IL RECUPERO ENERGETICO NELLE RETI DI DISTRIBUZIONE IDRICA

The water resource represents the "blue gold" of the third millennium, not only in developing countries but also in urbanized areas. Indeed, the per-capita water availability is progressively decreasing - also due to ongoing climate changes - and the lack of resources is provoking, on one hand, the huge exploitation of water bodies and, on the other hand, the search of new supply source, resulting in significant contrast with the principles of environmental sustainability. In the next future the stresses deriving from the water resources exploitation (water stress level) will become increasingly intense in most countries in the world, mainly due to climate change, with a related increase in the intensity of extreme phenomena, with particular regard to drought. During the last decades, to assure a sustainable exploitation of water resources, the topics concerning the water savings and the energy consumption efficiency are more and more current. Thus, as part of the Integrated Water Service (SII) management, a significant amount of energy is required for the supply, transport, use, treatment and release of waste water to the receiving water bodies. The amount of energy required to operate a water system certainly depends to a significant extent on the topographical characteristics of its networks, on the availability and quality of resources and on the type of treatments. On average, energy costs represent about 35% of the operating costs of the service. Today in Italy the total consumption of electricity in the SII is estimated to be around 7.5 billion kWh/year, corresponding to around 2.5% of national electricity consumption, a value in compliance with that of the other countries having infrastructure degree comparable to the Italian one. This rate will further increase over the next few years, as a consequence of the demand increase and the increasingly restrictive regulatory standards set for water treatment. The reduction of energy consumption in the urban water cycle is therefore significantly pivotal. First of all, it is certainly related to a higher conscious exploitation of water resources, based on the consumption decrease and the reduction of water losses in water systems, namely in Water Distribution Networks (WDNs). Indeed, in Italy and, especially in the Southern Region, they represent a significant rate of the inflow resources, strongly higher than the typical physiological values. Water losses in Italian WDNs are on average of about 40%, corresponding to 3.45 million cubic meters in 2015. In greater detail, real water losses, caused by the decay or corrosion of pipes, cracks or flaw joints and inefficiencies are equal to 38.3% of the input volumes in the network, whereas the apparent water losses, related to unauthorized consumptions and measurement errors, are equal to 3.1%. The most affected Italian Regions concern some areas of the South, with percentages even reaching 60%. In this field, the development of management practices aimed at the active control of the WDNs operations must be devoted to the reduction of excess pressures, in order to limit the excessive stresses of the components, capable of undermining their integrity, and to reduce water losses (due to the relationship between losses and pressures). The possibility of combining the pressure regulation (Pressure Management) with the small-scale hydropower generation in urban areas, therefore, represents a novel approach, achievable through the effective selection and management of devices for electrical generation in WDNs. In this paper, an in-depth analysis on the criteria for carrying out energy recovery in WDNs is provided, with specific reference to the assessment of both the currently available devices and the issues deriving from their application in the network. Specific attention is paid to the use of pumps running in reverse operations (Pumps As Turbines, PATs), by both providing their main peculiarities and discussing the theoretical and applicative literature approaches for their application in WDNs.