# Insights into PCDD/Fs and PAHs in Biomass Boilers Envisaging Risks of Ash Use as Fertilizers
## 2 Literature Review
### PCDD/Fs and TEQs; PAHs and BaP Equivalents
Nevertheless, the share of dl-PCBs in the WHO\({}_{1998}\)-TEQ(PCDD/F/dl-PCB) emissions from waste incineration plants was found to be less than 3% [71], and less than 16% for different industrial sources [76]. For biomass combustion emissions, the share of dl-PCBs on WHO\({}_{2005}\)-TEQ(PCDD/F/dl-PCB) was found to vary from 0.7-22% [73], and 5.8-23% [74]. In municipal solid wastes incineration (MSWI) fly ashes, the contribution of dl-PCBs to WHO\({}_{2005}\)-TEQ(PCDD/F/dl-PCB) was less than 5% [77]. The share of dl-PCBs on TEQs was found to increase along the flue gas paths equipped with APCDs. This is mainly due to the different adsorptions of PCDD/Fs and dl-PCBs on fly ashes that affect their partition between particle and gas phase [78]. The different volatilities of compounds may also influence the relative proportions of compounds along the exit flue gas path. In another matrix, such as soils and fertilizers, the relevance of dl-PCBs may however be quite different [79]. Also important for TEQ calculation is the strategy to account non-quantified or non-detected (ND) values of individual PCDD/F congeners, as different methodologies can be applied [80, 81, 82]. The simplest approach consists of the imputation method; attribution of fractions of the Estimated Quantification Level (EQL) for each congener: the Upper Bound (UB) with ND = EQL, the Medium Bound (MB) with ND = EQL/2, and the Lower Bound (LB) with ND = 0. Different options might lead to different TEQ values, which can hamper accurate risk assessment and evaluation of compliance with regulations [83]. [PERSON] and [PERSON] [84] also evaluated the influence of TEQ schemes and ND handling on TEQs of wood ashes. They verified that the impact of ND handling on TEQs increased with the number of non-detected congeners in wood ash. It was also shown that the ND options had higher influence than the TEQ scheme, for the lower concentrations of PCDD/Fs. Hence, accuracy of small concentrations is of prime importance, even for the evaluation of ash application in soils, for example, use of wood ashes with 1.4-840 ng TEQ/kg was estimated to increase soil's PCDD/F concentrations by 0.038-23 ng TEQ/kg a year [84]. EU regulations on the measurement of PCDD/Fs and PCBs in feed [85] and foodstuff [86] specifies that analytical reports must contain the levels of individual congeners measured, the TEQ values must be reported as LB, MB and UB and differences between LB and UB must be lower than 20%. Different approaches are in use, while the NCASI report [87] computes TEQ for wood boilers emissions assuming ND as zero, other studies [75, 88, 89] use the MB option. In the previous paper [56], the TEQ results presented were those provided by the analytical laboratory, which assumed the UB option. Reporting the ND handling and quantification/detection levels is frequently absent in the literature, which misleads the interpretation of results. Given the complexity of PCDD/Fs analysis, analytical sensibility may vary widely, depending on the sample characteristics and performance/focus of the laboratory (e.g., food control, environmental or waste analysis) [90]. The BAT/BEP guidelines [11]also mention that the reporting of PCDD/Fs and dl-PCBs should use LB and UB approaches for ND, and the required quantification levels should be a least 1/5 of the regulatory limits or levels of interest. PAHs are ubiquitous pollutants, comprising hundreds of lipophilic compounds with fused aromatic rings. They may have natural or anthropogenic origin, existing in many commonly used goods [91]. The United States Environmental Protection Agency (US-EPA) listed 16 PAHs compounds as priority pollutants for environmental risk assessment due to their toxic effects [92]. Table 2 presents the US-EPA 16 PAHs list and the acronyms used in this work.