In this study, the effects of photo-electrocatalysis (PEC) were evaluated as an innovative application of conventional photocatalysis (PC) to remediate water in a recirculating system for rainbow trout (Oncorhynchus mykiss) culture, in relation to fish welfare and health, with a multidisciplinary approach. Three tanks were employed, equipped with conventional biological filters as a control system, and three tanks equipped with the PEC purification system. The concentrations of ammonia, nitrite and nitrate ions in water were monitored, and the fish’s oxidative damage and stress response were evaluated in parallel. The water of the PEC-treated experimental group showed lower ammonia (TAN) and nitrite concentrations and higher nitrate concentration, possibly deriving from TAN oxidation through PEC, also leading to gaseous N₂. Histological analysis did not reveal any pathological alteration in the gills and liver of both groups. The superoxide dismutase (sod1), glutathione reductase (GR), glutathione peroxidase (GPx1), and Tumor necrosis factor (TNFα) gene expressions were significantly higher in the control group than in the PEC-treated group, while the Heat shock protein 70 (Hsp70) expression did not show any difference in the two groups. These results indicate that the use of PEC filters has a positive effect on water quality, compared to the use of conventional biological filters, inducing a high level of welfare in O. mykiss.

Aquaculture has rapidly expanded, surpassing capture fisheries and playing a vital role in global food security. However, this growth raises environmental concerns, especially regarding nitrogen waste accumulation in recirculating aquaculture systems (RASs). Nitrogen compounds from uneaten feed and fish excreta, mainly ammonia (NH₃) and nitrite (NO₂⁻), lead to water pollution, eutrophication, and greenhouse gas emissions. This study describes the setup and the efficiency of a new photo-electrocatalytic (PEC) system in reducing nitrogen waste in a high-density RAS for rainbow trout (30 kg/m³). The PEC system, an evolution of a pure photocatalytic system, was integrated in the units of the RAS and tested for the first time in field conditions, combining photocatalysis and electrochemical oxidation to convert toxic nitrogen species (NH₃) into less harmful nitrogen forms (NO₃⁻ and N₂), aiming to mitigate both water and atmospheric pollution. Over a 4-week period, water nitrogen compounds, ammonia and greenhouse gases (carbon dioxide, nitrous oxide and methane) emitted by water were continuously monitored in two groups of three tanks (PEC vs. control). Each tank was equipped as an independent RAS unit. PEC treatment led to significantly lower NH₃ concentrations (0.96 ± 0.2 mg/L vs. 1.78 ± 0.2 mg/L, p < 0.01), lower NO₂⁻ levels and higher NO₃⁻ levels (61.77 ± 2.14 mg/L vs. 53.10 ± 2.14 mg/L, p < 0.01) in water, indicating efficient nitrogen oxidation. Gaseous emissions were also reduced: NH₃ (1.49 vs. 2.64 mg/m²/day, p < 0.05) and N₂O (1.44 vs. 2.88 mg/m²/day, p < 0.05). These results support PEC technology as a promising solution for improving nitrogen management in intensive aquaculture. Although challenges remain in optimizing energy use and scalability, PEC offers a valuable strategy for reducing environmental impact while sustaining productivity in the aquaculture industry.