Dynamic modelling of bacterial growth in drinking water networks

Numerous biological and physicochemical reactions take place in drinking water distribution systems, and give rise to phenomena whereby the organoleptic or bacteriological characteristics of the distributed water are modified. Drinking water may contain residual biodegradable dissolved organic compounds which provide a primary source for the formation of a trophic chain inside the pipes. Bacterial biomasses develop mainly on the internal surface of the pipes, where they are relatively well protected from the action of chlorination agents. The detachment of these biomasses is responsible for most of the bacterial proliferation observed in water samples taken in distribution systems, and also contributes to the installation of undesirable metazoea such as Asellus aquaticus. Combatting these biological developments calls for the application of preventive and remedial treatments, and these can be studied more closely by the use of modelling. This article proposes a model for the study of the behaviour of bacterial biomasses in distribution networks, taking into account the various major parameters which govern their structure, such as the ratio of biodegradable dissolved organic carbon (BDOC), temperature, residual chlorine, pH and the hydraulic conditions of each pipe. The model makes use of the data supplied by the Piccolo hydraulic modelling software, which can provide predictive mapping of the situation of each section of the network. What is more, by taking into account the physicochemical and biological variations in the water at the intake to the network, this dynamic model forecasts the evolution of the variables depending on residence time but also on time, thus enabling better visualisation of a disruption in the system in real time. We discuss the influence of the expression of the detachment of fixed bacteria on solutions of the system of differential equations. Use of the model reveals threshold values of temperature and BDOC which can enable a natural limitation of bacterial biomasses in the network without the use of chlorine.