Report No DWI0213

Apr 1994

Field studies of a surface water derived distribution system, complemented by rigorously controlled laboratory modelling, showed that ammonia oxidation by biofilm autotrophic bacteria was the principal means of nitrite formation. This biologically mediated process may result in some samples at customers' taps not complying with the nitrite standard in the Water Supply (Water Quality) Regulations, 1989. Field studies confirmed that reducing the excess ammonia leaving water treatment works resulted in reduced nitrite formation in the distribution system. However, other factors relating to the distribution system also required elucidation to achieve compliance with the nitrite standard at all times.

The laboratory biofilm model was used to study the persistence of four microorganisms in distribution systems, Legionella pneumophila, Escherichia coli, Klebsiella oxytoca and Aeromonas hydrophila. These were all able to colonise and persist in biofilms, dependent on water temperature (5-25ºC). Various biofilm control measures were investigated. Although heterotrophic biofilm bacteria were more resistant to chlorine and UV irradiation, particularly at low temperatures, the species studied were clearly amenable to control using residual chlorine management. As these findings became known they were immediately put into -practise by the water company responsible.

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