Trophic models of constructed wetlands for wastewater treatment in northern Taiwan

• Main Authors: Ho-Mei Chang, 張禾玫
• Other Authors: Hsing-Juh Lin
• Format: Others
• Language: en_US
• Published: 2012
• Online Access: http://ndltd.ncl.edu.tw/handle/41353398655008447981

碩士 === 國立中興大學 === 生命科學系所 === 100 === Constructed wetlands are often built near the sources of pollution for water quality improvement of wastewater flowing into streams and rivers. Constructed wetlands are designed to mimic natural wetlands, therefore asides from wastewater treatment they can also provide a variety of ancillary benefits such as wildlife habitats. However, there has only been a little understanding of structure and function of constructed wetland ecosystems. The purpose of this study was to characterize the structure and function of constructed wetland ecosystems by constructing trophic models of twelve treatment cells in the Hsin-Hai Ⅱ and the Daniapi Constructed Wetlands using the Ecopath with Ecosim software system. Twelve treatment cell models were comprised of 8~22 compartments, including phytoplankton, periphyton, macrophytes, macroinvertebrates, fish, birds, etc. According to the trophic structure analysis, all treatment cells’ food webs consisted mostly of primary consumers whose effective trophic level were 2~3 while there were few consumers with higher trophic levels. The Lindeman Spine of treatment cells showed that primary producer and detritus dominated energy flows in most treatment cells and the mean transfer efficiencies of all treatment cells were low caused by the low biomasses of consumers with high trophic levels. According to ecosystem indices, the total system throughput of most treatment cells were mainly composed of exports and flows to detritus indicating that there were much net primary production which were not used. The net primary production: respiration ratio (P:R ratio) were higher than 1 indicated that all treatment cells were autotrophic, which implied that more organic matter was produced than consumed in both constructed wetlands. The low Finn’s cycling index indicated that the recycling of organic matter were unimportant in most treatment cells, likely resulted from the inflow of nutrient-rich wastewater in both constructed wetlands. These results suggested that the organic detritus should be removed regularly to maintain the operation of constructed wetlands. The number of compartments, the total system throughput, the total net primary production, the biomass of consumers and algae and the development capacity showed an increasing trend along the sequential treatment cells in both constructed wetlands. The high net primary production of algae in the late-phase treatment cells resulted in the detritivory: herbivory ratio (D:H ratio) which were lower than 1 suggested that herbivory flow was more important than detritivory flow. Key trophic group varied among treatment cells in both constructed wetlands, which changed from Chironomidae in the early-phase treatment cells to piscivorous birds in the final-phase treatment cells. Although many indices based on energy flows and biomass increased along the sequential treatment cells in both constructed wetlands, the comparative analysis with natural wetlands indicated that the final-phase treatment cell in both constructed wetlands presented lower biomasses of consumers, lower system omnivory index and lower mean transfer efficiencies than natural wetlands. Furthermore, the higher P:R ratio, higher primary production: biomass ratio (P:B ratio) and lower biomass: total system throughput ratio (B:T ratio) of both indicated that both constructed wetland ecosystems were still at a developing phase. Overall, the trophic models suggested that the Hsin-Hai Ⅱ and the Daniapi Constructed Wetlands were autotrophic ecosystems whose mean transfer efficiencies were low caused by the low biomass of consumers with high trophic levels. Many ecosystem indices of the constructed wetlands increased along the sequential treatment cells. However, comparative analysis of constructed wetlands with natural wetlands suggested that the low maturity of the final-phase treatment cells was due to the inflow of wastewater in both constructed wetlands.