A second cell may be added for more treatment. As wastewaters flow through the system, suspended solids and trace metals settle and are filtered. Plants and organic material also absorb trace metals. Organisms that live in water, on rocks, in soil, and on stems and roots of wetland plants use these organic materials and nutrients as food. Plants provide much of the oxygen needed by the organisms to live and grow.
Plant roots keep the rocks or soil loose so that water can flow through easily.
The lack of standard information for engineers is currently limiting the design and construction of effective low cost systems. Constructed wetlands may not effectively treat some types of complex pollutants. These systems also need more land than conventional systems. High land costs and lack of suitable land can make construction of large systems impractical. Sites which are relatively flat, have deep soils, and a low groundwater water table are needed for small scale stems.