The total and freely dissolved polycyclic aromatic hydrocarbons content in residues from biogas production

In the situation of increasing agricultural utilization of residues from biogas production (RBP) it is important to determine the concentration of contaminants, which could occur in these materials. The group of contaminants that requires special attention are polycyclic aromatic hydrocarbons (PAH). The objective of the study was to determine the total and freely dissolved (C_free) of PAHs in RBP from 6 different biogas plants operating under various temperature conditions and without or with the separation into the solid and liquid fractions. The freely dissolved PAHs were determined using polyoxymethylene (POM method). The total content of the Σ16 PAHs in RBP varied from 449 to 6147 μg/kg_dw, while that of C_free PAHs was at the level from 57 to 653 ng/L. No significant differences were noted in the content of the Σ16 PAHs (total) between the solid and the liquid fractions. This indicates that in the course of the separation, the PAHs are distributed proportionally between the fractions. However in the case of C_free, PAHs content in the solid fraction was over twice as high as in the liquid fraction. This was probably due to the greater affinity of the particles present in the liquid fraction to the analysed PAHs than to the particles of the solid fraction. Higher affinity to liquid fraction was also confirmed by the distribution coefficients K_TOC determined on the basis of C_free. - Highlights: • Total and freely dissolved PAHs concentration was determined in RBP. • Content of PAHs mainly depended on feedstock used for biogas production. • C_free PAHs in the solid fraction was over twice as high as in the liquid fraction. - Freely dissolved PAHs in the solid fraction is higher than in the liquid fraction of residues from biogas production.