Factors influencing the mobilisation of polycyclic aromatic hydrocarbons (PAHs) from the coal-tar lining of water mains

Coal-tar was commonly used as an internal lining for corrosion protection of water pipes from the 19th century. In this project the principal mechanisms leading to the occurrence of Polycyclic Aromatic Hydrocarbons (PAHs) in those water mains were investigated. The results showed that the occurrence of PAHs in a distribution system was linked to the presence of the disinfectants chlorine and chlorine dioxide. In laboratory experiments it was demonstrated that the coal-tar surface represents a substrate for the growth of biofilms which exhibits protective effects. Generally, hostile environmental conditions for microbiological activity of the biofilm such as disinfection, stagnation periods and anaerobic conditions could be identified as important factors which favour the occurrence of PAHs in drinking water. Immediately after stagnation periods an increase of PAH concentrations was observed. Furthermore, it was clearly shown that disturbances in the hydraulic regime such as water hammers, operation of valves and rapid increases in flow velocity can result in enhanced PAH concentrations by the destabilisation of the biofilm matrix or high shear forces affecting the pipe walls resulting in the release of particles highly contaminated with PAHs which may be responsible for the contamination of the drinking water.