Wastewater Treatment Plants as Environmental Barriers in Hyperarid Regions: A Comprehensive Evaluation of Their Performance, Groundwater Protection, and Reuse in Agriculture in the Algerian Sahara

Wastewater treatment plants (WWTPs) are increasingly considered critical infrastructure for environmental protection and combating climate change in regions suffering from severe water scarcity. The aim of this work is to provide a comprehensive and integrated evaluation of the performance of WWTPs in arid and hyperarid contexts, based on two representative experiences in the Algerian Sahara. The evaluation is based on an analysis of treatment performance (COD, BOD5, TSS), operational stability, and the agricultural suitability of the wastewater (electrical conductivity, SAR, RSC), in addition to the indirect effects on groundwater protection. The results show high and stable organic matter removal rates (>85–90%), demonstrating the effectiveness of biological processes under harsh climatic conditions. Despite these benefits, residual salinity and sodium carbonate remain the two main factors limiting the extent of long-term agricultural reuse, despite effective treatment. The international comparative analysis highlights the systemic nature of this dissociation in hyperarid environments and emphasizes the need to consider wastewater treatment plants as truly integrated environmental barriers.