Pollutants Transformation and Removal within Constructed Wetlands Hybrid Systems

In Taiwan, constructed wetlands have received tremendous interests for water quality enhancement due to insufficient sewage treatment and groundwater deterioration. The main objectives of this study were to investigate pollutants removal mechanisms and efficiency within the field-scale natural purification hybrid systems. The systems included an oxidation pond, two serial surface flow wetlands with a cascade in between, and a subsurface flow wetland. These water purification systems received secondary treated dormitory sewage as their inflow. The macrophytes within the study system were submergent plants including cattails and reeds. The average SS, BOD and COD percent removal efficiency was approximately 90.4, 80.4 and 59.9%, respectively. The ratio of BOD to COD decreased from 0.64 in the initial aerobic compartment to 0.2 in anoxic parts of the systems, indicating most biological degradable materials were decomposed in the aerobic oxidation pond and surface flow wetlands. Effective phosphorus removal was not achieved while percentage removal was around 21.5%. Total Kjeldahl nitrogen (TKN) and ammonium decreased from 7.42 to 2.15 and 5.12 to 1.32 mg/L, respectively, while nitrate nitrogen increased from 2.21 to 5.56 mg/L within the aerobic oxidation pond and surface flow wetlands. This result demonstrated nitrification occurring within aerobic units. The nitrate nitrogen continued to decrease from 5.56 to 1.21 mg/L within the anoxic subsurface wetlands through denitrification transformation. Total nitrogen removal was from 10.95 to 3.80 mg/L, with the percentage removal of total nitrogen around 65.29%. The primary nitrogen removal and transformation mechanisms within the studied wetland systems were nitrification within aerobic followed by denitrificaiton within anaerobic systems. The submergent macrophytes enhance aeration through oxygen transferring that attributing the higher organic matter removal and nitrification rate. The wetland systems are viable options of pollutants transformation and removal in tropical countries, while tertiary wastewater systems are too costly or unable to operate. Effluent of the studied hybrid constructed wetland systems can comply with surface water criteria and water reuse standards. Key-Words: Hybrid constructed wetlands, suspended solids, phosphorus, nitrification, denitrification