UV disinfection is cost-effective and easy to maintain for decentralized areas. However, to ensure its effectiveness, some parameters need to be considered. In this study, a general search of Web of Science articles was performed to determine the possible influence of these parameters on the reactivation of microorganisms in UV systems; in addition, different search strings were used focusing exclusively on wastewater treatment, UV systems and Advanced Oxidation Processes (AOPs). It was found that in order to maintain low transmittance, it is essential to remove suspended solids and reduce water hardness. It is recommended to control the zeta potential in the range of 0–5 mV to avoid the aggregation of particles and bacteria. Determining the appropriate UV dose is essential to mitigate the reactivation of microorganisms. A minimum dose of 40 mJ/cm² can contribute to effective disinfection and reduce the likelihood of reactivation. In addition, maintaining a residual chlorine level of at least 0.5 mg/L provides an additional barrier to reactivation. It is also important to optimize the design flow rate of the UV system as recommended for each individual unit. These measures, together with the combination of UV disinfection and chemical or AOP_S, can effectively reduce the reactivation.

Kraft mill effluents are characterized by their content of suspended solids, organic matter and color due to the presence of lignin, lignin derivatives and tannins. Additionally, Kraft mill effluents contain adsorbable organic halogens and wood extractive compounds (resin acids, fatty acids, phytosterol) and show high conductivity due to the chemical compounds used in the digestion process of pulp. Currently, Kraft mills are operating under the concept of a linear economy and, therefore, their effluents are generating serious toxicity effects, detected in daphnia, fish and biosensors. These effluents are treated by activated sludge and moving bed biofilm systems that are unable to remove recalcitrant organic matter, color and biological activity (toxicity) from effluents. Moreover, under climate change, these environmental effects are being exacerbated and some mills have had to stop their operation when the flows of aquatic ecosystems are lower. The aim of this review is to discuss the treatment of Kraft pulp mill effluents and their impact regarding the current practices and future perspectives towards sustainability under climate change. Kraft pulp mill sustainability involves the closure of water circuits in order to recirculate water and reduce the environmental impact, as well as the implementation of advanced technology for these purposes.

The disinfection process in wastewater treatment is key to the discharge and/or reuse of high-quality effluent. However, disinfection using ultraviolet (UV) light may be inefficient because bacteria possess mechanisms for repairing damaged DNA. This study aimed to assess the photoreactivation and dark repair of total coliform (TC) in wastewater effluent after UV-C disinfection treatment. Four UV-C doses (28.8, 53.1, 57.6, and 106.2 mJ/cm²) and two post-irradiation conditions (light vs. darkness) were applied. Reactivation was monitored after 2, 4, 6 and 24 h (25 °C). Similar TC inactivation efficiencies were observed for the three lowest UV-C doses, whereas the 106.2 mJ/cm² dose achieved the greatest reduction (1.1 Log of TC), decreasing TC concentrations from 3.1 × 10⁵ ± 3.5 × 10⁵ to 1.2 × 10⁵ ± 1.4 × 10⁵ MPN/100 mL. Reactivation assays revealed substantial bacterial recovery after UV treatment, with 24 h survival rates up to 2.3 × 10³ under light and 9.2 × 10² in darkness. Photoreactivation and dark repair assays revealed substantial variability in bacterial recovery after UV treatment depending on UV-C dose, post-irradiation condition and incubation time. In general, bacterial recovery was still detected even at the 106.2 mJ/cm² dose, particularly after 24 h of incubation (178–604%). These findings suggest that effective organic matter removal before UV-C disinfection is critical to improve UV transmittance, reduce shielding effects, and limit subsequent bacterial recovery.