Pollutant removal within hybrid constructed wetland systems in tropical regions.

Hybrid constructed wetlands have received tremendous interests for water quality enhancement due to insufficient sewage treatment and groundwater deterioration in Taiwan. The main objectives of this study were to investigate pollutant removal efficiencies and mechanisms within field-scale hybrid natural purification systems. The studied hybrid constructed wetland systems include an oxidation pond, two serial surface flow wetlands with a cascade in between, and a subsurface flow wetland receiving secondary treated dormitory sewage. The average SS, BOD and COD percent removal efficiency was 86.7, 86.5 and 57.8%, respectively. The ratio of BOD to COD decreased from 0.65 in the initial aerobic compartment to 0.21 in anoxic parts of the systems, indicating most biological degradable materials were decomposed in the aerobic oxidation pond and surface flow wetlands. Heavy metal removal percentages of copper and zinc were 72.9 and 68.3%, respectively. Sedimentation and plant uptake are the possible sinks for metals retention. Significant phosphorus removal was not achieved in this study. Total Kjeldahl nitrogen (TKN) and ammonium decreased from 4.08 to 1.43 and 3.74 to 1.21 mg/L, respectively, while nitrate nitrogen increased from 1.91 to 3.85 mg/L within the aerobic oxidation pond and surface flow wetlands. This result demonstrated nitrification occurring within aerobic compartments. The nitrate nitrogen continued to decrease from 3.85 to 1.43 mg/L within the anoxic subsurface wetlands mainly through denitrification transformation. Total nitrogen removal was from 7.61 to 3.61 mg/L, with the percentage removal of total nitrogen around 52.6%. The primary nitrogen removal and transformation mechanisms within the studied wetland systems were nitrification within aerobic followed by denitrificaiton within anaerobic systems. The emergent macrophytes enhance aeration through oxygen transferring that attributing the higher organic matter removal and nitrification rate. The hybrid 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 purified systems can comply with local surface water criteria rendering for groundwater recharge.