The role of biofilms in the mobilisation of polycyclic aromatic hydrocarbons (PAHs) from the coal-tar lining of water pipes

The influence of biofilm formation on the coal-tar lining of water pipes in drinking water distribution systems was investigated in batch experiments as well as in a continuously flown circular reactor system. The nutrient source for the growth of the biofilm was only the drinking water and the coal-tar coating on the slides. The growth and existence of the biofilm was examined using different techniques. Leaching experiments showed clearly that the biofilm has protective characteristics because enhanced PAH concentrations could be detected after the removal of the biofilm from the slides. Moreover, the dosage of chlorine as a disinfectant during the reactor experiments gave indication that the chlorination resulted in a destabilisation of the biofilm which lead to elevated PAH concentrations. The results are in correspondence with observations made in real distribution systems where enhanced PAH concentrations were observed during disinfection processes. The examination of coal-tar coated water mains revealed that in addition to the protective effect of a biofilm a vast amount of the PAHs was adsorbed on particles embedded in the biofilm, e.g. from the iron oxidising bacteria Gallionella.