Analysis And Prediction Of Eutrophication Of Typical Urban Malodorous-black River After Treating

Previous researches about eutrophication are mainly concentrated on the phenomenon in ocean, lake and reservoir, while eutrophication in tidal plain river networks, especially in urban malodorous rivers, has received less attention. Malodorous black is a common problem for urban aquatic environment. In recent years, along with the implementation of river treatment and restoration, some secondary pollution issues such as algal blooms and duckweed flooding break out in urban malodorous rivers. Based on these backgrounds, this dissertation took urban rivers in Wenzhou City, a typical urban malodorous-black river network, as the research object. The research combined the two separated concepts on the first time, namely malodorous-black and eutrophication, and systematically studied the eutrophication evolution in urban malodorous rivers from the aspects of investigation, assessment, mechanism and prediction, hopefully to provide beneficial references for the theory of malodorous-black transforming to secondary eutrophication as well as the forecast and management of eutrophication in urban rivers. The main work and results of this research are as follows:1. Through data collecting, field observing and monitoring, the spatial-temporal variation of water quality factors as well as malodorous-black pollution level of the Wenruitang River basin were investigated. The results showed that①almost all the sampling sites had the problem of malodorous-black, especially worst for river section in Lucheng District. No distinct water self-purification gradient was observed in the whole basin;②slow water velocity, abundant nutrients and suitable temperature and pH all contributed to the potential algae bloom in the whole basin. According to the findings of the investigation, six rivers (Qinfen River, Jiushanwai River, Xiaonanmen River, Chan River, Shanxia River and Puzhouheng River) in the key managing region by the local government were selected to be further studied due to their different malodorous-black backgrounds and significant eutrophication characteristics.2. This dissertation discussed the evolution pattern of the emerging secondary eutrophication after the implementation of river treatment engineering projects from perspectives of water quality, plankton and sediment.①Typical malodorous rivers were always in a level of abundant nutrients, and ecological water diversion would affect water quality factors significantly.②Chlorella and Bacillariophyceae were predominant phytoplankton species, Cyanobacteria and Euglenophyceae were at second place, other kinds of algae were relatively less. The distribution of phytoplankton in six rivers exhibited general features. The predominant pollution tolerance species were as follows:Ankistrodesmus falcatus, Chlorella, Pandorina, Scenedesmus quadricanda which were classified to Chlorophyta. Cryptomonas ovata, Cyclotella comta and Euglena viridis. Along with the treatment measures of malodorous-black rivers, algae bloom broke out frequently in the six studied rivers.③The temporal variation of alkaline phosphatase activity of sediments from different sampling sites was in accordance with the variation of total phosphorus in surface water, urease activity of sediments was in accordance with total nitrogen in surface water.3. A comprehensive assessment index system was established and utilized for evaluating eutrophi cation after the treatment of malodorous river.①Despite the river treatment, pollution load was still very high and all the rivers were in a state of eutrophication. Results indicated that algae bloom could also happen when river was malodorous-black. It means that river treatment should take measures to strengthen the reduction of malodorous-black and prevent algae bloom simultaneously.②The spatial distribution of organic contaminants presented an order like SX>CH>QF> JS>XN>PZ. Ecological water diversion can significantly change the concentrations of ammonium and organic matters in river water, but such change can only maintain a short time due to the exogenous pollution. To solve this problem, it requires optimizing water gates operation scheme, and strengthening sewage collecting at the same time in order to ensure the effectiveness and durability of ecological water diversion.③The result of assessment for small rivers by biodiversity index method was not in accordance with that by algal density and dominant species method. It might be attributed to the interference caused by the treatment engineering and abundant nitrogen and phosphorus existing in rivers. 4. Multiple linear regression and gray correlation analysis were adopted to determine the major pollutants of the rivers and the main factors causing algae bloom.①The order of the factors influencing Chl-a in Wenruitang River is lg(DO), pH, CODCr, NH3-N, TN, SD and WT. Since phosphorus was very abundant in Wenruitang River, nitrogen would be the key impacting factor of algae growth in typical malodorous-black rivers, phosphorus would be the controlling factor of eutrophication in these rivers.②Phosphorus was not the critical factor for influencing enzyme activity. Water temperature, total nitrogen and pH were most closely related to sediment phosphatase activity. Total nitrogen, Chl-a and DO were most closely related to sediment urease activity. On the other aspect, it indicates that controlling sediment microbial activity might affect the content of total nitrogen in the surface water to some extent.5. Prediction models for assessing Chl-a and phosphorus in Wenruitang River were preliminarily constructed.①It is feasible to forecast the eutrophication level of Wenruitang River through a BP neural network model. This model is simple and easy, with high prediction accuracy, and can provide suggestions for evaluating the eutrophication level of Wenruitang River.②ARMA time sequencing model fits to predict data series with mild fluctuation and certain pattern. However, this model can also be applied to Jiushanwai River which was under large scale treatment and restoration then.③Current technical methods and management strategies cannot meet the objective of controlling phosphorus, thus, our government should exercise macro-control and strengthen treatment efforts.