Occurrence and removal of polycyclic aromatic hydrocarbons in the wastewater treatment process.

The occurrence and removal of polycyclic aromatic hydrocarbons (PAHs) was examined at different stages at a modified anaerobic-anoxic-oxic biological wastewater treatment plant in Tai'an City in eastern China. Twelve PAHs were detectable, with low molecular weight PAHs found at different treatment stages in wastewater and sludge. The most frequent and abundant PAHs in the influents were different in May and November. In May, 3- and 4-ringed PAH compounds were predominant, accounting for 53.5% of the ∑PAHs. However, in November, 2-ringed PAH compounds were most frequent and abundant, accounting for 90.1% of the ∑PAHs. In May and November, the removal efficiencies of individual PAH compounds ranged from 72.4 to 99.4% and from 21.1% to 100%, respectively, in the total treatment process. A significant relationship was observed between the removal efficiency and log K(ow) of the PAH compounds in the grit stage (when the values of log K(ow) were higher than approximately 5), suggesting that these compounds were principally removed through sorption to sludge particles and transfer to sludge processing systems. The residence time impacted the removal efficiency in the biological treatment process at the primary and secondary stages. In addition, the removal efficiencies depended dramatically on seasonal variations.