**1. Introduction**

A water supply system (WSS) is a set of civil infrastructures (tanks, pipes and others), hydro-mechanical facilities, electrical equipment and services that extracts, conveys and distributes water to consumers. This distribution must be compatible with the demand in both quantity and quality [1]. A sub-grid of a water supply network, also termed a "district metered area", is a part of the network between two nodes where the head pressure is controlled. This pressure is maintained constant or follows a certain schedule [2] to ensure that there is an optimal pressure distribution in all the branches downstream. This is normally ensured by pressure reducing valves (PRVs), but recent studies have proposed their replacement by turbines. Pumps operated as turbines and Kaplan turbines are the technologies proposed by [3,4] for low-head sites. The study presented in [4] has shown that there is an economical interest in micro-hydropower in WSSs. This study was performed at a regional scale, identifying existing infrastructure such as PRVs and reservoirs where there is potential for energy recovery. Moreover, there is potential for energy production within the urban network itself. Besides the locations where PRVs are installed, there are also areas within the sub-grids that have more pressure than necessary, even if not excessive, because of their connection to other higher areas [5].

The use of PRVs or even micro-turbines in a WSS may seem illogical, since most systems need pumping. Nevertheless, the use of operational pressure control has proven to be cost-effective for reducing leakage problems [1,6]. Water loss occurs due to the structural deterioration of the pipes [7]. Local energy dissipation is seen to reduce the frequency of pipe bursts [8], ensure the quality standards of water [9] and protect roads and building foundations from underground cavities originating from water losses [10]. From the perspective of hydropower, the implementation of production units in existing infrastructure has the economic advantage of low costs [11] given the synergies with the existing facilities of the WSS, of consumption dependent discharges, which are predictable and almost guaranteed, and of benefiting several countries with respect to legal and financial incentives for self-generation and self-consumption (e.g., feed-in tariffs provided for renewable energies). However, it is worth mentioning that the feed-in-tariff regulations vary considerably between countries and, for example, in the UK, a micro-hydropower plant (MHP) downstream from any pumping station is not eligible for the subvention [12]. In addition to the abovementioned synergies with the existing WSS facilities, the energy produced may justify the installation of a grid-connected generator or local consumption [13]; the latter provides financial savings by avoiding external consumption and expensive electrical connections [14]. The installation of these systems is highly dependent on the costs associated with the turbine, construction and grid connection [4].

Examples of solutions for the installation of MHPs in WSSs can be found in the literature. For example, the installation of PATs as replacement for a PRV in the an urban water distribution system of Pompei, with discharges between 20 and 50 L/s and heads between 35 and 90 m, would produce between 20 and 94 MWh/year [3]. In Portland, Oregon, a PRV was replaced by a 10" (approx. 250 mm) micro turbine that generates 150 MWh/year with 30 kW of installed power [15]. In Hong Kong, an eight blade spherical inline turbine is expected to produce 700 kWh/year in the city's water main pipes [16]. Finally, the installation of a turbine replacing a break pressure tank in Kildare in Ireland could generate approximately 237 MWh/year from 200 kPa to 17,910 m3/day [17].

Although the potential for energy production within urban WSSs exists, it is difficult to quantify unless focusing on a particular location. Distribution networks are complex systems, usually composed of multiple loops and asymmetric consumption in both time and space. The optimal location of turbines within such systems is a subject of research [5,18,19]. Moreover, before enduring deep analyses and simulations of turbine operation and water demand scenarios, a measure of the excess energy available within the city network, indicating the potential for that network to be used for hydropower, would be useful for WSS managers.

In this research, a method for the evaluation of the potential for hydropower in a city is presented. This method is then applied to a case study city and compared with the actual energy production with a proposed hydropower scheme. The energy production is estimated using an algorithm for the identification of optimal location of the micro-hydropower units considering the whole network where infrastructure may or may not exist. In Section 2, the two methodologies are presented, for potential assessment and for the actual energy production simulation. The case study, the city of Fribourg in Switzerland, and the hydropower scheme are described in Section 3. The results are presented and discussed in Section 4. Section 5 presents a straightforward methodology that is proposed for the presented hydropower scheme and the main conclusions are drawn in Section 6.
