Distribution of Per- and Polyfluoroalkyl Substances (PFASs) in a Waste-to-Energy Plant—Tracking PFASs in Internal Residual Streams

Per- and polyfluoroalkyl substances (PFASs) constitute a diverse group of man-made chemicals characterized by their water- and oil-repellent properties and persistency. Given their widespread use in consumer products, PFASs will inevitably be present in waste streams sent to Waste-to-Energy (WtE) plants. We have previously observed a subset of PFASs in residual streams (ashes, treated process water, and flue gas) from a WtE plant. However, the transport and distribution of PFASs inside the WtE plant have remained unaddressed. This study is part of a comprehensive investigation to create a synoptic overview of the distribution of PFASs in WtE residues. PFASs were found in all sample types except for boiler ash. The total levels of 18 individual PFASs (Σ18PFASs) in untreated flue gas ranged from 5.2 to 9.5 ng m–3, decreasing with 35% ± 10% after wet flue gas treatment. Σ18PFASs in the condensate ranged from 46 to 50 ng L–1, of which perfluorohexanoic acid (PFHxA) made up 90% on a ng L–1 basis. PFHxA was also dominant in filter ash, where Σ18PFASs ranged from 0.28 to 0.79 ng g–1. This study shows that flue gas treatment can capture some PFASs and transfer them into WtE residues.


Section S1. Plant description.
The Waste-to-Energy (WtE) plant is utilized for energy recovery from waste, with an uptake area of 150-400 km.The waste fuel (except for a portion of waste that require pre-crushing) is delivered directly into the waste bunker to reduce need for handling from a hygienic and logistic perspective.A schematics of the plant layout can be found in Figure 1 in the main article.A condensed description of the plant operation is given below (a full detailed description can be found in Björklund et.al (2023) 1 ).
To distribute moisture content and break up mono-fractions, manual fuel mixing is performed in the waste bunker using the waste handling crane.The crane operator adds approximately four 5-ton loads of fuel to the fuel hopper each hour, from where the moving grate boiler is fed continuously by a hydraulic pusher system.After the incineration (minimum 2 s residence time at 850 °C in accordance to Swedish law, SFS 2013:253 §32) the flue gas goes through two empty passes to reduce temperature (from > 800 °C to ca 600 °C) before reaching the superheaters, where boiler ash is collected.After the superheater, and an economizer, activated carbon is added to the flue gas (220 °C) which is then allowed to pass through textile filters to capture dioxins and ash.After the textile filters, the flue gas passes a second economizer before being quenched with water to reduce the temperature.After the quench, the flue gases pass through an acid scrubber, where water is used to wash out the HCl, NH3 and Hg from the flue gases.A portion of water from the acid scrubber is directed to the internal water treatment, while the majority is recirculated in the scrubber.The flue gases continue into the SO2 scrubber, where the flue gases are sprayed with a slaked lime mixture (Ca(OH)2) that forces the SO2 from the gas phase to form gypsum (CaSO4).After the SO2 scrubber, the flue gases are led through a condensate scrubber, removing some of the excess water and the remaining heat.Lastly, the flue gases are reheated slightly before they are released through the stack.All stack gas measuring equipment are placed directly before the gases are released into the air.
During 2021, the plant logged no hours of operations with emissions exceeding legislative limits and had very good adherence to the environmental requirements outlined in Swedish law (SFS 2013:253).In Table S1 the adherence to SFS 2013:253 with regard to emissions of metals, acids, ammonia and dioxins to recipient water and air through treated process water and flue gas is outlined.The emissions are below the legislative limits for all measured contaminants.
Table S1.Emissions of metals, acids, ammonia and dioxins in treated process water and flue gas from the waste incineration plant where sampling was conducted.The internal water treatment consists of two main sections -the first one removes heavy metals and other contaminants from the acidic scrubber water, while the second one removes harmful substances from the flue gas condensate.The first section has a flow that varies between 1.5 to 20 m 3 •h -1 , and consists of pH-adjustment, a CO2-stripper, precipitation of heavy metals using TMT-15 (15 % water solution of C3N3S3Na3; CAS 17766-26-6, Algol Chemicals, Esbo, Finland), flocculation using a sulphur group functionalized organic polymer Kurifloc 6504 (Kurita Europe, Mannheim, Germany) and a lamella clarifier.After the lamella clarifier, water from the condensate scrubber is introduced to the water treatment and another set of precipitation, flocculation and lamella clarification is performed at a flow rate between 4 to 14 m 3 •h -1 .The setup is constructed to allow the acidic scrubber condensate to be treated twice, even if a notable dilution occurs when the condensate is introduced into the process.
The final stage of the water treatment process is a sand filter.Solid waste from the process water treatment (from the precipitation steps and lamella clarifiers) is mixed with boiler ash and filter ash to form self-hardening APCR.The APCR is landfilled at a hazardous waste landfill.
The addition of sludge during the Sludge:MSWI sampling campaign had no detrimental effect on the incineration, district heating production or electricity generation.The average incineration temperature during the Sludge:MSWI campaign was 910 -1089 ℃, and during the MSWI campaign it was 899 -1082 ℃, both well above the legislative limit.
Flue gas sampling was conducted before the quench over a period of six hours per sampling occasion, maintaining a flow rate of approximately 16 L min -1 .In total, six sampling occasions were conducted: three with the typical municipal solid waste mix, and three with the addition of 5-8wt% sludge from a wastewater treatment plant to the waste fuel.
The sampling method is based on method EN 1948:1 (designed for dioxin sampling) (Fig. S1).The sampling train consists of a cooled glass probe (8 mm i.To facilitate comparison of concentrations in the flue gas, the volume of sampled flue gas was normalized to dry gas, 0°C and 1 atm pressure according to Equation S1.

Section S4. Instrumental parameters, quality assurance
Table S3.Target compounds included in the analysis, their chemical formula, parent ion, quantification ion and the corresponding internal and recovery standard.Concentrations below the method detection limit is marked by <LOD.Numbers in italics are above LOD but below limit of quantification (LOQ).Results <LOD were treated as zero when calculating sums and averages.*Compound was quantified using the internal standard closest in retention time and levels should be considered semi-quantitative.
d.) that is inserted in the centre of the flue gas duct.The flue gases are led to a impinge bottle containing 250 mL of MilliQ water.Prior to the sampling, the MilliQ water was spiked with 3 ng isotopically labelled standard to account for losses during sampling.The gases are then led to an impinge bottle containing 200 mL of 0.1 M sodium hydroxide.Both bottles are placed in an ice bath to facilitate the condensation of any water vapor present in the flue gas.

Table S4 .
Limit of detection per sample matrix.

Table S5 .
Concentrations of PFAS in all samples.

Table S6 .
Concentration of PFAS in field blanks.

Table S7 .
Average internal standard recoveries per sample matrix.