Tricks and tracks of hybrid constructed wetlands: critical review on emerging contaminant removal pathways

Emerging contaminants (ECs), including pharmaceuticals, antibiotics, personal care products, endocrine-disrupting compounds and persistent industrial chemicals, are increasingly detected in wastewater effluents because many are poorly removed by conventional treatment and may form biologically active transformation products. This review critically evaluates hybrid constructed wetlands (HCWs) as engineered biodegradation platforms for EC attenuation, with emphasis on microbial pathways, redox-zonation and substrate-mediated transformation. HCWs integrate vertical-flow, horizontal subsurface-flow and free-water surface units to create coupled aerobic, anoxic, anaerobic, rhizospheric and photic microenvironments. These zones support filtration and sorption, but importantly promote biodegradation, cometabolism, nitrification-denitrification, anaerobic ammonium-oxidation and sulfur-driven autotrophic-denitrification. Anammox bacteria, including Candidatus Brocadia, Candidatus Jettenia, and Candidatus Scalindua, contribute to low-carbon nitrogen removal by coupling ammonium oxidation with nitrite reduction. Sulfur-oxidizing denitrifiers Thiobacillus, Sulfurimonas and Sulfuritalea further stabilize anoxic electron-transfer conditions under carbon-limited wastewater. Although these communities do-not directly mineralize all ECs, they regulate redox gradients, biofilm activity and enzymatic environments that favour transformation of biodegradable and cometabolizable compounds. Conventional gravel and sand provide hydraulic support and show limited dissolved EC-removal, generally below 20-30%, whereas biochar, zeolite and LECA improve removal to 60-90% by enhancing sorption and microbial colonization. Functionalized media, including TiO₂-coated substrates, Fe/Mn materials and nano-zero-valent (nZVI) iron, can achieve 70-98% removal for selected recalcitrant ECs. Field systems commonly report 50-85% removal, whereas laboratory systems often exceed 90%, indicating a scale-up gap. This review highlights microbial responses to HCW, redox conditions, substrates, rhizosphere-interactions and wastewater characteristics, while identifying future needs in transformation-product tracking, and functional-gene monitoring for SDG 6-oriented wastewater treatment.