Toxicity assessment of wastewater after advanced oxidation processes for emerging contaminants degradation

Abstract The dilution paradigm, “the solution to pollution is dilution,” failed, and current wastewater treatment technologies are generally missing the treatment of the so-called contaminants of emerging concerns (CECs). Among CECs, a huge variety of substances can be found such as pharmaceuticals, personal care products, engineered nanomaterials, perfluorooctane sulfonate and perfluorooctanoic acid, endocrine disrupting compounds, textile dyes, surfactants, and UV-B filters from sunscreen creams. But how to reduce/remove CECs? How to meet the goal of environmental sustainability reducing discharge's dilution phenomena and maximizing treated wastewater reuse? How to provide a “toxicity”-free treated wastewater to be discharged into surface waters or addressed to specific reuse activities? The problem of “residual” toxicity in treated wastewater could be a huge problem. Whole effluent toxicity and whole effluent assessment firstly introduced the need to monitor wastewater toxicity, and now toxicity plays an important role in wastewater treatment technology efficiency definition as best available technologies. Advanced oxidation processes (AOPs) have recently emerged as very interesting alternatives for water treatment, including water detoxification and disinfection as well. AOPs are physical–chemical processes enabling to produce deep changes in contaminants chemical structure due to generation of free radicals' such as hydroxyl radicals (HO⋅) with catalysts (e.g., metal-supported catalysts, clays, carbon materials, or semiconductors or composite material). Anyway, the elimination of mother compounds (CECs) does not necessarily result in toxicity removal too. It may be true that the targeted compound can be eliminated, but the photocatalytic degradation can produce intermediates and by-products, which can still exert adverse biological effects. When applied as tertiary treatment, processes can push ahead the degradation of CECs toward high mineralization levels (>99%). Nevertheless, the main focus should be the toxicity removal from the effluent to ease its safe reuse that can be reached even at lower mineralization level to optimize costs and benefits. Currently, only few studies investigated AOPs treating CECs following not only the traditional parameters, but also toxicity. General results suggested that a case-by-case approach is a necessary coupling-specific AOPs technique to a targeted series of recalcitrant compounds.