From Structure to Performance: Multi-Technical Study of the Purification Potential of Middle Atlas Pozzolan and a Local Gravel for Application in a Multi-Soil Layering (MSL) Filter

Two natural materials, pozzolan from Aguelmous (Middle Atlas, Morocco) and local gravel (3–6 mm), were selected and compared as permeable layers in a multi-soil layering (MSL) system treating domestic wastewater. This comparison was motivated by the need to evaluate a widely used conventional material in Morocco (gravel) against a highly porous volcanic pozzolan expected to improve pollutant removal due to its superior structural and physicochemical properties. The study combined physicochemical characterization, adsorption experiments using methylene blue (MB) and methyl orange (MO), and pilot-scale multi-soil layering (MSL) system tests under vertical-flow conditions. Pozzolan exhibited a highly porous structure with a specific surface area of 13.77 m2·g−1 and a total porosity of 70.12%, compared with 1.92 m2·g−1 and 45.49% for gravel, respectively. These properties resulted in higher adsorption performance, with MB removal efficiencies of 66.60% for pozzolan versus 45.35% for gravel at 16.5 mg·L−1, and MO removal efficiencies of 69.12% versus 58.55% at 10 mg·L−1. Correspondingly, pozzolan showed higher adsorption capacities for MB (9.976 mg·g−1) and MO (1.534 mg·g−1) than gravel (0.829 and 0.750 mg·g−1, respectively). In pilot-scale operation, pozzolan-based filters achieved higher removal efficiencies for BOD5 (82.01%), COD (86.14%), ammoniacal nitrogen (89.48%), and total phosphorus (66.30%) compared with gravel (75.56%, 78.24%, 61.56%, and 50.42%, respectively). The enhanced ammoniacal nitrogen removal in filter F2 (6–15 mm) is attributed to favorable aerobic conditions within the system, which likely promoted nitrification due to improved oxygen availability. However, the absence of sufficient anoxic zones limited denitrification, leading to nitrate (NO3−) accumulation. Overall, the results indicate that natural pozzolan is a highly suitable permeable material for enhanced pollutant removal, making it a promising alternative to conventional gravel in decentralized MSL wastewater treatment systems.