Enhancing Oxygen Transfer in Subsurface-Flow Constructed Wetlands

Conventional subsurface-flow constructed wetlands (SFCW) for wastewater treatment are typically designed as horizontal plug-flow vegetated beds. Limited oxygen, provided mostly by slow transfer through air–water interface, turns these systems anaerobic, thus requiring a long hydraulic retention time (HRT) to achieve desired effluent quality. A SFCW treatment system with biofield effluent disposal has been designed and constructed for the highway I-70 rest area station near Greenfield, Indiana. A number of features were incorporated into the design of this system in an effort to enhance oxygen transfer. They include 1) a pair of parallel wetlands that can be operated in a cyclic “draw-and-fill” scheme – allowing filling of one wetland while draining the other, and 2) a vertical filter with coarse gravel at the front section of each wetland cell. It is expected these modifications can significantly improve biological oxygen demand (BOD) and nitrogen removal, therefore reducing the HRT of the system. This paper presents details of the modified design, flow and treatment data from the first year of operation, and early experience gained during this period.