ABSTRACT
During the last century, Spanish water policy focused on increasing the hydraulic supply and on the construction of new infrastructures of regulation (dams) and distribution (water networks). However, in this century, continuously increasing water demand has become more and more difficult to satisfy. Water is an abundant but not inexhaustible resource. Due to climate change, droughts are more frequent and water use requires higher levels of efficiency. Consequently, water policy is now focused on minimising supply and renovating existing networks.

Several studies have looked at the question of the renovation of pipes in water distribution systems. These commonly deal with the use of new pipes, the determination of the order of renovation, the calculation of the optimum renovation period for mains systems and the minimisation of the economic cost of renewal.

This thesis begins with a review of economic investment in pipe renovation. It then goes on to present the main characteristics of the most commonly used pipe materials along with their respective pros and cons. The new trenchless technologies are also looked at and economically analysed. In addition, there is an overview of the latest studies on decision-support systems to prioritize renovation.

Thereafter, the influence of the cost of water and energy and how they help on the optimum renovation of pipes is analysed here. Thus, a review of the traditional methodology is presented. A complete cost model (i.e. one consisting of not only the usual costs) is shown. For the first time the social, variable and opportunity costs are considered. Social costs are those caused by disruptions arising from repair works, such as traffic problems, pavement damage, loss of productivity, etc. Variable costs are related to water lost through leakage and its effect on production and the environment, and the expenditure resulting from increased energy use when pressurizing leaked water. Lastly, opportunity costs represent the potential savings made by sharing the cost of renovation with another operator, which, in practice can be rendered as a profit or a negative cost. The key factors are identified and the influences of water and energy costs are highlighted.



Consequently, the quantification of water and energy becomes essential to the optimisation of pipe renovation. The first is calculated with a water balance and the second with a new methodology. Both audits are linked so as to calculate the energy audit of water networks which requires solving the water balance and the mathematical model of a system contained within a control volume defined at will. This is the first tool to allow the quantification of the energy consumed but also indicates how that energy is used. These results are useful when it comes to adopting the proper decisions as regards the renovation, operation and management of water distribution systems. The key objective of the audit is to determine the amount of energy lost through leaks and, if previous stages of the water cycle are considered, to calculate relation between energy and water loss.

Interaction between water and energy increases when climate change is taken into account. Water is an energy-consuming agent, the higher consumption of the first the higher that of the second. Moreover, energy consumption produces greenhouse gas emissions, which will condition the future availability of water. So, these three concepts are inextricably linked. Taking into account the results of the water audit and using the water to air model (Wolff, 2004), the carbon credits earned by the management of a water distribution system may be evaluated, and comparing two cases, a leak-free network and a real one, the energy savings and emission reductions resulting from eliminating leaks may be calculated.



i