土木環境工学ジャーナル

A Study and Analysis on the Physical Shading Effect of Water Quality Control in Constructed Wetlands

Abstract

Yeh TY, Wu MH, Cheng CY and Hsu YH

This study mainly utilized model basins to conduct relevant water quality analyses and tests to understand the influence of the shading effect of wetland plants in the natural water purification system on algae control and eutrophication of water bodies. The model basin tests are based on the physical shading effect in order to evaluate and further understand the influence of algae control and water quality parameters. The physical shading tests were conducted at different rates (100, 70, 50, 30, and 0%) and the research results indicate that the initial dissolved oxygen concentration of each group is between 5 - 7 mg/L. On Day 5, the average dissolved oxygen concentration of the 100 (aerobic) , 100 (anaerobic), 70, 50, 30 and 0% groups is respectively 9.22 ± 1.24, 8.54 ± 1.23, 18.8 ± 3.74, 21.71 ± 0.41, 22 ± 0, and 22 ± 0 mg/L while that of the 70, 50, 30, and 0% groups significantly increases. The initial concentration of Chlorophyll a of the 100 (aerobic) , 100 (anaerobic), 70, 50, 30 and 0% groups is, respectively, 5.27 ± 0.93, 5.92 ± 0.00, 7.9 ± 5.59, 5.92 ± 0.00, 7.9 ± 5.59 and 13.82 ± 2.79 μg/L. On Day 5, the average concentration of Chlorophyll a is, respectively, 57.27 ± 11.17, 9.87 ± 2.79, 244.86 ± 50.34, 280.4 ± 32.92, 329.77 ± 15.55 and 250.78 ± 32.21 μg/L. In general, the physical shading effect can prevent photosynthesis by algae in water by absorbing light and converting light into chemical energy for self growth and proliferation that cause a rapid increase in the number of algae. The physical shading effect, however, cannot reduce nutrients; this leads to a high nutrient level in water bodies during the latter stages of the tests. After the introduction of the Carlson Tropic State Index (CTSI), the completely shaded water bodies at the initial stage of tests are mesotrophic water bodies that contain neutral nutrients. The partially shaded water bodies are eutrophic. In the latter stages of the tests, the former turned eutrophic while the latter turned mesotrophic. The main cause lies in the relatively higher weight CTSI placed on total phosphorous content.