Environmental dredging is an important measure to treat water because it can completely remove the polluted bottom sediment and eliminate the potential factors affecting water quality. However, disturbance and improper disposal of contaminated sediment in environmental protection dredging projects will lead to secondary environmental pollution. Various technologies to reduce sediment disturbance and resource utilization of sediment have become research hotspots. Reducing pollution sediment disturbance is mainly achieved by improving dredging equipment. At present, the main environmental protection dredging equipment used in China is the cutter suction dredge and environmental protection reamer. The resource utilization of sediment can not only improve the secondary pollution to the environment but also turn waste into treasure and increase resources. At present, the main ways of resource utilization include land utilization, building materials, filling materials, sewage treatment materials and hydrogen production technology.

Most of the rivers flowing through cities in China are seriously polluted, especially the bottom sediment. When the external pollution sources are controlled, once the water environment of rivers and lakes is changed, the nitrogen and phosphorus nutrient elements, heavy metals and refractory organic compounds deposited in the sediment will be released into the water again, affecting the water quality of the overlying water, and forming secondary pollution. In addition, sediment is the main living place and food source of benthic organisms and the pollutants can directly or indirectly produce toxic and harmful effects on benthic organisms or overlying aquatic organisms. Moreover, it further influences terrestrial organisms and human health through biological enrichment, food chain amplification and other processes. Sediment dredging can permanently remove contaminated sediment from the water body and prevent the secondary pollution of contaminated sediment to the water body, so it is commonly used to treat the water body.

Dredging is divided into engineering dredging and environmental dredging. The main purpose of engineering dredging is navigation, capacity enhancement and fishery application. Environmental dredging aims to improve water quality. At present, environmental pollution is becoming more and more serious, and people’s environmental awareness is becoming stronger, so the prospect of environmental dredging is broad.

1. Environmental dredging impacts

Dredging can effectively reduce the concentration of pollutants such as nutrients, heavy metals and persistent organic compounds in sediments. However, during the dredging process, pollutants will be released into the water, and the interface process after dredging may have a great impact on the dredging effect. Whether dredging techniques can fundamentally improve the water environment remains a matter of great debate. The environmental impacts of dredging are mainly as follows:

(1) Sediment is the main storage site of hydrophobic pollutants, a large part of which adsorbs on suspended particles and settles into the sediment. Dredging projects cause sediments to be re-suspended so that pollutants are released into the water as suspended particles, increasing the level of pollution in the water.

(2) The sediment can absorb pollutants in the water and release pollutants into the water. When the environment changes little, the adsorption and release of pollutants in the water reach a dynamic balance. If the dredging depth is lower than the polluted sediment depth and the polluted sediment is not completely removed, the new sludge layer will be exposed to the sediment surface, breaking the dynamic balance between sludge and pollutants and aggravating water pollution. Excessive digging will damage the original soil and increase the project cost.

(3) In the process of sediment transport, leakage will cause secondary pollution to the water body and may pollute other environments.

(4) Contaminated sediment usually contains a lot of pollutants. If the dredged sediment is simply stacked without any measures, on the one hand, it will occupy a large amount of land, and on the other hand, it will produce secondary pollution due to the erosion of rain.

(5) Sediment dredging leads to changes in the physical and chemical properties of sediments (such as grain size composition and organic matter content), which can lead to the replacement of benthic populations and thus change the composition of benthic communities. In the aquatic ecosystem, the microorganism is not only an important part of the benthic community but also plays a key control role in material degradation, transformation and energy flow in the aquatic ecosystem. The sediment dredging changes the physical and chemical properties of the surface sediments, destroys the stable structure of the surface microbial community before dredging, and changes the community composition or community structure of the microbial community in the sediment.

(6) Oil spills from dredgers during dredging and the discharge of household waste can also pollute water bodies.

(7) The construction process of dredging will produce some noise pollution to the surrounding environment.

2. Correlation processing characteristics and optimization utilization techniques

In order to minimize the negative impact of dredging on the environment, the following points should be controlled during contaminated sediment dredging: (1) Reduce the agitation of contaminated sediment and take anti-proliferation and leakage measures to ensure a high concentration of inhalation, so as to avoid the pollution of suspended pollutants to the surrounding water. (2) Monitoring work should be done well to improve dredging accuracy, so as to completely remove the contaminated sediment, ensure the environmental protection dredging effect, and reduce the over dredging quantity as far as possible, so as to reduce the project cost. (3) Reduce leakage during transportation and avoid secondary pollution to water body and another environment. (4) Dredging sludge should be treated safely to avoid secondary pollution near the storage yard. And it can be appropriately processed into available resources. (5) The dredging period should avoid the larval regeneration and breeding period of benthic organisms (usually the warmer months), and winter is the season with the least environmental risk for dredging of sediment, so the dredging project should be carried out in winter. (6) Before the construction, the dredger shall be comprehensively inspected to prevent oil leakage, and the household garbage generated by the dredger shall not be directly discharged into the water body to prevent the pollution of the water body. (7) Do a good job in anti-noise work, avoid night construction, appropriate installation of sound-absorbing, and soundproof material. The characteristics of environmental protection dredging are introduced in detail from the aspects of resource utilization of dredging equipment and sediment.

 2.1 Dredging equipment

The choice of dredging equipment directly determines the form of dredging: different dredging equipment has different disturbance degrees, different diffusion and leakage performance and different suction concentration. Therefore, the pollution caused by the construction of the surrounding water is different. Currently, the research and development of international environmental dredging equipment fall into two categories: The first is to use the original dredger for transformation to meet the requirements of environmental protection construction. The second is a special environmental dredging equipment. Using the original dredger to carry on the transformation is the common method at present, which is economical and practical. The reform of dredger focuses on dredging equipment and positioning control, that is, replacing the original dredging equipment with an environment-friendly reamer, and equipping with advanced high-precision positioning and monitoring system, so as to reduce the secondary pollution in the dredging process and meet the requirements of environmental protection dredging. From the technical point of view, the design and construction of special environmental protection equipment is not difficult. But from the economic point of view, the design and construction of special equipment is much higher than the transformation of traditional dredging equipment, and different projects need to build different equipment. The economy is worse for smaller projects.

Environmental dredging equipment is more advanced by using suction dredger with an environmental reamer. In this way, the surface sediments are sucked up by pipes under the action of mud pumps and transported to the land for a long distance. All these dredging vessels are equipped with an automatic control system and monitoring system and adopt DGPS for precise 3d positioning, which greatly improves the dredging accuracy. This method can reduce the re-suspension of sediment and release of pollutants.

 2.2 Sediment dredging

The conventional disposal methods of dredging sediment mainly include stacking, blowing and ocean dumping. These disposal methods not only solve the problem of dredging outlet but also generate various problems. Stacking treatment is mainly the problem of stacking. Stacking will take up a lot of lands, and may produce secondary pollution. Blowout treatment is a method of comprehensive utilization of soil, which transports the excavated soil to the filling point. After water loss, the silt is left to increase the elevation of the filled land. However, the backfill foundation is usually very weak, which is not conducive to the later development and use of the backfill land. Meanwhile, backfill construction is prone to cause secondary pollution of the surrounding environment. Marine slinging, though cheap, can pollute the marine environment and damage fishing grounds. Many countries have banned it. Dredging mud contains rich secondary minerals and is a good renewable resource. As the pollution source, dredging mud can be comprehensively developed and reused through a series of harmless and stable treatment methods, which can not only avoid the harm to the environment but also produce newly available resources.

3. Conclusion

Sediment deposition affects the normal operation of the waterway, and the contaminated sediment will release pollutants into the water, making the water quality worse. Environmental dredging has become an important measure to control the polluted sediment in the waterway because it can remove the polluted sediment directly and thoroughly from the water. However, the sediment disturbance, sludge leakage and unreasonable sludge treatment caused by environmental protection dredging will also cause certain pollution to the environment. With the enhancement of people’s awareness of environmental protection, the prevention and control of water pollution caused by the dredging process have been gradually developed.