How do constructed wetlands treat wastewater?

Constructed wetlands are engineered systems using natural processes involving vegetation, soil, and microorganisms for wastewater treatment. Types include: Surface flow wetlands - water flows over saturated substrate; mimics natural marshes; good for wildlife habitat but requires large area. Subsurface flow (horizontal) - water flows through gravel/sand bed below surface; reduces odors and mosquitoes; better pathogen removal. Vertical flow - water percolates vertically through bed; better aeration; often used with horizontal flow in hybrid systems. Treatment mechanisms: Organic removal - microbial degradation in biofilms on substrate and roots. Nitrogen removal - nitrification in aerobic zones, denitrification in anoxic zones. Phosphorus removal - soil adsorption, plant uptake, precipitation. Pathogen removal - UV exposure, predation, filtration, adsorption. Plants (Phragmites, Typha, Scirpus) provide oxygen to root zone, surface for biofilms, and nutrient uptake. Suitable for small communities, polishing, and stormwater treatment.