Performance of pilot-scale horizontal subsurface flow constructed wetlands in wastewater treatment

Constructed wetland technology for municipal, industrial and agricultural wastewater treatment is becoming very popular all over the world. Although constructed wetlands are extensively used today, the precise mechanisms responsible for removal of organic matter and nutrients and the factors responsible for performance variations are not fully understood. One of the reasons for this fact is the lack of studies in pilot or laboratory-scale systems, where these factors could be examined better through controlled experiments. With this in mind, we have designed, constructed and are operating now in our laboratory five small-scale subsurface flow constructed wetlands of dimensions 3 m long, 0.75 m wide and 0.5 m deep each. The five wetlands have three different porous media and two vegetation types. The aim is to identify the effects of vegetation type, porous medium type and other factors on the performance of these systems, and derive appropriate experimental parameters and models describing the function of these systems under Mediterranean climate conditions. The units received synthetic wastewater from January 2004 until July 2004 with 14 days residence time and from August 2004 until March 2005 with 8 days residence time. Water quality samples were collected at the inlet, two points along each unit and the outlet of each unit, and were analyzed in the laboratory for BOD, COD, TKN, ammonia, nitrate, nitrite, TP, and ortho-phosphate. Typical BOD removal varied from 80 to 95% depending on season. In general, the performance of the units in BOD, COD, TKN and phosphorus removal depends on temperature, vegetation type and porous medium type. This paper presents the results of the first year of operation of these pilot units.