Which option describes a natural, passive method of treating wastewater using plants and soil in wetlands?

• A. Wetland-Based Treatment Systems
• B. Secondary Sewage Treatment
• C. Tertiary Sewage Treatment
• D. Septic Tanks

Answer: (A)

Natural, passive wastewater treatment using wetlands relies on a designed or existing wetland system where plants, soil, and microbial communities work together to clean effluent as it moves slowly through the wetland. The plants create an active root zone that supplies oxygen and release organic compounds that feed microbes, while the soil and root-filled channels physically filter sediments and adsorb nutrients and contaminants. Microbes in the rhizosphere degrade organic matter and transform nutrients, so the overall process reduces biochemical oxygen demand, nutrients, and some pathogens without requiring mechanical energy input. Because the treatment depends on natural processes and gravity-driven flow rather than pumps or chemical steps, it’s considered passive and well-suited for decentralized or small-scale applications.

Other options describe more engineered or energy-intensive processes. Secondary treatment relies on aerated, biological degradation in mechanical systems, not a wetland. Tertiary treatment adds advanced polishing steps, often chemical or physical, beyond the basics. Septic tanks provide anaerobic digestion in a buried tank rather than a vegetation-involved wetland system.