Per- and polyfluoroalkyl substances in municipal solid waste and their fate in incineration

Project description

Per- and polyfluoroalkyl substances (PFAS) is a large and diverse group of chemicals, encompassing >4700 individual substances depending on definition. PFAS are used in a wide range of products which, at their end-of-life, end up in waste incineration. However, the fate of PFAS in the incineration process is unknown, as are the degradation products that incineration of PFAS-containing waste could potentially give rise to, or what risks are of release into the environment. The few degradation studies that exist in literature indicate incomplete degradation, and that there is a risk of short-chain PFAS being formed, including the very potent greenhouse gases CF4 and C2F6.

In ongoing collaboration with Umeå Energi, we have started filling this knowledge gap. We have detected PFAS in all residual streams from Umeå Energi's waste boiler at Dåva, which shows the need for in-depth studies to explore what happens in the incineration process. In this project, we will study the breakdown and transformation of PFAS in waste incineration, with the aim of better understanding the conversion reactions, and in a longer perspective generating a mass balance. This knowledge is important for assessing the risk of PFAS-related emissions from waste incineration.

This PhD project will address the following research questions: i) To what degree do PFAS degrade and/or transform in a waste boiler during normal operation? Which products/byproducts are formed?;  ii) What combustion conditions are required for effective destruction of PFAS in a waste boiler?; and  iii) In which fraction(s) do the PFAS that pass through the waste boiler intact end up, and where do we find PFAS that are only partially degraded?

Theoretically, complete combustion of PFAS generates only CO2, H2O and HF as end products. However, the strong C-F bonds require a lot of energy to break (Tsang et al., 1998, doi.org/10.1080/00102209808952095), which is the underlying reason for the high stability and useful properties of PFAS substances. We hypothesize that most of the PFAS substances that we will be able to measure in the collected samples will only partially degrade, and that chemical alterations of the head group, and C-C rupture (i.e. generating shorter-chain PFAS) will be the main modes of conversion.

To understand the fate of PFAS in the waste incineration process, a mass balance is essential. In this project, we will conduct combustion trials using fuel materials with a known amount of PFAS, and thoroughly characterize the resulting residuals. This knowledge can subsequently be used to estimate the amounts of PFAS that enter full-scale waste incineration, and to what degree they are destroyed. There are currently no studies on these parameters, and this project will therefore contribute will knowledge critical for future handling of PFAS-containing materials.

In Sweden, combustible waste materials are incinerated in modern boilers at high temperature (flue gases retained at a temperature of ≥850 °C for at least two seconds), a process widely assumed to break down organic contaminants. For PFAS however, literature data addressing this issue is scarce, to say the least, and the few studies that exist only partly support this assumption. Given the high stability of the C-F bond, there are obvious concerns whether waste incineration is consistently operated under conditions that ensure full mineralization of PFAS. A study by Winchell et al. (2021) showed that complete combustion of C8 PFAS, such as PFOA and PFOS, requires temperatures of at least 1000 °C (doi.org/10.1002/wer.1483). Other than the data that our group is generating within the PhD project of Sofie Björklund (IDS/Umeå Energi), we are only aware of less than a handful studies reporting data from full-scale studies (e.g. Wang et al., 2020, doi.org/10.1016/j.scitotenv.2019.135832) and in those the extent of the sampling as well as the analytical protocol is also very limited. This research is both in the absolute research front and highly relevant from a societal perspective.