Natural Wetlands ... Treatment Systems

To comprehend the constructed wetlands treatment process, the workings of natural wetlands must be understood. Natural wetlands are variously called swamps, marshes, bogs, fens, or wet meadows. The wetlands these terms define are not necessarily all the same. Plant types, water and geographic conditions vary, creating different kinds of wetlands.

Wetlands are transitional areas between water and land. The term wetlands  means "those areas that are inundated or saturated by surface or groundwater at a frequency and duration sufficient to support, and that under normal circumstances do support, a prevalence of vegetation typically adapted for life in saturated soil conditions," according to the 1977 Clean Water Act.
Occurring in low-lying areas, wetlands receive runoff water and overflow from rivers and streams that accumulate at or near the rate of discharge. Various wetland biological mechanisms or processes evolved over geologic time that treats inflows. These mechanisms trap sediments and break down a wide range of pollutants into less toxic or inert substances.
Wetlands have an remarkable ability to treat wastewater. Water moves slowly through wetlands, as shallow flows, saturated substrates or both. Slow flow and shallow waters cause sediments to settle and become anoxic. The slow flows prolong contact times between the water and surfaces within the wetland allowing mechanisms to remediate substances that would not be possible otherwise.
The accumulated organic and inorganic materials within a wetland form a complex mass. This mass along with anoxic and oxic zones promotes a varied community of microorganisms, to break down or transform a wide variety of substances.
Dense growths of vascular plants adapted to saturated conditions often thrive in wetlands and contribute marginally to its treatment capacity. Along with slowing the flow of water, the vegetation creates microenvironments and provides the microbial community attachment sites, and oxygenated microhabitats through transport of oxygen from the atmosphere to the rizosphere (root zone of the plants) allowing nitrification and other chemolithotrophic process to occur. Further, plants die back in the fall and accumulate as litter. This creates additional material and exchange sites as well as providing a source of fixed carbon, nitrogen and phosphorous to fuel microbial processes.