# Techno-Economic Analysis of a Solar Thermal Plant for Large-Scale Water Pasteurization
## 1 Introduction  
High quality standards for drinkable water supply are essential to health protection and personal hygiene [1]. Securing safety of drinking water is particularly critical in large-scale community distribution, to prevent outbreaks of waterborne diseases [2]. Community water systems, in general, mostly resort to groundwater sources, where the major contaminants may include: heavy metals, such as arsenic, cadmium, chromium, nickel and lead; organic compounds, such as chloroform, gasoline, pesticides and herbicides; pathogens, such as protozoa, bacteria and viruses and emerging pollutants [3; 4; 5]. Water treatment lines must be then properly designed to eliminate specific target contaminants, which generally require different purification techniques and protocols [6; 7; 8]. In this sense, microbiological inactivation--i.e., disinfection--accounts for most violations with respect to the prescribed quality standards [3]. Conventional water disinfection technologies rely on chlorination, ozonation and treatment by ultraviolet (UV) radiation [9]. Chlorination is one of the most effective techniques and, thanks to the reduced operational costs, is widely adopted. Water disinfection is obtained by the hypochlorous acidformed from chlorine when introduced in water. However, chlorination may give rise to disinfection by-products that could be harmful to the human health and environment, besides affecting the taste and smell of the treated water [10; 11]. Ozonation represents a valid alternative to chlorination, thanks to its increased effectiveness against viruses and bacteria. In addition, ozone increases the dissolved oxygen, improving the water quality. However, ozone is corrosive, extremely irritating and eventually toxic, very sensitive to dosages and not cheap [12; 13]. Finally, UV treatment inactivates the exposed microorganisms and viruses by damaging their DNA/RNA, which reduces their functionality and ability to reproduce. This treatment has the advantage to avoid harmful by-products; however, it requires important capital and operational costs for electricity and consumables [14; 15]. An alternative method to overcome some of the drawbacks of the previous techniques relies on the use of heat for water treatment, commonly known as pasteurization. This method has been traditionally adopted for inactivation of pathogens and microorganisms in milk and food. Heating is indeed effective in eliminating pathogens; however, this effectiveness strongly depends on the treatment temperature and exposure time [16; 17]. The high throughput of modern systems for community water treatment typically requires short treatment times, thus resulting in higher temperatures and thus energy-intensive processes. In this sense, renewable energy sources and waste-heat recovery have the potential to reduce the environmental footprint and allow design of sustainable water treatments, as already demonstrated in a variety of different applications [18; 19; 20; 21; 22; 23]. In the case of solar thermal energy, continuous water processing is limited by the intermittent availability of the solar irradiance. This issue can be overcome using thermal energy storage [24; 25; 26] and ancillary power units, opportunely integrated in the plant layout. In this view, here we develop a complete lumped-component model of large-scale solar thermal water disinfection based on a pasteurization protocol validated by experiments. This model is employed to size the components of the plant and determine its operating parameters given the mass flow rate of water to be treated. We also report an economic analysis of the proposed plant layout, which allows to optimize the unit cost of water purification based on specific design variables. The article is organized as follows. In Section 2 we first select and experimentally test the target pasteurization protocol. In Section 3 we develop the plant model and discuss its technical implementation. Section 4 is dedicated to the economic analysis of the proposed solutions for different possible scenarios.