Ultraviolet disinfection of effluent from subsurface flow constructed wetlands

Subsurface flow constructed wetlands are becoming increasingly common for on-site treatment of domestic wastewater. Before spray application, wetland effluent must be disinfected. Traditionally, tablet chlorination has been used, but an alternative is needed to increase dependability. Consequently, we investigated the use of ultraviolet light disinfection of effluent from constructed wetlands. Two low pressure (254 nm) and two medium pressure ultraviolet bulbs (190 to 400 nm) were used for disinfection. Upon installation, all units disinfected effluent successfully. After 2 weeks in operation, three of the four units did not reduce fecal coliform populations below 200 in 100 ml of effluent due to decreased light intensity from films that developed on the bulbs. One unit consistently disinfected water to populations lower than 200 in 100 ml for 1 year without maintenance. Ultraviolet disinfection, as utilized, was not consistently suitable for disinfection of effluent from subsurface flow constructed wetlands because of coatings that developed on the bulbs and blocked the light.