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Cooling towers (CTs) are used to dissipate excess heat from water by evaporation, common in large facilities as hospital, companies, and hotels. The main risk attributed to CTs is represented by Legionella, a Gram-negative bacterium associated with a severe form of pneumonia known as Legionnaires' disease (LD). The infection route is by inhalation of aerosols reaching the lower respiratory tract. Despite several events associated with CTs, the knowledge in this field is still limited. The aim of this study was to develop a predictive model of bioaerosol dispersion using PM10 particles as a proxy, to generate risk maps of Legionella spread in the surrounding area in several weather and microbiological conditions. The Legionella contamination in the CT basin was 40938 ± 24523 cfu/L, with four peaks independent of the season, associated with an increase in air minimum temperature values (+1–2 °C) and a high relative humidity (66–100%) preceded by rainfall (0.2–30.6 mm/day). The model revealed that the most extensive bioaerosol spread is predicted in winter and summer, with an increase in Legionella risk at a distance of up to 1.5 km from the CT. This method represents a novel integrated approach for the prevention and management of LD risk in CTs.

The abatement performance of photocatalytic coatings on NOx concentrations in real-world street canyons remains an open question considering the very different conclusions reached by the few previous field studies. To fill this gap, an intensive experimental campaign was carried out in summer 2018 in the outskirts of Bologna in Italy. The experimental design involved two parallel street canyons in an open-air controlled environment fully instrumented for measuring air pollutant concentrations, meteorological and turbulence variables in presence of a photocatalytic coating. Specifically, the coating utilized TiO2 photocatalysts. Several controlled pollutant release experiments with a known pollutant source were conducted within the two canyons, of which one coated with TiO2 and the other free (reference canyon). The comparison of observations in the two canyons indicated a distinct behavior of airflows, leading to higher concentrations in the coated canyon. Three independent methods were developed to demonstrate the abatement performance of the coatings in an open-air controlled environment. The first method involved the comparison of the two canyons before and after the application of the coating; the second one regarded the derivation of normalized concentrations to remove the effect of the different geometry and dilution volume of the canyons; the third method involved dispersion modeling simulations conducted during the controlled pollutant release experiments. All the three methods demonstrated a photocatalytic reduction of NOx in the range of 14–21%, suggesting the potential effect of photocatalysis under real weather conditions.

Motivated by the increasing interest on passive control solutions to lower pollutant concentrations in cities, this paper introduces a novel methodology to demonstrate the potential of photocatalytic coatings in abating air pollution in real-world urban environments. The methodology introduced in this paper is based on an original application of Computational Fluid Dynamic (CFD) modelling to simulate the effect of photocatalytic coatings in real yet simplified urban setting. The numerical approach is validated against observations gathered during an ad-hoc designed intensive experimental campaign performed in a real urban area in the city of Bologna, Italy (44.5075 N, 11.3514E), under semi-controlled conditions. Comparison of the model output with observations show a concentration reduction in the range 10–20%. After validation and choice of the proper model set-up, numerical simulations are analyzed by focusing on the mechanisms enhancing the flow circulation within the canyon, an effect that may increase the effect of coatings within street canyons. Results show that application of photocatalytic coatings can give pollutant reductions up to 50% in a confined region close to the walls. A parametrization for the pollutant reduction within the street canyon is suggested to summarize these results, providing a characterization of the photocatalytic coatings performances as a function of the geometric characteristic of the street canyon.