# Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers
## 1 Introduction  
One of the objectives of the United Nations' Sustainable Development Goals (UN SDGs) is the worldwide minimization of release of chemicals to air, water, and soil [1]. The most recent EU Regulation on Persistent Organic Pollutants (POPs) [2], which recast the previous regulation [3] on the EU POPs Implementation Plan, reinforces the need of reduction and elimination of POPs to protect the environment and human life. This regulation aligns with the United Nations (UN) instruments; the Aarhus Protocol on Long-Range Transboundary Air Pollution on POPs (POPs Protocol) [4], the Stockholm Convention on POPs (POPs Convention) [5] and the Basel Convention on the control of transboundary movements of hazardous wastes and disposal [6]. Polycyclic Aromatic Hydrocarbons (PAHs), polychlorinated biphenyls (PCBs) and polychlorinated dibenzo-p-dioxins (PCDDs) and polychlorinated dibenzofurans (PCDFs), also named as dioxins or PCDD/Fs, are among the dozens of highly toxic POPs envisaged in several regulations. According to annex C of the Stockholm POPs Convention, the PCDD/Fs are considered as \"Unintentional\" POPs (UPOPs) since they are formed unintentionally in various anthropogenic activities, including \"Firinginstallations for wood and other biomass fuels\". For waste management, the EU POPs Regulation [2] aligns with the Basel Convention; contaminated wastes with levels of POPs above certain limits, \"low POPs content\" (LPCL), are \"POP wastes\" and must be disposed or treated. For PCDD/Fs, the provisional LPCL (expressed as Toxicity Equivalents-TEQ), 15,000 ng TEQ/kg, is considered too high and not protective. In fact, practices such as uncontrolled dumping and unaware spread of contaminated residues in soils (sludge, ashes, etc.) [7; 8; 9], highlight the need to define more precautionary limits. The UNEP Standardized Toolkit [10] was developed within the Stockholm Convention on POPs, for identification and quantification of releases of PCDD/Fs and other UPOPs. Five emission (EF) vectors were considered for several activities: EFair, EFsoil, EFwater, EFproducts and EFresidues, revealing that the greatest dissemination routes for PCDD/Fs are air emissions (45%), residues, including ashes (34%), and land accumulation (11%). Several guidelines on \"Best Available Technology/Best Environmental Practices\" (BAT/BEPs) were also developed in accordance with article 5 and annex C of the POPs Convention, to prevent formation and release of UPOPs [11]. The BAT/BEP \"Guidance/Guidelines for Firing Installations for Wood and Other Biomass Fuels\" recommends that combustion of contaminated biomass, such as construction wood waste, should be avoided in biomass furnaces to minimize formation of PCDD/Fs and PAHs. It also specifies that fly ashes from biomass combustion have to be properly landfilled. The implementation of technologies to reduce air emissions from industrial installations according to BAT/BEPs and BREFs [12] of the Industrial Emissions Directive [13] is foreseen to increase amounts and hazardous of ashes contaminated with POPs. As very Persistent, very Bioaccumulative and Toxic pollutant (PBT), PCDD/Fs were broadly discussed within the EU \"Strategy for a Non-Toxic Environment\"; a sub-study on very persistent chemicals [14] highlights the risks of their accumulation, concentration and dispersion, namely through reuse/recycling of streams within the Circular Economy. These concerns will be addressed in the upcoming European Green Deal framework [15]. The Community Strategy for PCDD/Fs and PCBs [16] recognized that bioaccumulation via the food chain is the most critical pathway of human exposure to PCDD/Fs and PCBs and that contamination of food processing animals and edible plants occurs mainly via uptake from soils. Given the evidence of environmental legacy and threats, [7; 8; 17; 18; 19; 20; 21], PCDD/Fs and PCBs are included in the group of most relevant soil pollutants, among other POPs.