Science Puzzle
Wetland Filtration
A constructed wetland can take in heavily polluted agricultural runoff and produce water clean enough to return to a river or use for irrigation, without any chemical treatment plants or electricity.
How does a wetland clean the water?
The Answer
A wetland uses three interlocking processes. Physical filtration: as water moves slowly through gravel, sediment and plant roots, suspended particles settle out and are trapped. The slow flow rate is essential; fast-moving water carries particles instead of dropping them.
Biological uptake: the roots of reed plants and other wetland vegetation absorb nitrogen and phosphorus directly from the water, the same nutrients that cause algae blooms in rivers. The plants incorporate these into their tissues, removing them from the water permanently until the plants are harvested.
Microbial degradation: the sediment layer hosts enormous communities of bacteria that break down organic compounds, including many pollutants and pathogens, into simpler, harmless substances. These bacteria thrive in the slow, nutrient-rich environment that the wetland provides.
Together these three processes, physical, biological and microbial, replicate and often outperform expensive industrial water treatment, at a fraction of the energy cost.
The principle: Wetland biofiltration. Physical settling, biological nutrient uptake by plants, and microbial decomposition in sediment combine to clean water without chemicals or electricity.