Synergistic Removal of various emergent pollutants in Biochar-Enhanced Constructed Wetlands: A Pilot-Scale Nature-Based Solution for Municipal Wastewater Treatment Improvements

Chaimae Maldou¹Yasmina El Janoussi¹Dhafer Mohammed M. Al Salah²Majida Lahrouni¹Naaila Ouazzani¹Tonni Agustiono Kurnaiwan³Ayoub El Ghadraoui⁴Amine Elleuch⁵John Poté⁶Faissal AZIZ^1*

• ¹Faculty of Science Semlalia of Marrakech, Laboratory of Water Sciences, Microbial Biotechnologies and Natural Resources Sustainability, BP 2390, 40000,., Cadi Ayyad University, Marrakech, Morocco
• ²Joint Centers of Excellence Program, Prince Turki the 1st St, Riyadh, 11442, Saudi Arabia, King Abdulaziz City for Science And Technology, Riyadh, Saudi Arabia
• ³College of Environment and Ecology, Xiamen University, Xiamen, 361102, Fujian, China., Xiamen University International College, Xiamen, China
• ⁴Leibniz-Institut fur Gewasserokologie und Binnenfischerei im Forschungsverbund Berlin eV, Berlin, Germany
• ⁵Laboratory of Plant Biotechnology Applied to Crop Improvement,, Universite de Sfax Faculte des Sciences de Sfax, Sfax, Tunisia
• ⁶Faculty of Sciences, Department F.-A. Forel for Environmental and Aquatic Sciences and Institute for Environmental Sciences, 1211 , 4,., Universite de Geneve, Geneva, Switzerland

The primary drawback of reusing treated wastewater for irrigation is that most wastewater treatment plants (WWTPs) remain unable to remove most emerging pollutants detected in recent decades, necessitating a strategy to improve these WWTPs' performance. In this regard, the present study aims to evaluate the effectiveness of an enhanced pilot vertical-flow biochar-enriched constructed wetland for treating municipal wastewater pretreated with natural lagoons. Three key harmful micropollutants are addressed: heavy metals (trace elements), pharmaceutical residues, and antibiotic-resistant bacteria (ARB). The system utilizes a biochar layer to enhance treatment efficiency by increasing adsorption capacity and supporting beneficial microorganisms, in conjunction with Phragmites australis plants that facilitate oxygen transfer and nutrient uptake. To evaluate the treatment system's performance, Inductively Coupled Plasma Mass Spectrometry (ICP-MS) was used to measure heavy metal concentrations. A molecular approach using quantitative polymerase chain reaction (qPCR) 2 was used to quantify antibiotic resistance genes (ARGs) in water samples before and after treatment, including β-lactamases TEM, CTX-M, and SHV. The performance obtained is remarkable for the well-known toxic metals, reducing their concentrations from 16.35 ± 0.82 μg L⁻¹ for arsenic, 0.0936 ± 0.0047 μg L⁻¹ for cadmium, and 0.6036 ± 0.030 μg L⁻¹ for lead to 2.75 ± 0.14 μg L⁻¹, 0.0014 ± 0.000070 μg L⁻¹, and 0.089 ± 0.0045 μg L⁻¹, respectively. Furthermore, the results showed a significantly reduced copy number of the 16S rRNA gene and of the resistance-conferring genes. Pharmaceutical residues (doxycycline, ciprofloxacin, enoxacin, enrofloxacin, norfloxacin, ofloxacin, amoxicillin and ampicillin) were also effectively removed after treatment, with removal rates ranging from 63% to over 99% depending on the compound. For example, ibuprofen and ciprofloxacin were reduced by 96–97% (from 7.310 to 238 ng L⁻¹ and from 2.300 to 83 ng L⁻¹, respectively). Even recalcitrant molecules such as carbamazepine and diclofenac have been reduced by more than 60%, confirming the high efficiency of the biochar-enriched artificial wetland system. These results highlight the considerable potential of this nature-based solution as a promising approach to addressing the global concern of enhancing WWTP removal of emergent pollutants. It therefore offers a sustainable, environmentally friendly alternative for wastewater treatment reuse in irrigation.