Stratégies hydrauliques pour améliorer la qualité de l'eau potable en réseau de distribution

by Simard, Andréanne

This Master’s project presents a methodology that can effectively identify distribution zones within a drinking water distribution network in order to ensure sufficient concentrations of residual chlorine at the dead-ends of a distribution system. The project was carried out in two phases. The first phase of the project consisted in identifying water distribution zones within the network with a tracer study. The second phase was designed to predict concentrations of residual chlorine within the distribution network, as well as make recommendations in hydraulic strategies to improve these concentrations in problematic areas of the network. The sector selected for the study is a residential district of Quebec City supplied with drinking water directly from the main supply pipes from the water treatment plant and by re-chlorinated water from a reservoir. The water distribution network is characterized by several dead-ends due to the absence of interconnexions with the bordering networks to the north and east and is also the last sector to be supplied on the eastern side of the plant’s service area. Unfortunately, water consumption is very low in these areas resulting in long standing time in the water pipes. These conditions lead to the degradation of water quality.

The proposed methodology engages three strategies: a hydraulic characterization of the distribution network, a tracer study and a water quality characterization study. The hydraulic and water quality models were calibrated with original data characterizing the spatial and temporal water quality variations within the network. An intensive sampling campaign was conducted to obtain the necessary data for the calibration of the models. The tracer study was conducted by injecting CaCl2 brine at the reservoir outlet in order to increase calcium concentrations in water from the reservoir. It was then possible to associate sampling points at which calcium concentrations showed an increase during the test as being supplied by the reservoir. A water quality characterization study was conducted simultaneously with the tracer study, making it possible to validate the distribution zones identified. This validation was made possible by the difference between chlorine values at the two entry points: water from the reservoir has distinctively higher levels due to re-chlorination. In addition, it was possible to identify distribution zones at points where the calcium concentration had remained constant during the tracer test.

Thereafter, the hydraulic and water quality models were calibrated satisfactorily providing a tool for evaluating hydraulic strategies to boost chlorine concentrations in problematic areas. Five types of hydraulic strategies were explored: (1) valve closures on major pipelines, (2) the addition of a re-circulating pump with chlorine injection, (3) the regulation of pressure at the main entry points of the sector, (4) the opening of an interconnexion with the east neighbouring network, and (5) a combination of strategies 2 and 4. Following the various simulations, two hydraulic strategies were more efficient than the others: the addition of a re-circulation pump with the injection of a hypochlorite solution and the opening of the interconnexion with the east neighbouring network.