Vegetation effects on fecal bacteria, BOD, and suspended solid removal in constructed wetlands treating domestic wastewater

Abstract

Constructed wetlands have emerged as a viable alternative for secondary treatment of domestic wastewater in areas with landscape limitations, poor soil conditions, and high water tables, which limit installation of full-scale adsorption fields. Existing information on the effects of macrophytes on treatment performance is contradictory and mostly derived from greenhouse mesocosm experiments. This study investigated the removal efficiency of fecal bacteria, biological oxygen demand (BOD), and total suspended solids (TSS) in 12 constructed wetlands treating secondary effluent from single household domestic wastewater in Kentucky. The wetlands were monoculture systems planted to cattails ( Typha latifolia L.) or fescue ( Festuca arundinacea Schreb.), polyculture systems planted with a variety of flowering plants, and unplanted systems. Influent and effluent samples were taken on a monthly basis over a period of 1 year and analyzed for fecal coliforms (FC), fecal streptococci (FS), BOD and TSS. The findings suggested no significant differences ( P<0.05) in the average yearly removal of fecal bacteria (>93%) between systems, with the vegetated systems performing best during warmer months and the unplanted systems performing best during the winter. The vegetated systems showed significantly greater ( P<0.05) removal efficiencies for BOD (>75%) and TSS (>88%) than the unplanted systems (63 and 46%, respectively) throughout the year. Overall, the polyculture systems seemed to provide the best and most consistent treatment for all wastewater parameters, while being least susceptible to seasonal variations. The performance of the cattail systems may improve by harvesting the plants at the end of the growing season, thus reducing additional BOD and TSS inputs from decaying biomass litter. The fescue systems were generally inferior to the polyculture and cattail systems because of their shallow rooting zone and limited biofilm surface area, while the unplanted systems were completely inefficient for BOD and TSS removal and should not be recommended.