Citation: Lanzetta, A.; Papirio, S.; Oliva, A.; Cesaro, A.; Pucci, L.; Capasso, E.M.; Esposito, G.; Pirozzi, F. Ozonation Processes for Color Removal from Urban and Leather Tanning Wastewater. Water 2023, 15, 2362. https://doi.org/10.3390/ w15132362 Academic Editors: Elia Judith Martínez Torres and Silvia Fiore Received: 16 May 2023 Revised: 16 June 2023 Accepted: 25 June 2023 Published: 27 June 2023 Copyright: © 2023 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). water Article Ozonation Processes for Color Removal from Urban and Leather Tanning Wastewater Anna Lanzetta 1 , Stefano Papirio 1 , Armando Oliva 1 , Alessandra Cesaro 1, * , Luca Pucci 2 , Emanuele Mariano Capasso 2 , Giovanni Esposito 1 and Francesco Pirozzi 1 1 Department of Civil, Architectural and Environmental Engineering, University of Napoli Federico II, Via Claudio 21, 80125 Napoli, Italy; anna.lanzetta@unina.it (A.L.); stefano.papirio@unina.it (S.P.); armando.oliva@unina.it (A.O.); giovanni.esposito1@unina.it (G.E.); francesco.pirozzi@unina.it (F.P.) 2 Gori Spa, 80056 Ercolano, Italy; lpucci@goriacqua.com (L.P.); ecapasso@goriacqua.com (E.M.C.) * Correspondence: alessandra.cesaro@unina.it Abstract: The need to ensure adequate levels of both human and environmental health protection calls for the identification of efficient wastewater treatment processes that target the removal of conventional pollutants as well as emerging contaminants, including synthetic dyes. This study explores the potential of ozone for use in the decolorization of both pretreated tannery wastewater and urban wastewater effluents. Different ozone contact times (15, 30, and 45 min) were applied to such wastewater samples at an uncontrolled (7.5–8.2) pH as well as at a pH adjusted to 10. The highest color removal efficiencies (REs) (>90%) were obtained with urban wastewater after a 45 min contact time and at pH 10. Under the same conditions, a COD RE from 31 to 51% was achieved. Even without pH adjustment, color and COD REs ranging from 81 to 92% and 39 to 41%, respectively, were obtained. A preliminary evaluation of the costs associated with the implementation of an ozonation stage within the wastewater treatment plant (WWTP) generating the urban wastewater used here was carried out. The energy cost of the ozone treatment would account for a 69% increase in the total energy cost of the WWTP, suggesting the need to identify proper strategies to enhance the cost-competitiveness of this technology. Keywords: advanced oxidation processes; color removal; emerging contaminants; tannery wastewater; ozone 1. Introduction Significant population growth, urbanization, and the global water crisis have gener- ated an increasing demand for freshwater worldwide. The proper treatment of wastewater plays a crucial role in supporting and maintaining the quality of aquatic ecosystem resources as well as human health. The main determinants of the quality of receiving freshwater bodies are chemical compounds and particles in water, such as nutrients, heavy metals, and micro-organisms, whose concentrations depend on several factors, such as the origin of the treated wastewater [1]. However, the presence of emerging contaminants (ECs), mainly organics, including endocrine disruptors, pesticides, pharmaceuticals, hormones, toxins, and industrial-related synthetic dyes [2,3], may compromise the water quality due to the complexity associated with their treatment [4,5]. Normally, wastewater treatment plants (WWTPs) are designed and operated to re- move mainly ammonia, total suspended solids (TSS), and the chemical oxygen demand (COD) and to produce final high-quality effluents to be discharged in accordance with the legally established standards. However, conventional WWTP processes are poorly efficient for the removal of ECs [5], these being highly persistent and requiring particular operating or environmental conditions due to their physicochemical characteristics [5]. Although EC removal efficiencies (REs) in the ranges of 20–50% and 30–70% have been reported dur- ing primary and secondary treatments, respectively, in most cases, conventional WWTPs Water 2023, 15, 2362. https://doi.org/10.3390/w15132362 https://www.mdpi.com/journal/water