Biochar Synthesis, Characterization, and Application in Constructed Wetlands Systems for Removing Organic Matter, Nutrients, Pathogens, and Pharmaceutical Pollutants

Growing pressures on water resources and inadequate wastewater treatment infrastructure in many regions call for low-cost, efficient, and sustainable solutions capable of addressing both conventional and emerging contaminants. This thesis investigates the valorisation through pyrolysis processes of local agro-industrial residues, mainly argan press cakes (white and black), olive pomace, and lignocellulosic materials, into biochar for decentralized wastewater treatment systems. Furthermore, sewage sludge together with sawdust has been used as feedstock for biochar production via a co-pyrolysis process. The research combined feedstock characterization, pyrolysis optimization, and performance evaluation in filtration and constructed wetland systems, with a focus on both classical pollution parameters and pharmaceutical contaminants. Detailed physicochemical and thermal analyses showed that the selected agro-industrial residues contained high carbon percentages (52–61%) and exhibited good energy potential, making them suitable feedstocks for pyrolysis. Biochar yields ranged from 8% to 34% depending on feedstock and pyrolysis temperature, with fixed carbon increasing and volatile matter decreasing as temperature rose. At 800 °C, olive pomace biochar achieved the highest specific surface area (22 m²/g) and methylene blue adsorption capacity (432 mg/g), confirming the strong effect of pyrolysis conditions on sorption properties. In the second phase, biochar was incorporated at different proportions (0%, 10%, 25%, and 50%) into filtration systems and pilot-scale vertical flow constructed wetlands (VF-CWs) treating domestic wastewater. Results demonstrated a significant enhancement of treatment performance compared to unamended systems. The 10% biochar dose provided the best balance between efficiency and hydraulic performance, achieving up to 92% removal of total suspended solids (TSS), 83% of chemical oxygen demand (COD), 87% of organic nitrogen, and 44% of total phosphorus, with faecal indicator contamination removal reaching 3–4 log units. Hydraulic conductivity was maintained, indicating good substrate stability. In the third phase, the removal of contaminants of emerging concern was investigated. Biochar-amended VF-CWs exhibited high removal efficiencies for several pharmaceutical compounds, including clarithromycin, erythromycin, and ketoprofen (up to 99–100%), while others, such as carbamazepine and fluoxetine showed low to moderate removal (≤35%). These findings reflect both the sorption properties of biochar and the differential biodegradability of pharmaceuticals. Biochar effectively improved micropollutant retention without compromising system operation, confirming its suitability as an integrated, low-cost substrate. Overall, this work demonstrates the technical feasibility and environmental relevance of converting local biomass residues into high-performance biochar for sustainable wastewater treatment. It provides robust experimental evidence that biochar can substantially enhance the efficiency of decentralized systems, contributing to improved water quality, waste valorisation, and climate co-benefits through carbon sequestration. These results support the development of scalable, resource-efficient sanitation solutions for rural and peri-urban communities facing both biomass accumulation and water pollution challenges. Particularly interesting results were obtained in vertical systems by replacing conventional substrates (sand and gravel) with biochar obtained from the co-pyrolysis of sewage sludge and sawdust, and subsequently chemically and thermally activated (surface area approximately 400 m2/g). In this study, which lasted approximately eight months, the biochar systems showed removal rates almost double those of conventional systems, achieving a mean removal of about 70% over the whole period and thus highlighting very promising aspects of their practical application.