation in Sewage Sludge – Peer Reviewed Study
In a study published by State University of New York, researchers in the Journal of Hazardous Materials (Zhang et al., 2022), demonstrated positive effects of sorbent addition in reducing PFAS leaching from sewage sludge.
In this study, RemBind 100, RemBind 100X, GAC and biochar were evaluated with respect to their effects on PFAS stabilisation in sewage sludge. Generally, at the three tested doses, 0.1%, 1% and 2% RemBind 100X showed the best stabilisation performance, while biochar showed a limited capability for stabilising PFAS in sludge.
Over time, the leachable PFAS gradually increased for sludge amended with GAC, but fluctuated in the water leachate of the sludge added with RemBind 100X during this 115-day study, implying a dynamic adsorption/desorption process and potential degradation of PFAS precursors during the experimental period.
A further study by the same researchers published in Environmental Pollution (Zhang and Liang, 2022), showed that the application of sorbents significantly reduced PFAS bioavailability to Timothy grass grown in biosolids-amended agricultural soil. When RemBind 100X or GAC were used at a 2% dosage, reductions of >96% in total PFAS in the grass shoots were observed.
This study goes some way to validating the notion of in-situ application of sorbents to reduce the bioavailability of PFAS to crops and reducing PFAS transport to sensitive biota and water receptors.
References
Zhang, W. and Liang, Y. (2022) Changing bioavailability of per- and polyfluoroalkyl substances (PFAS) to plant in biosolids amended soil through stabilization or mobilization. Env. Pollution, 308, 119724.
https://doi.org/10.1016/j.envpol.2022.119724
Zhang, W., Jiang, T., Liang, Y., (2022). Stabilization of per-and polyfluoroalkyl substances (PFAS) in sewage sludge using different sorbents. J. Hazard. Mater. Adv. 6, 100089.
https://doi.org/10.1016/j.hazadv.2022.100089