Prediction of Temperature Distribution in a Drinking Water Network
by Lucyna Magda
According to the Dutch Drinking Water Directive the temperature of drinking water should remain below 25°C at the customers’ taps to prevent microorganisms proliferating in the distribution mains and to ensure a supply of water that is qualitatively and aesthetically stable. However, the water temperature in a distribution network is difficult to control as it depends on multiple factors including: climatic conditions, surface conditions, soil characteristics and drinking water discharges.
Recent studies demonstrate that the rising global mean temperatures as projected by the International Panel on Climate Change are likely to be manifested in an increase of the temperature of the surface water systems and of the soil. Therefore, it is likely that also the temperature of drinking water will increase and drinking water supply companies may find themselves in a situation of temperatures approaching legally imposed standards.
The principal goal of this research was to explore how network hydraulic influences the temperature of the drinking water. The secondary aim was to establish linkages between climate conditions, network characteristics and the drinking water temperature. To achieve these goals, we have used a model instrument consisting of two heat transfer models representing weather and soil conditions, and two network models calculating hydraulics and water quality. In order to validate the models an experiment was organised. An experiment was conducted to observe the soil at the three different depths below the surface simultaneously with the drinking water temperature. The observation unit was repeated at four different locations in a network, that featured different surface covers and pipe characteristics.
The evaluation of a capability of a combined weather and soil-diffusion models to estimate pipe wall temperatures has led to inaccurate results; however the models we have used carried useful information for network calculations. Secondly, numerical network simulations at the heat wave conditions, indicate importance of the residence time and gradient between the soil and drinking water temperatures. However results from this case study demonstrated that the gradient between the soil, pipe wall and the drinking water, during the measuring period (autumn-winter), was small. It was difficult to determine a clear influence of the hydraulic on the diurnal cycle of the drinking water-soil temperature exchange.
This study proved an applicability of the model instrument to predict a temperature of the drinking water. However, heat transfer modelling approach requires additional improvements towards models’ accuracy, and an account of precipitation and soil moisture. The reliability of the methodology used in this thesis, should be verified for summer conditions when the air, net global radiation and soil temperature exhibit stronger diurnal fluctuations.
To obtain information on a large-scale network variability local or point measurements should be contrasted with groundwater, soil temperatures and surface conditions (GIS analysis). Drinking water companies, in order to address the overheating problem, should start to monitor water temperature near the small diameter pipes at the peripheral parts of a network which, seemingly, are most vulnerable to extraneous heat surges.