Among the critical issues that prevent the reuse of wastewater treatment plants (WWTPs) effluents in a circular economy perspective, the microbiological component plays a key role causing infections and diseases. To date, the use of conventional chemical oxidants (e.g., chlorine) represent the main applied process for wastewater (WW) disinfection following a series of operational advantages. However, toxicity linked to the production of highly dangerous disinfection by-products (DBPs) has been widely demonstrated. Therefore, in recent years, there is an increasing attention to implement sustainable processes, which can simultaneously guarantee the microbiological quality of the WWs treated and the protection of both humans and the environment. This review focuses on treatments based on ultraviolet radiation (UV) alone or in combination with other processes (sonophotolysis, photocatalysis and photoelectrocatalysis with both natural and artificial light) without the dosage of chemical oxidants. The strengths of these technologies and the most significant critical issues are reported. To date, the use of synthetic waters in laboratory tests despite real waters, the capital and operative costs and the limited, or absent, experience of full-scale plant management (especially for UV-based combined processes) represent the main limits to their application on a larger scale. Although further in-depth studies are required to ensure full applicability of UV-based combined processes in WWTPs for reuse of their purified effluents, excellent prospects are presented thanks to an absent environmental impact in terms of DBPs formation and excellent disinfection yields of microorganisms (in most cases higher than 3-log reduction).

The formation of persistent foams can be a critical problem in wastewater treatment plants (WWTPs) as it could lead to a series of operational problems, especially the reduction of the overall system performance. To date, the effects of foaming in the WWTPs are a problem that is currently very common and shared, but which to date is treated mainly only at the management level and still too little studied through a globally shared scientific method: the complexity of the phenomenon and the systems have led to numerous partially contradictory descriptions and hypotheses over the years. The goal must be to suggest future research directions and indicate promising strategies to prevent or control the formation of foams in WWTPs. This study examines and investigates the problem of foams by a methodological approach of research through a review on the state of the art: the factors influencing the formation of foams are described first (such as surfactants and/or extracellular polymeric substances (EPSs)), then the known methods for the evaluation of foaming, both direct and indirect, are presented, with the aim of identifying the correct and best (from the management point of view) control and/or prevention strategies to be applied in the future in WWTPs.

Interest in the presence of microplastics (MPs) in wastewater has grown significantly in recent years. In fact, wastewater treatment plants (WWTPs) represent the last barrier before the discharge of MPs into an aquatic ecosystem. The research has highlighted how MPs are in part effectively removed from the waters and accumulated inside the sewage sludge (SeS) produced by the WWTP, being a cause for concern, especially in the case of agricultural reuse. This work aims to analyze the existing literature on the (i) methodical procedure for MPs analysis (thermal, spectroscopic, optical analyses), (ii) qualitative and quantitative presence of MPs in SeS, (iii) effect on sludge properties, and (iv) the possible accumulation in amended soils. Based on the results already obtained in the literature, this work aims to provide critical insights to stimulate interest in the topic and direct future research on aspects that should be deepened. In particular, it emerges that there is a clear need for standardization of the collection methods and the analytical techniques for identifying and quantifying MPs, since their physico-chemical characterization and the study on aging and on the response towards acid or basic pre-treatments are fundamental for the understanding of microplastics ecotoxicological potential.

In drinking water, high concentrations of fluoride and arsenic can have adverse effects on human health. Waste deriving from the rice industry (rice husk, rice straw, rice bran) can be promising adsorbent materials, because they are (i) produced in large quantities in many parts of the world, (ii) recoverable in a circular economy perspective, (iii) at low cost if compared to expensive conventional activated carbon, and (iv) easily manageable even in developing countries. For the removal of fluoride, rice husk and rice straw allowed to obtain adsorption capacities in the range of 7.9–15.2 mg/g. Using rice husk for arsenic adsorption, excellent results were achieved with adsorption capacities above 19 mg/g. The best results both for fluorides and arsenic (>50 mg/g) were found with metal- or chemical-modified rice straw and rice husk. Identifying the next steps of future research to ensure the upscaling of biochar from recovered by-products, it is fundamental to perform: (i) tests on real waters for multicomponent adsorption; (ii) experiments with pilot plants in continuous operation; (iii) cost analysis/real applicability of modification treatments such as metal coupling or chemical synthesis; (iv) more studies on the biochar stability and on its regeneration or recovery after use.

This work evaluates the efficiency of three biochar samples toward the adsorption of manganese, iron, and selenium present in a sample of urban wastewater. The biochar was produced from the pyrolysis of rice husks at 350 °C for 6 h (RHB) and subsequently modified using HCl (RHB_HCl) or NaOH (RHB_NaOH) to increase its surface area. The RHB_NaOH sample exhibited the highest removal efficiency for the three metals. The metals’ adsorption removal efficiency for RHB_NaOH was in the order Mn (76%), Se (66%), and Fe (66%), while for RHB_HCl, it was Fe (59%), Mn (30%), and Se (26%). The results show that the as-prepared RHB can remove the metals, even if in low amounts (Fe (48%), Mn (3%), and Se (39%)). The adsorption removal for the three types of adsorbents follows the Langmuir isotherm model. Pseudo-first-order and pseudo-second-order models were used to determine the adsorption mechanism for each of the three adsorbents. Both models showed a good fit with R² (>0.9) for the RHB_NaOH and RHB sorption of Fe, Mn, and Se. Overall, this work demonstrates the potential of biochar for the removal of metals from real wastewater.

Per- and polyfluoroalkyl substances (PFAS) constitute a group of organofluorine chemical synthetic compounds widely used in industries and manufacturing due to their hydrophobic properties. However, PFAS have been found to cause negative human health outcomes. Therefore, a strong interest in the possible removal of these compounds from wastewater (WW) has been shown. This work aims to present a systematic analysis of the scientific literature related to the innovative and alternative adsorbent materials that can be used for treating PFAS-contaminated WW. Moreover, the adsorption processes are considered, focusing the attention on virgin adsorbent materials and biochar as adsorbents. Virgin adsorbent materials comprise conventional adsorbent materials, functional clays, metal–organic frameworks, and functionalized organic polymers. Biochar includes materials obtained from agricultural or food residues and from sewage sludge. The review shows that conventional treatment units using virgin adsorbent materials are characterized by high adsorption capacity, but also high costs. In addition, the refunctionalization of adsorbent materials is difficult to obtain. On the contrary, biochar, which is a residual product of other production processes, appears to be a cost-effective solution.