Study on nitrogen release characteristics and tail water control during the whole process of dredging sediment disposal

The discharge of tail water in the process of dredged sediment disposal is a key step in ecological dredging projects. Tail water treatment in dredged sediment disposal constitutes an important component of ecological dredging. At present, research on the treatment technology of tail water mainly focuses on end treatment, that is, tail water is generally regarded as a single treatment target, and the tail water purification research on the targeted removal of key pollution factors is carried out. However, the understanding of pollutant release characteristics and water quality changes throughout the whole process of sediment disposal "Lake→Sludge Inlet→Sludge Storage Tank→Drug Dosing Tank→Sedimentation Tank" is relatively insufficient. Based on the first-stage ecological dredging project of Gehu Lake, this study takes nitrogen pollutants as the research object and investigates nitrogen release characteristics and water quality responses during the entire sediment disposal process through multi-frequency monitoring, sampling, and laboratory experiments. The results show that when dredged sediment is first put into the storage tank, porosity increases and reduction of the overlying water is strong, making it a hotspot for nitrogen release. From the front end to the back end of the reservoir, with the gradual increase of dissolved oxygen level and the decrease of particle size, the nitrogen release capacity of the sediment gradually decreased. In the dosing tank–sedimentation tank stage, nitrogen release levels rebounded compared with those at the end of the sludge storage tank, which was attributed to the increase in the nitrogen concentration gradient at the sludge–water interface following dosing treatment. The concentrations of total nitrogen and nitrate nitrogen in the overlying water showed a cumulative effect along the process, while the concentration of ammonia nitrogen first increased and then decreased. This pattern was attributed to the improvement in water transparency at the back end, where ammonia nitrogen was gradually converted into nitrate nitrogen through nitrification. In this project, back-end treatment measures were tested in the tail water treatment of sediment disposal, including pollutant flocculation and sedimentation through dosing, as well as extending the length of flow path per unit area in the sedimentation tank. These measures effectively enabled COD_Mn and ammonia nitrogen to meet discharge standards and significantly reduced SS concentrations. A key factor for ammonia nitrogen reaching the standard was the improvement in overlying water transparency, which promoted conversion of ammonia nitrogen into nitrate nitrogen through nitrification. Although dosing and natural sedimentation in the sedimentation tank improved tail water quality, the concentrations of nitrate nitrogen and total nitrogen still exceeded the standard, and the concentration of solid suspended matter is intermittently exceeding the standard. The release potential of ammonia nitrogen and total nitrogen at the early front end of the storage tank was 22.28 times and 1.35 times that of Gehu Lake, respectively, and 3 times and 1.68 times that at the back end of the storage tank. By closing the front end of the reservoir for 164 hours, the total nitrogen load of the water body can be reduced by 30%–50%. If comprehensive treatment is supplemented with vegetation planting and other measures, it may offer a fundamental pathway to achieve the standard discharge of subsequent tail water. If standing time affects the progress of the project, manual interventions can be further taken at the front end, such as dosing treatment or vegetation planting and restoration, thereby effectively shortening the required retention time. The research results provide strong support for the discharge of tail water in the process of river and lake dredged sediment disposal, and provide a scientific decision-making basis for the river and lake ecological dredging projects of water conservancy departments.