Study On Field Experiment And Mechanism Of Eutrophic Landscape Water Restoration Using Plants

Eutrophication of landscape water is one of the ubiquitous water environmental problems in many cities in China. Major landscape waters in Xi'an were selected as the study cases, characteristics, causes and trend of pollution of these landscape waters were systemically analyzed. Characteristics and mechanisms of releasing of nitrogen and phosphorus from polluted sediment were experimentally studied, evolution of eutrophication of shallow urban landscape water was numerically modeled. Using Changan Lake as the representative landscape water in Xi'an, pilot experiment of ecological water restoration in situ was performed, removals and mechanisms of TN?TP?NH3-N and COD by aquatic plants were explored. Main conclusions of this study are as follows:1. Changan Lake, Xi'an moat and Xingqing Lake were selected as built representatives, Qujiang Pool and Taiye Pool in Daming Palace National Heritage Park were selected as newly-built representatives, the status and pollution causes of various landscape waters were experimentally studied.2. Phosphorus is the major regulating target for the eutrophication of landscape water, which is closely related to the growth of algae. Experimental results of pollutant release from sediment shows: main reasons for releasing of phosphorus were microbial activities and higher pH values under anaerobic condition. Under unsterilized condition, the maximum release rate of phosphorus was 2.14 mg/(d·m²) at controlled pH value of 6.5, the maximum release rate of phosphorus could be greatly increased to 8.6 mg/(d·m²) at pH range of 7.5-8.6. Under sterilized condition, the release intensity of phosphorus was very low, but when the pH value was adjusted to 9.1, the release of phosphorus was clearly observed under both aerobic and anaerobic conditions, and the release rate was 9.51 mg/(d·m²). In the polluted sediment, the measured result of alkaline phosphatase activity agreed with the characteristics of release of phosphorus.3. Under anaerobic condition, due to the decrease of surrounding oxidation-reduction potential (ORP) caused by microbial activities which took sulphate radical as their electronic acceptor, phosphorus and soluble ferrous were significantly released from the sediment. Analysis result of phosphorus forms in the sediment, the Fe-P and O-P activated by microbial activities were more easily released. Experimental results also showed that the release of soluble ferrous is positively correlated with the release of phosphorus with a correlation factor of 0.9224.4. Under unsterilized condition, the variation characteristics of TN and NH3-N were consistent in the water above the sediment, and their contents were positively correlated with the content of organic matter (R2=0.8966). Under sterilized condition, the content of TN is poorly correlated with the content of organic matter (R²=0.6929).5. Water quality modeling was performed with WASP water quality model for the water in Qujiang Pool, proper model parameters were determined, and the modeling results of NH₃-N?DO?TP?NO₃-N agreed well with the measured ones.6. Based on the measuring results of stress tolerance, adaptability and root oxidizing ability, the optimal water plants for ecological floating bed restoration in situ were Canna Incida, fleur-de-lis and Cyperus alternifolius. The best emergent aquatic plant was rhizoma calami.7. Judging from the experimental results of pollutant removal and the state of the plant growth, the removal rate of phosphorus in landscape water was positively correlated with the growth of plant, which indicates that the absorption by plant is the dominant contribution to the removal of phosphorus. By comparing the purification efficiencies using combinations of plant, the highest removal rate up to 95.7% was obtained using both fleur-de-lis and Cyperus alternifolius, removal rates of 65%?82.8% could be achieved using both Canna Incida and Cyperus alternifolius. All the combinations of plant led to stable removal of TN, the highest removal rate of 92.3% could be achieved using combination of reed, silverreed and rhizoma calami, removal rates of 87.1%?89.2%, 86.2%?89.9% and 82.9%?89.2% could be maintained using combinations of Canna Incida and cattail, Canna Incida and Cyperus alternifolius, fleur-de-lis and Cyperus alternifolius respectively.8. The contents of water, TN and TP in seven bodies of water plant were measured. The results showed the TN content was the highest at 1.72% in the body of Canna Incida, the TP content was the highest at 0.75% in the body of rhizoma calami. The TN content in plant's body was in the order of Canna Incida > rhizoma calami > fleur-de-lis > cattail > Cyperus alternifolius >reed> silverreed?Taking the removal of phosphorus as target, the efficiency of improving the quality of eutrophic waters using floating bed plant was in the order of Canna Incida > Cyperus alternifolius > fleur-de-lis > cattail, the increment of phosphorus per unit floating bed area was 26.6g/m² using Canna Incida.