Caffeine removal in wastewater: a comprehensive review of current treatment plants and small-scale innovations.

As an emerging pollutant, caffeine has increasingly attracted public attention. Although typically metabolized in the human body, approximately 5% of ingested caffeine is excreted through urine, ending in wastewater treatment plants (WWTPs). Given the extensive global consumer market, the daily quantity of caffeine reaching these WWTPs is significant. Understanding the fate of caffeine through the three treatment stages in WWTPs is thus crucial. The primary treatment stage, mainly focused on removing large solids and grit from wastewater, does not effectively and directly eliminate caffeine from the sewage. Conversely, the secondary treatment stage plays a central role in caffeine removal, heavily depending on microbiological activities under aerobic, anaerobic and anoxic conditions. Tertiary treatment procedures such as adsorption, advanced oxidation processes, membrane separation and constructed wetlands can further remove caffeine before the safe discharge of effluent into the environment. This review concludes that the caffeine removal capacities can vary significantly across different WWTPs due to differences in treatment methods, cost-efficiency and operational constraints. Hence, there is an ongoing need for continued research and optimization of wastewater treatment processes concerning caffeine and similar emerging pollutants.