Progress in environmental dredging of river sediment

With the increasing number of environmental dredging projects, the environmental protection dredging of river sediment has become a worldwide concern. The removal of nutrients, heavy metals and persistent organic compounds from sediment by dredging are obvious. Environmental dredging of sediment is time-effective for controlling river pollution. Dredging methods, dredging depth and dredging equipment are the main factors for dredging project design. This paper reviews the research progress of environmental dredging of sediment in order to deepen the understanding of environmental dredging of sediment.

Channel sediment is not only the central link of the nutrient cycle but also the main accumulation reservoir of nutrients, persistent organic pollutants and heavy metals. Even if exogenous sources are effectively controlled, the release of sediments by biological or physical factors may still lead to the poor state of water eutrophication or deterioration of water quality for a long period of time. Sediment dredging completely removes pollutants from the water system, which can greatly reduce the contribution rate of sediment to the overlying water, so as to solve the secondary pollution caused by endogenous release under biological or physical effects.

1.The concept of environmental dredging

Environmental dredging is the dredging projects aim at environmental protection. Environmental dredging refers to the technical method of removing surface sediments rich in nutrients, toxic chemicals and toxin bacteria by manual or mechanical means to reduce the endogenous load and pollution risk of sediment. Environmental protection dredging refers to a kind of dredging technique and technology that has the least impact on the environment and surrounding water during dredging, transportation and disposal of soil with no region. According to Kleeberg A and Kohl J G, environmental dredging is a technique to remove phosphorous rich surface sediments and control the release of phosphorus in order to obtain phosphorus constraints for algae growth. In conclusion, environmental dredging is an environmentally friendly dredging technique that USES mechanical methods to remove polluted sediment from rivers and lakes to provide phosphorus restriction conditions for algae growth.

2.Reference factors for the design of environmental dredging project

Environmental dredging is a new industry developed in the recent 30 years, and it is the edge engineering technology of water conservancy engineering, environmental engineering and dredging engineering. It is necessary to consider the technical feasibility and economic rationality of the dredging construction and meet the requirements of environmental protection. The main factors considered in the design of the project scheme are as follows.

2.1 Dredging depth

Usually, the polluted sediment thickness in the water is uneven and varies greatly. In dredging, not only the polluted bottom sediment should be removed, but also the over-excavation of non-polluted bottom sediment should be reduced as far as possible, so as to avoid damage, and at the same time, the treatment quantity and treatment cost of polluted bottom sediment should be reduced. Therefore, the dredging precision of dredging equipment is much higher than that of general channel dredging or water conservancy dredging. The bottom mud structure can be divided into three layers from the top to the bottom: the upper mud layer, the middle mud layer, the bottom soil layer. The upper floating layer is the main factor that most easily pollutes the overlying water body in the bottom mud and is the main object of environmental protection dredging. The middle silt layer should also be removed during dredging. The old soil layer at the bottom is a natural structure layer, which should be retained when dredging. Based on the analysis of the layered structure of sediment and its pollution characteristics, the dredging thickness of sediment is usually determined by the “inflection point method”, that is, the “inflection point” (the point where the pollutant concentration suddenly decreases) is found from the vertical distribution characteristics of pollutants along the thickness direction of sediment, and the thickness above the “inflection point” is taken as the dredging thickness. The determination of dredging depth should consider comprehensively the effects of removing endogenous pollution, controlling on higher aquatic plants and benthic animals, as well as the benefits to ecological restoration. In areas with low water tables, dredging should be prevented from damaging the impermeable layer of water and causing leakage. In addition, the impact of dredging on river safety should be considered and evaluated, such as the damage and collapse of shoreline and coastal buildings caused by dredging.

 2.2 Dredging measures

Dredging measures of sediment can be divided into two basic types: dry bed dredging and water dredging. Dry bed dredging is the draining of water prior to dredging. It is usually suitable for small ponds, reservoirs, or small river channels, and the sediment can be removed by manual cleaning, mechanical (bulldozer) cleaning, or hydraulic (water gun with a buoy and dredge pump) cleaning. Dry bed dredging is convenient for human and mechanical operation, and it is quite thorough, but it has a great impact on the ecology of the water body, and sometimes affects the safety of shoreline and surrounding buildings. In contrast, the operation with water has a wide range of applications, including river dredging and lake dredging.

 2.3 Dredging equipment

River dredging project has the characteristics of technology-intensive, capital intensive and management intensive. River dredging project relies on the dredging equipment (dredging vessel) that belongs to the non-standard equipment with high technical content. There are basically two types of dredging equipment for water operation: special dredging equipment and conventional dredger transformation. In the conventional dredger transformation, the main purpose is to develop the environmental protection reamer for dredging. The selection of dredging equipment needs to consider the availability of equipment, project time requirements, sediment transport distance, discharge head and physical and chemical characteristics of sediment, etc. The appropriate equipment should also be selected according to specific local conditions.

3. Major problems in the environmental dredging process presently

 3.1 Sediment disturbance

During dredging, mechanical agitation causes sediment to hang up. Sediments are the main storage sites for hydrophobic pops. The sediment overhang causes the release of ammonia and phosphates, which “fertilizes” the water and makes dredging less effective. For example, the concentration of herbicides in the water after dredging at Brighton harbor is higher than before. In addition, toxic substances such as heavy metals that are re-suspended due to disturbance can be potentially harmful to local aquatic life.

 3.2 Secondary contamination of sediment

Under the condition of leaching, the heavy metals, nitrogen, phosphorus and organic pollutants contained in the sediment may diffuse and transfer, causing secondary pollution to the environment. Especially the rosy metal in the bottom sediment has environmental persistence. The improper disposal will cause a serious consequence which is difficult to make up. Some developed countries even require an ecological risk assessment of contaminated sediment before dredging.

 3.3 Destruction of the physical structure at the bottom of the channel

The urban river, especially the river with a long history, has formed a good natural impermeable layer at the bottom because of historical accumulation. If it is not properly disposed of in the dredging process of bottom sediment, it will lead to the destruction of the natural impermeable layer, and thus destroy the physical structure at the bottom of the river. For example, the Sanjiadian reservoir in Beijing broke the seepage prevention layer at the bottom of the reservoir during the dredging process, resulting in serious leakage accidents.

 3.4 Destruction of benthic ecosystems

Due to the interference of various factors, environmental protection dredging will damage the benthic ecosystem. After the completion of the dredging project and before the establishment of the new benthic cattle system, the river ecosystem is fragile and prone to cause algal bloom.

4.Major directions for environmental dredging research

 4.1 Research on dredging technology of hydraulic mud suction and centrifugal dehydration

According to the characteristics of sediment pollution and the conditions of river dredging construction in the central built-up area, the combined dredging technology of hydraulic suction and site dewatering of dredged mud is an applicable technology for the cleaning of polluted sediment in urban river channels. The advantages are as follows: (1) Hydraulic dredging systems can effectively dredge the pollution accumulates sediment with a high content of organic matter and close specific gravity to water. (2) On-site centrifugal dewatering can greatly reduce the amount of dredged material to be transported and absorbed, which is suitable for the construction environment of the built-up area of the center. (3) The inherent shearing and particle hydraulic classification function of centrifugal dewatering can make the logistics after centrifugal dewatering split according to the degree of pollution, which can significantly reduce the amount of material to be disposed of. The technology is being rolled out in engineering practice.

 4.2 Precise improvement of cutter suction dredger

The research on precise dredging and high-concentration dredging technology of contaminated sediment is carried out abroad, the core of which is the development and introduction of advanced dredging equipment. At present, the more advanced environmental dredging method is to use the cutter suction dredger, which is directed by the pipe under the action of the mud pump to lift the surface sediment and long-distance transport to the land accumulation area (yard). High-precision technologies such as RTK-DGPS and underwater camera are gradually being widely used in the field of dredging in developed countries.

 4.3 Study on post-treatment of dredging sediment

In order to avoid secondary pollution, the research of sediment post-treatment focuses on the following aspects.

4.3.1 Disposal and utilization of contaminated sediment

The treatment of polluted bottom sludge has abandoned the previous single method of disposal of soil and is gradually developing towards the direction of resource utilization, reduction and harmless utilization. The main methods are storage and closure, containerization of contaminated sludge, bioremediation, and resource utilization. The combination of sludge disposal and comprehensive utilization should be encouraged when possible, and the waste bottom mud should be used to exchange soil with mud, make bricks, improve soil and fill lowland, etc. and to turn waste into treasure, which is not only beneficial to the protection of the ecological environment but also can produce certain economic and social benefits.

4.3.2 Storage yard surplus water treatment

The excess water discharged from the storage yard after the polluted sediment is deposited naturally is called surplus water. The remaining water contains a large number of organic compounds, nitrogen, phosphorus, heavy metals and other pollutants, which attach to fine particles and are suspended in the remaining water. The methods of residual water treatment include filtration, centrifugation, flocculation and sedimentation. Flocculation and sedimentation is the most commonly used method for residual water treatment. The advantages of flocculation precipitation method are that it does not need power, has great flexibility of operation, has strong adaptability to the change of water quality and quantity, is convenient for the supply of agents, and the treatment facilities cover a small area, and the facilities are simple, low cost and easy to build.

4.3.3 The desiccation of dredging mud

Polluted sediment belongs to silty soil with high organic content and the natural drying and consolidation process of silty soil is slow. Manual enhanced dehydration measures can be used when the drying process needs to be accelerated. The common methods of accelerating solidification in blowing-filling engineering include vacuum preloading method, stack-loading preloading method, dosing sedimentation method, mechanical dewatering method, active dewatering method in the storage yard, etc. In recent years, geotechnical bag filtration and dehydration technology have been gradually used in the sludge dehydration treatment of rivers, lakes and seas.

 4.4 The combination of dredging and ecological restoration

At present, the main research direction of river and lake eutrophication control is to combine dredging with river ecological reconstruction from the perspective of river ecological protection, and to give full consideration to the growth conditions and distribution of large aquatic plants when determining the dredging area and dredging depth.