Temperature and infiltration characterization of a constructed wetland for wastewater treatment

Graduation date: 2010The City of Woodburn, Oregon's Wastewater Treatment Plant (WWTP) has been investigating several natural alternatives for improving effluent quality. Based on its current National Pollution Discharge Elimination System (NPDES) permit, the WWTP will require additional reduction of temperature and ammonia levels in the summer, especially in the critical month of September.\ud \ud The present research focuses on the performance of a 0.15 ha constructed pilot wetland during the summer and early fall of 2009. The wetland was intended to demonstrate the feasibility of using treatment wetlands to lower temperature and ammonia during an annual operational period (approximately June through October).\ud Point sensors and a fiber optic distributed temperature sensor (DTS) were used to assess temperature treatment during the study. Infiltration was monitored to characterize the hydrogeologic behavior of the site. A wetland water budget was used to determine system-wide infiltration, and heat pulses applied to a subsurface fiber optic cable were used to assess infiltrative variability.\ud \ud The results showed that temperature reduction in the pilot wetland was marginal over the study period. In the September critical month, significantly more treatment occurred in the WWTP storage lagoon than in the wetland (about 4°C cooling compared to 1.2°C in the wetland). Decreasing the hydraulic retention time from 2.5 to 0.5 days in mid-September did not change the average temperature treatment. DTS data demonstrated that all temperature reduction occurred in the first half of the wetland. Infiltration was greater than outflow for most of the study, and steadily decreased through time. The highest and lowest infiltration velocities were within a factor of 2, and this range also declined between November 2008 and June 2009. Potential explanations for decreasing infiltration include soil clogging and settling. The study showed the utility of DTS for assessing the spatial and temporal variability of infiltration processes