Constructed wetland for water quality improvement: a case study from Taiwan

摘要

In Taiwan, more than 20% of the major rivers are mildly to heavily polluted by domestic,nindustrial, and agricultural wastewaters due to the low percentage of sewers connected tonwastewater treatment plants. Thus, constructed or engineered wetlands have been adoptednas the major alternatives to clean up polluted rivers. Constructed wetlands are also appliednas the tertiary wastewater treatment systems for the wastewater polishment to meet waternreuse standards with lower operational costs. The studied Kaoping River Rail Bridge ConstructednWetland (KRRBCW) is the largest constructed wetland in Taiwan. It is a multi-function wetlandnand is used for polluted creek water purification and secondary wastewater polishment beforenit is discharged into the Kaoping River. Although constructed wetlands are feasible forncontaminated water treatment, wetland sediments are usually the sinks for organics and metals.nIn this study, water and sediment samples were collected from the major wetland basins innKRRBCW. The investigation results show that more than 97% of total coliforms (TC), 55% ofnbiochemical oxygen demand (BOD), and 30% of nutrients [e.g. total nitrogen (TN), totalnphosphorus (TP)] were removed via the constructed wetland system. However, results fromnthe sediment analyses show that wetland sediments contained high concentrations of metalsn(e.g. Cu, Fe, Zn, Cr, and Mn), organic contents (sediment oxygen demand ¼ 1.7 to 7.6 g O2/m2nd),nand nutrients (up to 18.7 g/kg of TN and 1.22 g/kg of TN). Thus, sediments should be excavatednperiodically to prevent the release the pollutants into the wetland system and causing thendeterioration of wetland water quality. Results of polymerase chain reaction (PCR), denaturingngradient gel electrophoresis (DGGE), and nucleotide sequence analysis reveal that a variation innmicrobial diversity in the wetland systems was observed. Results from the DGGE analysis indicatenthat all sediment samples contained significant amounts of microbial ribospecies, which mightncontribute to the carbon degradation and nitrogen removal. Gradual disappearance of E. coli wasnalso observed along the flow courses through natural attenuation mechanisms.n|