The lake is the main reservoir of sediment pollutants. Water pollution is divided into endogenous pollution and exogenous pollution. Studies show that even if the polluted water is effectively controlled from exogenous sources, the nutrient salts in the sediment will be released gradually, which will still make the rivers and lakes eutrophication. Therefore, when the exogenous pollution of rivers and lakes is basically controlled, it is necessary to take measures of environmental protection dredging to remove the serious pollution and release heavy sediment. Environmental dredging is one of the most effective ways to remove endogenous pollution quickly. However, due to the influence of dredging precision, dredging diffusion, dredging sludge drying, the resources of sediment dehydration utilization, resource utilization of sediment dehydration, residual water treatment, biological activation of the new interface layer after dredging, back silting and other problems. It is still difficult to achieve the expected effect in practical engineering.

With the development of human society, the speed of urbanization is getting faster and faster, the industrialization degree is higher and the increasing influence of human activities on the natural environment. As a result of a weak human environmental protection meaning before, a lot of industrial wastewater and living sewage discharged into rivers and lakes, many rivers and lakes water bodies of soil pollution, cause the eutrophication of water bodies or adverse effects such as water quality deterioration and has the serious influence human survival environment.

1. Environmental impact factors of dredging

Environmental dredging aims to remove pollutants from the water. It is a science which combines the technology of removing the internal pollutant from polluted water with the technology of ecological restoration. Environmental dredging technology is a comprehensive science of environmental engineering, port and navigation engineering and water conservancy engineering. In the process of environmental protection dredging, there’re some technical defects of dredging accuracy and the diffusion in the dredging process, drying of dredged sediment, sediment over the dehydration of resource utilization, water treatment.

 1.1 Dredging accuracy

The aim of environmental dredging is to remove contaminated sediment from the water, and the depth of excavation depends on the distribution of contaminated sediment. On the other hand, it is necessary to avoid the over digging of non-polluted sediment as far as possible and control the treatment quantity and cost of polluted sediment within a certain range. Therefore, the dredging precision of environmental dredging equipment is much higher than that of ordinary waterway or water conservancy dredging equipment. The bottom mud is composed of floating mud layer, silt layer and old soil layer from top to bottom. Environmental dredging requires dredging the upper layer of mud which is prone to contaminate the overlying water. The intermediate silt layer is also removed during dredging. The old soil layer at the bottom is a natural structural layer, which should be retained during dredging. It is not difficult to see that environmental dredging has a high technical standard for dredging accuracy, and the dredging depth error is controlled within 5cm~10cm. Normally, the accuracy that the dredging control system can demonstrate and achieve refers to the positioning accuracy of the dredging head, that is, the locating point of the dredging head. It cannot be understood that the depth reached by dredging is the corresponding depth of the dredging head. According to the basic information such as the angle of the dredging head support, the vertical position of the ship (determined by RTK-GPS slipway) and the structural parameters of the ship, the depth of the dredging head was calculated by comprehensive processing.

It is proved by practice that the error of the ship’s vertical position and the accuracy of RTK-GPS itself are the important factors affecting the positioning accuracy of digging head. Therefore, to improve the positioning accuracy of the digging head and optimize the accuracy of RTK-GPS are system technologies, the key lies in avoiding the vertical error of ships and the accuracy of RTK-GPS itself.

At present, the most commonly used methods for vertical positioning of dredging vessels are as follows: (1) The conversion and positioning of the water surface have certain limitations. Generally, it can meet the requirements of environmental protection dredging under suitable working conditions, but its accuracy will be affected to some extent when the water environment is narrow, closed, or stormy. (2) RTK-GPS positioning, using RTK-GPS positioning can reach the accuracy of water surface conversion positioning. Its advantages can overcome the limitations of water surface conversion positioning.

In addition, dredging accuracy is affected by other influencing factors, such as reamer lateral displacement, reamer speed, mud pump flow, dredging sediment physical properties, etc. There are many factors affecting the precision of environmental protection dredging. To enhance the application scope of the system, comprehensive research should be carried out from various factors.

 1.2 Sludge diffuses during dredging

Due to the disturbance of the excavation head and the flow of water, the particles of the dyed sediment will become smaller after being dredged. The sludge with smaller particles is easy to diffuse in the water, and the sludge particles are not inhaled by the mud suction pipe to diffuse around the reamer head, which is easy to cause secondary pollution and pollute the water quality of the surrounding waters, which is called diffusion.

1.2.1 The common problems in the dredging process of traditional dredging technology

First is the sediment disturbance during dredging. In the course of dredging, sediment will appear suspended diffusion phenomenon under the disturbance of external force. Many non-hydrophilic and persistent organic pollutants accumulated in the contaminated sediment. The suspension of sediments causes the release of ammonia and phosphates, resulting in eutrophication and secondary pollution of the water body, which cannot achieve the ideal environmental protection dredging effect. There are cases in the UK showing that the concentration of herbicide in the water after dredging is higher than that before dredging due to the disturbance of the sediment during dredging. The reason is that the sediment disturbance in the dredging process leads to the sludge suspension diffusion, and most of these suspended diffusion sludge contains some heavy metals and toxic and harmful substances with the sludge particles into the water, leaving pollution hazard to the local aquatic environment.

Second is the secondary contamination of dredged sediment. The dredged sludge is stored in the sludge storage yard before drying. In the rainy season, when the polluted sludge is washed by the rain, the toxic and harmful sludge in the sludge may be transferred and spread with the rain, releasing the toxic and harmful sludge into the surrounding environment, which is easy to cause secondary pollution. In particular, heavy metals contained in contaminated sediment have a lasting negative impact on the environment. Therefore, dredged sediment should be properly disposed of. If not handled properly, it will seriously affect the surrounding ecological environment. While Germany, Japan, the United States, and other developed countries continue to improve the technology of environmental protection dredging, they even require the implementation of ecological risk assessment before dredging.

The third is the physical structure at the bottom of the channel is destroyed during dredging. In general, the ancient river course accumulates for a long time and forms a fixed natural impervious layer. If the precision of dredging is not well controlled, the natural impervious layer may be destroyed by humans, thus damaging the original impervious structure at the bottom of the river course.

1.2.2 Key technologies to control the diffusion

(1) Research on the law of natural sedimentation of sludge particles during dredging.

(2) Research on the relationship between centrifugal force in the turning process of reamer cutting and the negative pressure in the suction process of the mud pump.

(3) Research on the law of sediment diffusion during environmental dredging.

(4) Research on environmental protection dredging construction technology: the capacity of mud pump, the capacity of the dredging head, reduce the residual amount of contaminated sediment dredging and limit the diffusion range of sediment particles.

 1.3 The dewatering and drying of dredged sediment

The storage yard treatment is one of the effective measures for the disposal of a large number of environmental dredging sludge. The storage yard sediment is characterized by small solid content, fine particles, negative electricity, and a large amount of nitrogen and phosphorus nutrients. The natural drying period of dredged sediment is long, which is because the dredged mud is mostly silty clay with poor permeability. There is no transpiration of water in the middle and bottom of the storage yard without external force. Because the infiltration rate of contaminated sediment is quite slow, the mud layer of the whole storage yard is in the state of saturation for a long time, the bearing capacity is always low, and its solid content cannot meet the transportation requirements, leading to the dredging sediment cannot be removed in time. In the event of precipitation, the dredging sludge drying rate is further reduced, and secondary pollution will be caused by the erosion and immersion of rainwater. A large amount of polluted sediment is stacked in the storage yard for a long time, which occupies land resources, and is not conducive to the subsequent treatment and resource utilization of dredging sediment, affecting or even restricting the extensive development of environmental protection dredging of lake reservoir sediment. Therefore, the improvement of the rapid dewatering and drying technology of dredging sediment is beneficial to the large-scale implementation of the environmental protection dredging project.

At present, the main methods of sediment dewatering in engineering practice include physical dewatering and chemical dewatering. The physical method is mainly used to remove capillary and binding water from the sludge. The chemical dewatering method changes the particle size of sediment by adding flocculants. Adding flocculants can increase the floc size, improve the sedimentation rate of sludge floc, and facilitate the rapid dewatering and drying of sludge. Increasing the effect of sediment dewatering and reducing the cost of sediment dewatering will promote the wide application of environmental protection dredging technology. It has been proved that chemical dehydration alone and physical dehydration alone cannot achieve the desired result. The chemical method of adding flocculants combined with the physical drainage method has a better treatment effect. The addition of flocculants increases the particle size of bottom sludge and improves the drainage channel of bottom sludge in the storage yard, which is conducive to physical dehydration. It increases the sludge drying rate and reduces the occupation time of the sludge storage yard, shortens the engineering cycle and indirectly reduces the treatment cost. The combination of chemical and physical dehydration gives full play to the advantages of the low cost of flocculants and the good effect of physical drying. In urban lake dredging projects, the limitations of sediment storage area, time limit, and economic index examination are all factors to be considered in environmental dredging projects. Chemical flocculation dehydration combined with physical dehydration is a very cost-effective dredging measure, which is also the development trend of large-scale environmental protection dredging technology applications in the future.