Occurrence and mass balance of polycyclic aromatic hydrocarbons in the Thessaloniki sewage treatment plant

The occurrence and the mass balance of the 16 EPA. polycyclic aromatic hydrocarbons (PAHs) were investigated in a conventional activated sludge treatment plant. Polycyclic aromatic hydrocarbon concentrations were determined in wastewater and sludge samples from different treatment stages. The concentrations of PAHs in raw sewage were statistically analyzed to examine whether temporal factors (sampling period, day and hour of sampling) had a significant effect on the observed variance. Sampling period was the only factor that showed a significant impart. Principal component analysis (PCA) was used to assess similarities of the wastewater PAH pattern with possible source matrices (domestic effluents, urban runoff, and atmospheric particulates). The composition of wastewaters was found to be particularly similar to that of domestic effluents. Mass balances of PAHs were calculated for individual stages of wastewater treatment (primary treatment, secondary treatment, and chlorination), as well as for the whole treatment process. Relatively good mass balances were calculated in the primary clarifier for most PAHs. Lower molecu lar weight PAHs (Np, Ace, Ph, An, and Fl) showed substantial losses (>40%), in the secondary treatment probably due to biodegradation and volatilization. Good closures for all PAHs were found in the chlorination stage. Total mass balances indicated that only the lower PAHs are effectively removed, whereas the heavier compounds are rather resistant to the treatment process. Sorption was found to be the dominating removal process of PAHs-especially of the heavier ones-during primary treatment, while their removal during the secondary treatment is also governed by other processes, such as biodegradation and/or volatilization.