(See endnote 35)
A subsurface flow wetland is specially designed to treat some types of wastewater, and it consists of a bed or channel with coarse rock or gravel and planted with vegetation usually found in marshes. The water surface is kept below the top surface of the medium which helps prevent the growth of mosquitoes as well as odors. Since it had been noted that in natural wetlands the quality of the water improves due to physical, chemical, and biochemical reactions, this type of system has been designed and installed to work at even higher rates of success than the natural wetlands. The advantages of this system are that it provides effective treatment without requiring the use of mechanical equipment, sources of energy, or a skilled operator. This helps contain cost. It can be used year-round, has no residual biosolids, and removes BOD, COD, total suspended solids, and metals effectively. The disadvantages of this system are: the requirement for large land areas; in cold climates the amount of BOD, ammonia, and nitrogen is decreased because of the low winter water temperatures; and the amount of Escherichia coli removed may not be sufficient to meet standards of water for discharge.