Equilibrium sampling reveals increasing thermodynamic potential of polycyclic aromatic hydrocarbons during sewage sludge digestion.

The reuse of digested sludge from wastewater treatment plants (WWTPs) as soil fertilizer poses a risk for contamination of soil and water environments. The present study provides a new approach for investigating the exposure of hydrophobic organic chemicals in sewage sludge. The methodology of equilibrium sampling with multiple thicknesses of silicone was successfully validated and applied to complex sludge matrices. Polycyclic aromatic hydrocarbon (PAH) concentrations in silicone (Csilicone) were determined and compared across four WWTPs. Activity ratios (ARs), defined as Csilicone at equilibrium with digested sludge (final product) over Csilicone at equilibrium with secondary sludge (intermediate product), were in the range 0.85-20 with all except one AR>1. These ARs thus revealed increased thermodynamic potential of both parent and alkylated PAHs in digested sludge compared with secondary sludge, and thereby higher exposure of PAHs in sludge after digestion than before digestion. This observation can be explained by the concept of "solvent depletion" as organic matter decreased by a factor of 1.3 during digestion, resulting in reduced sorptive capacity and increased freely dissolved concentrations (Cfree). The PAHs with logKow > 6 had ARs close to 1.3, whereas PAHs with logKow < 6 showed higher ARs than the organic matter decrease factor of 1.3. Cfree in digested sludge were higher than reported in rural soil and generally consistent with levels reported for Baltic Sea sediment.