Mobilization of PAHs from the coal-tar lining of water mains as a result of disinfection

Coal-tar was commonly used as an internal lining for corrosion protection of water pipes from the 19th century up to the present, In this project the principal mechanisms for the occurrence of PAHs (Polycyclic Aromatic Hydrocarbons) in drinking water distribution systems 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 the most important factors to favour the occurrence of PAHs in the drinking water. 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. Immediately after stagnation periods PAH concentrations increased to levels which exceeded the prescribed concentrations of the EC-guideline of 200 ng/l. The destabilisation of the biofilm matrix resulted in the release of particles highly contaminated with PAHs which may be responsible for the contamination of the drinking water.