Experience with Subsurface-Flow Constructed Wetlands for Municipal Wastewater Treatment in the Midwest

This paper documents the performance of a subsurface-flow constructed wetlands system during its initial years of operation under variable loading and operating conditions associated with a northern midwestern U.S. climate. The results indicate that effective and sufficient CW seasonal removals of TSS, VSS, CBOD₅, COD, and fecal coliform were achieved. Wastewater temperatures seemed to affect CBOD₅ and COD removal rates. Nitrogen and phosphorus reductions were not as effective and varied seasonally, as well as with wastewater temperature. The addition of a sand filter, to aid in further nitrification and disinfection following CW treatment, markedly improved the performance of the wetlands system. After a few years of operation, the remarkable performance of the CW system was dampened by apparent clogging and subsequent eruption of wastewater at the head-end of the treatment cells. While clogging was partially caused by biomass build-up in the wetlands substrate, visual observations suggest that excessive vegetation coupled with relaxed maintenance may also be responsible for clogging.