# Techno-Economic Analysis of a Solar Thermal Plant for Large-Scale Water Pasteurization
## 2 Experimental Pasteurization Tests  
The starting point for the design and optimization of the solar thermal plant for water treatment is the identification of the target operating parameters for the pasteurization protocol. In this section, we experimentally test the effectiveness of different protocols for water pasteurization, which will be then employed to build the lumped-component model of the plant. The key parameters, that is, the treatment temperature and the residence time, are commonly chosen according to the Safety Zone in the temperature-time diagram for water pasteurization [27], which allows to easily identify the proper treatment to be adopted according to the target pathogens to inactivate. Here, we consider one point on the inactivation curve for enteric viruses (540 s at 70 \({}^{\circ}\)C), which is the most conservative choice for short-time pasteurization processes, and other three points obtained by regression at higher temperatures (114 s at 75 \({}^{\circ}\)C, 30 s at 80 \({}^{\circ}\)C and 8 s at 85 \({}^{\circ}\)C). The set up for the experimental tests has been designed considering two main requirements: water has to be maintained at uniform temperature during the pasteurization; the heating up phase of water to the treatment temperature has to be fast, to avoid spurious effects on the observed disinfection results. The equipment for the experimental pasteurization tests is shown in Figure 1 (left). Four aluminum test tubes--positioned into a tailor-made 3D-printed bracket (1) to easily handle the water samples during the experiment (see a detail of the CAD drawing in the inset)--allow to carry out four repetitions per test. Each tube is filled with a reduced amount of sample water per test (10 mL) to reduce thermal inertia during the tests. The temperature of each sample is measured using a thermocouple (K-type,RS Pro [28]; temperature range -75 to 260 \({}^{\circ}\)C and accuracy \(\pm\)1.5 \({}^{\circ}\)C), which is positioned at the center of the water sample using a tailor-made 3-D printed gasket. This latter gasket has been designed to hold the tip of the thermocouple in the bulk water, thus measuring the mean temperature of the sample. All the thermocouples are connected to a laptop (4) for digital acquisition of the temperature signals (LabVIEW(c), National Instruments, Austin, TX, USA). The water samples are first heated up quickly to the target treatment temperature using a (2) thermostatic water bath (CORIO CD-300F, Julabo, Seelbach, Germany), which is maintained at a controlled temperature of 95 \({}^{\circ}\)C. The samples are then quickly moved into (3) an industrial furnace for thermostatic applications (TypeM120-VF, MPM Instruments, Bernareggio, Italy), which is maintained at the tested treatment temperature and used for the pasteurization. After the target resident time, an ice bucket is used to cool down the water samples to the environment temperature. Each test is repeated three times, in order to obtain the necessary amount of water for the microbiological analysis of the results (provided by the SMAT laboratories in Torino, Italy). All the tools in contact with the water samples (i.e., aluminum tubes, thermocouples and related brackets) are sanitized after each test, by immersing them for 120 s into a beaker filled with 700 mL of water maintained at a constant temperature of 95 \({}^{\circ}\)C thanks to a (5) digital hot plate stirrer (AREX Digital, VELP Scientific, Usmate Velate MB, Italy). The temperature of the water samples during the experimental tests for the four selected pasteurization protocols are shown in Figure 1 (right). The solid lines represent the average value of the four repetitions for one protocol, while the semi-transparent band the minimum-maximum range observed. The three stages of the adopted thermal protocol can be clearly noticed from the reported temperature transients.