A comprehensive review of treatment technologies for greywater contaminants: an emphasis on surfactants.

Surfactants are considered a key pollutant in greywater (GW). However, there is no consensus on the best technologies for their removal. This review provides a comprehensive analysis of the main lines of GW research. Recent literature on GW characteristics, surfactant characteristics, toxicity and treatment technologies are analyzed along with case studies of GW reuse. The bibliometric analysis shows that 69 % of the studies on GW involve surfactants, and physicochemical technologies predominate with 13.5 % of the total studies. However, the toxicity of surfactants has been scarcely addressed (10 % of the articles). Anionic surfactants represent 60 % of the world production of surfactants. Concentrations in GW range from 7 to 436 mg/L and their toxicity measured as lethal concentration 50 (LC₅₀) varies between 0.027 and 92 mg/L. Physicochemical treatments show a removal efficiency between 22 and 99 % of anionic surfactants, while biological and advanced systems show between 53 and 99 % removal efficiency. Furthermore, combined treatments show removal efficiencies between 78 and 99 %. However, high costs of advanced systems limit their application on a large scale. In this sense, low-cost alternatives such as the use of reused adsorbent materials, natural solutions and disinfection systems could offer up to 98 % surfactant removal. Regarding effluent toxicity, more research is needed, since limited studies address this gap. Moreover, it is unclear whether there will be a synergistic or antagonistic effect on the toxicity caused by surfactants and other pollutants on organisms and species. Along with this, the study of surfactant transformation products by different technologies and their toxicity is presented as a new research gap.