Transformations in dissolved organic carbon through constructed wetlands

Abstract

Constructed wetlands have emerged as a viable option for addressing a wide range of water quality problems, especially in treating wastewater effluent. This paper presents longitudinal profiles in dissolved organic carbon (DOC) concentrations and structural characteristics across a full-scale wastewater treatment wetland receiving lagoon-treated wastewater (DOC=15–25 mg/L). DOC removal through the wetland varied seasonally, achieving a maximum net removal of 47% in February and minimum net removal of 9% in June. During summer months, when the wetland plants were actively growing, DOC decreased across the first half of the wetland and then increased through the second half of the wetland. Specific ultraviolet absorbance at 254 nm always increased across the wetland, with the largest increases (>130%) occurring during summer months. DOC lability decreased across the wetland. DOC reactivity to form trihalomethanes was also reduced on both an absolute and per carbon mass basis. Laboratory experiments employing a series of wetland microcosms with HRTs ranging from 1.6 to 7.4 days were employed to determine the amount of DOC leached from Typha wetland plant material. During fifty-six day steady-state experiments, roughly 5–8% of the total Typha biomass added was leached as DOC, 45–60% remained in the reactor as accumulated biomass, the remainder of the carbon (30–50%) exited as particulate organic carbon or was microbially respired. We hypothesized that DOC in the wastewater effluent biodegraded over the first-half of the wetland, and that DOC leaching from plant material occurred throughout the wetland. A DOC-wetland model was developed, and the results suggested that the percentage of plant-derived DOC increases with longer HRTs, and while the overall DOC concentration exiting a wetland may only be slightly lower than influent levels that a majority of the DOC, which contains a large percentage of refractory DOC, could be plant-derived. Wetlands with short HRTs would reduce the amount of DOC leached from plant material.