Nitrogen Transformation within Constructed Wetlands Purifying Secondary Treated Sewage

Most of domestic wastewater is either untreated or only primarily settled by septic tanks in Taiwan. Manure produced by concentrated swine operations is also one of the primary pollution sources of watercourses. Recently, constructed wetlands have received tremendous interests as one of the contemporary alternatives employing to enhance the water quality regarding nitrogen nutrients due to the deterioration of groundwater quality. Wetlands can be employed to enhance water quality, with saving in infrastructure construction costs as well as providing economic, environmental, and social benefits. The main objectives of this study were to investigate the nitrogen nutrients removal mechanisms and efficiency within the field scale natural purification systems. The systems included an oxidation pond, two serial surface flow wetlands with a cascade in between, and a subsurface flow system. These water purification systems received secondary treated sewage as their inflow. The macrophytes within the study system were submergent plants including cattails and reeds. The average BOD and COD percent removal efficiency was approximately 86 and 42%, respectively. The ratio of BOD to COD decreased from 0.63 in the initial aerobic compartment to 0.16 in anoxic parts of the systems, indicating most biological degradable materials were decomposed in the aerobic oxidation pond and surface flow wetlands. Total Kjeldahl nitrogen (TKN) and ammonium decreased from 7.58 to 2.45 and 5.46 to 1.86 mg/L, respectively, while nitrate nitrogen increased from 1.86 to 5.68 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 10.62 to 3.78 mg/L, with the percentage removal of total nitrogen around 64.3%. The primary nitrogen removal and transformation mechanisms within the studied wetland systems were nitrification within aerobic followed by denitrificaiton within anaerobic systems. pH of the systems ranged from 6.5 to 7.8 which indicate the ammonia volatilization is negligible. The submergent macrophytes enhance aeration through oxygen transferring that attributing the higher organic matter removal and nitrification rate. The wetland systems are prominent options of nitrogen nutrients removal in tropical countries, while tertiary wastewater systems are too costly or unable to operate. Results of this study will be useful for applying similar treatment train natural purification systems to purify secondary treated sewage in tropical regions. Key-Words: Constructed wetlands, nitrogen removal, nitrification, denitrification