Emission of Per- and Polyfluoroalkyl Substances from a Waste-to-Energy Plant—Occurrence in Ashes, Treated Process Water, and First Observation in Flue Gas

Per- and polyfluoroalkyl substances (PFASs) are a large group of compounds commonly used as industrial chemicals and constituents of consumer products, e.g., as surfactants and surface protectors. When products containing PFASs reach their end of life, some end up in waste streams sent to waste-to-energy (WtE) plants. However, the fate of PFASs in WtE processes is largely unknown, as is their potential to enter the environment via ash, gypsum, treated process water, and flue gas. This study forms part of a comprehensive investigation of the occurrence and distribution of PFASs in WtE residues. Sampling was performed during incineration of two different waste mixes: normal municipal solid waste incineration (MSWI) and incineration of a waste mix with 5–8 wt % sewage sludge added to the MSWI (referred to as Sludge:MSWI). PFASs were identified in all examined residues, with short-chain (C4–C7) perfluorocarboxylic acids being the most abundant. Total levels of extractable PFASs were higher during Sludge:MSWI than during MSWI, with the total annual release estimated to be 47 and 13 g, respectively. Furthermore, PFASs were detected in flue gas for the first time (4.0–5.6 ng m–3). Our results demonstrate that some PFASs are not fully degraded by the high temperatures during WtE conversion and can be emitted from the plant via ash, gypsum, treated process water, and flue gas.

Section S1. Detailed plant description.
The waste incineration plant is used for energy recovery from waste, with a 150-400 km uptake area. 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. The plant layout can be found in Figure 1 in the main article. A detailed description of the plant operation is given below: 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 is also used for adding 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, see SFS 2013:253 §32) the hot flue gases go through two empty passes to reduce temperature (from > 800 °C to ca 600 °C) before reaching the superheaters, where the heat is transferred to superheated steam which is used for electricity and district heat production. After the superheater, and an economizer, activated carbon is added to the warm flue gases (220 °C) which are then allowed to pass through textile filters to remove dioxins and ash. This ash is combined with the superheater ash, and constitute the dry part of the air pollution control residue (APCR). 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 the 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 process 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) which is collected as a separate waste stream. 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.
The process water treatment consists of two main sections -the first section removes heavy metals and other contaminants from the HCl-scrubber water while the second section removes harmful substances from the final flue gas condensate step. The first section, which has a flow that varies between 1.5 to 2.0 m 3 •h -1 , 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 finally a lamella clarifier. Water from the first section is mixed with the water from the condensate scrubber and go through another set of precipitation, flocculation and lamella clarification at a rate between 4 to 14 m 3 •h -1 . The setup is constructed to allow the considerably more contaminated HCl-scrubber condensate to be treated twice, even if a notable dilution occurs when the final condensate is introduced into the process. The final stage of the water treatment process is a sand filter. Solid phase waste from the process water treatment (from the precipitation steps and lamella clarifiers) is mixed with the ash from superheater and textile filters to form self-hardening APCR. The APCR is landfilled at a hazardous waste landfill.

S3
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). For example, CO did not exceed the 150 mg CO•Nm -3 limit for more than seven 10-minute periods/24 hours during the year (these are the terms of the legislation). The NOx emissions exceeded legislative limits (200 mg•Nm -3 ) during five 30-minute periods, SO2 emissions exceeded legislative limits (50 mg•Nm -3 ) for thirty 30-minute periods, and TOC emissions exceeded legislative limits (10 mg•Nm -3 ) for two 30-minute periods. The 2013:253 allows NOx, SO2 and TOC emissions to exceed the boundary value for less than 3% of the running time, or 468 30-minute periods out of the 7800 hours of operating time. The plant operates within these limits, and 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.  Figure S1. Flue gas sampling train.
Flue gas sampling was conducted in the stack during 6 hours per sampling occasion, at a rate of approximately 16 L min -1 . A total of 6 sampling occasions were performed, three during which the waste fuel mix incinerated was normal municipal solid waste, and three during which 5-8wt% of sludge from a wastewater treatment plant was added to the fuel mix.
The sampling train used was based on method EN 1948:1 (designed for dioxin sampling) (Fig. S1). The sampling train consists of a cooled glass probe (8 mm i.d.) which is inserted into the centre of the flue gas duct. The flue gases are led to a impinger bottle (2L) containing 250 mL of MilliQ water. The MilliQ water was spiked with 3 ng labelled standard prior to sampling to account for losses during sampling. The gases are then led to an impinger bottle (2L) containing 200 mL 0.1 M sodium hydroxide. The bottles are placed in an ice bath to promote condensation of water vapor present in the flue gas. Following the impinger bottles is a filter holder containing a pre-cleaned activated carbon disc (0.3 g; Futamura Chemical CO. LTD. Nagoya. Aichi. Japan) mounted between two polyurethane foam filters.
To facilitate comparison of levels in flue gas, the sampled flue gas volume was normalized to dry gas, 0°C and 1 atm pressure (see eq. S1).