Small and Medium River Sand Dredging and Silt Treatment Technology

Silting of small and medium-sized rivers in my country is relatively common, and the original flood regulation, disaster prevention, and mitigation capabilities of rivers have been weakened. In recent years, we have strengthened the management of small and medium-sized rivers and rural rivers, and river siltation projects, including river sand dredging, have been widely implemented as the main measures. Taking Jiangsu Province as an example, from 2003 to 2014, the investment in river siltation exceeded 4 billion yuan, and the cumulative river siltation volume exceeded 3.5 billion cubic meters. At present, river siltation has expanded from the traditional water conservancy project goal of improving the flood control and drainage capacity of rivers to ecological water conservancy goals, that is, the goals of many river siltation projects include reducing endogenous pollution in rivers and providing guarantees for improving river water quality.

Historically, rural river siltation projects were mostly manual operations, and large river siltation equipment and siltation ships were basically built for large river siltation projects in ports, waterways or large rivers, and could not enter medium and small rivers for construction. Therefore, the existing river siltation equipment is generally suitable for medium and small river siltation projects. Another problem of dredging rural rivers is the treatment of silt. In the past, river mud was generally dug in winter, and there was a habit of composting the dug river mud and using it as fertilizer. This habit naturally solved the problem of the destination of silt. Now, due to the development of township enterprises, some rural rivers are polluted by industrial sewage discharge, and the change of rural lifestyle has also caused some small industrial waste and domestic garbage to be abandoned in the river. For these reasons, some river mud has become “sludge” and is no longer suitable for direct return to the field or composting as fertilizer. On the other hand, the use of chemical fertilizers in agricultural production is becoming more and more common, and the demand for fertilizers originally made from river mud is gradually lost.

In the past, my country had many sand mining projects for waterways, ports, and large inland rivers, with mature technical methods and mechanical equipment. During the “15th Five-Year Plan”, in response to the problem of removing pollutants from the bottom mud in the management of rivers and lakes, a number of “environmentally friendly river sand mining” projects were carried out, that is, projects to remove pollutants accumulated in the bottom mud of lakes and rivers from the lakes through dredging. Compared with engineering dredging, the thickness of the bottom mud of environmental river sand mining is very thin. For example, the thickness of the bottom mud of Taihu Lake environmental dredging project is mostly 20-40cm. Therefore, the positioning and operating system accuracy of the river dredging equipment itself are very high. In eutrophic lakes such as Taihu Lake and Dianchi Lake, large-scale environmental river dredging projects have been carried out to remove nutrients rich in nitrogen and phosphorus in the bottom mud.

Environmental dredging is also commonly carried out in urban rivers. Before the Guangzhou Asian Games, urban rivers were generally dredged to improve the water quality of rivers. Urban rivers such as Shanghai Suzhou River and Nanjing Qinhuai River were also generally dredged. Compared with lakes, urban river environmental dredging projects can be slightly more extensive, because rivers have the characteristics of fluidity and exchangeability, and the requirements for construction accuracy and prevention of secondary pollution are not as sensitive as lakes. At present, environmental dredging projects generally adopt the field pile mode for sludge treatment. The sludge produced by river dredging is first stored in the yard and used for land restoration or governance after sedimentation. As for how to implement the river dredging projects of small and medium-sized rivers and rural rivers, what kind of river dredging machinery and technology to use, and how to deal with the silt after removal, there is currently no relatively standardized or recognized method. In response to this problem, this article collects and organizes the dredging methods of my country’s rivers, especially small and medium-sized rivers. It introduces river dredging technology, as well as the technology and methods of sludge treatment and utilization, and analyzes the advantages and disadvantages and scope of application of various methods. It is hoped that some technical references will be provided for rural river dredging projects.

Commonly used river dredging technology

Currently, most river dredging projects have the purpose of improving water quality, so they still belong to the category of “environmental river dredging”. In addition, there are two relatively close terms in engineering, “dredging” and “dredging”. In order to distinguish from large-scale river dredging projects such as waterway and port dredging, engineers from Ocean Pump Industry suggest that the dredging and excavation projects of small and medium-sized rivers and rural rivers be collectively referred to as “dredging projects”, highlighting the concept of removing pollutants in the bottom mud and the engineering purpose of solving the siltation problem. The current river dredging project has the characteristics of systematic construction. Before dredging the river, a preliminary sediment survey should be carried out to determine the morphological characteristics of the riverbed through measurement, and to determine the characteristics of pollutants in the sediment and whether they exceed the environmental quality standards through sediment sampling and analysis. For small and medium-sized rivers, especially rivers in rural areas, the amount of engineering is relatively small, and these preliminary work is easy to be overlooked. However, doing some simple preliminary work well is helpful for the smooth implementation of the entire river dredging project and achieving the expected results. On the basis of the preliminary work, according to the quantity, scope, nature and surrounding conditions of the sediment, the process schemes of the main engineering links such as river dredging, transportation, sludge disposal, and tailwater treatment are determined, and the river dredging process and equipment are selected according to local conditions. The sludge generated by river dredging is properly disposed of to prevent secondary pollution. Due to the large number of river sand mining projects in my country’s ports, waterways, inland rivers, and lakes in recent years, river sand mining and dredging technologies have developed greatly, and equipment capabilities have also been greatly improved. However, special river sand mining ships and equipment that can enter small and medium-sized rivers and rural rivers are not common. The most commonly used river sand mining technologies for small and medium-sized rivers are as follows.

 Treatment Technology of River Sludge

River sand dredging will inevitably produce a large amount of sludge. These sludge generally has high water content and low strength. Some of the sludge may contain toxic and harmful substances. These toxic and harmful substances are easily leached out after being washed by rainwater, thereby causing secondary pollution to the surrounding water environment. Therefore, it is necessary to properly dispose of the sludge after river sand dredging. The treatment method of sludge is affected by the basic physical and chemical properties of the sludge itself. These basic properties mainly include the initial moisture content of the sludge (the mass ratio of water to dry soil, the same below), the content of clay particles, the content of organic matter, the types of clay minerals, the pollutants type and contamination degree. In the actual sludge treatment project, the appropriate treatment scheme can be selected according to the basic properties of the sludge and the existing treatment conditions. Throughout the domestic and foreign sludge treatment and disposal technologies, the following classifications can be made according to different standards.

1. Drainage and dredging

For rivers with small flow and no flood control, drainage, and navigation functions, drainage and dredging refers to the construction of temporary cofferdams in the river construction section, and then dry excavation or hydraulic excavation after drainage. After drainage, it can be divided into two processes: dry excavation and hydraulic flushing dredging.

1.1. Dry excavation. After the water in the operation area is drained, excavators are used to dig silt in most cases. The excavated silt is directly transported by muck trucks or piled up at temporary storage points on the shore. If the river channel and pond dam have a certain width, there will be a distance between the construction area and the mud storage point, and the silt needs to be transported to the shore storage point by transfer equipment. Generally, an extrusion mud pump, that is, a concrete delivery pump, is used to transport fluid plastic silt, and the transportation distance can reach 200-300m. There are also engineering examples of using belt conveyors for short-distance transportation. The advantages of dry dredging are thorough dredging, easy quality assurance, and low requirements for equipment and technology; the silt produced by river sand dredging has a low moisture content and is easy to handle later.

2. Hydraulic flushing and dredging: Use the high-pressure water gun of the hydraulic flushing machine to flush the riverbed sludge, stir the riverbed sludge into mud, collect the flowing mud in the pre-set low-lying area, pump it with a mud pump, and transport it through a pipeline to the shore yard or pit. Hydraulic flushing and dredging requires simple riverbed dredging equipment, convenient transportation, and low construction cost, but the mud concentration formed by this method is low, which increases the difficulty of subsequent treatment and the construction environment is also relatively harsh.

In general, the construction of riverbed dredging and drainage is intuitive, the quality is easy to ensure, and it is also easy to handle the situation where the dredged material contacts the riverbed.

3. The development direction of river channel dredging and silt treatment technology

With the development of society and the emphasis on ecological environmental protection, more and more urban and rural rivers will carry out river channel dredging and silt cleaning projects. The large amount of silt produced by river dredging occupies a large area of ​​storage yards, so new requirements are put forward for dredging and silt treatment technology.

The latest river dredging technologies are:

3.1. High-concentration in-situ environmentally friendly river dredging method. Due to the current environmentally friendly river dredging method, the concentration of dredged silt is about 15%~20%, the volume of water molecules is much larger than that of soil particles, and the volume of dredged slurry is about 4~5 times that of particles. High-cement slurry often requires a larger storage yard for placement. Many river dredging projects are seriously restricted by the problem of storage yards. High-concentration in-situ environmentally friendly river sand dredging can reduce the volume expansion rate of mud during river sand dredging, reduce the water content of mud during intermediate transportation, and directly turn mud into soil that can be used for filling. Therefore, in order to save land and reduce the cost of river sand dredging and silt treatment, high-concentration in-situ environmentally friendly river sand dredging technology has become a future development trend.

3.2. Rapid silt removal technology in storage yards. At present, most of the silt produced by inland river sand dredging is stored in storage yards. After the silt yard has been treated with the foundation and the problem of long-term swamp state has been solved, it can be used for construction, landscape and farmland. This foundation treatment process is the silt consolidation and drainage process. The silt has a high mud content and poor permeability. It takes a long time to consolidate under the action of its own weight, and the strength after self-weight consolidation is low. The problem of rapid drainage and consolidation of silt needs to be solved urgently. The vacuum preloading method and the pile loading preloading method used on the soft clay foundation are often difficult to play a good effect on the silt. The silt has a high moisture content and is in a flowing state. The effective stress between particles is very low. Under the high-pressure vacuum state, the silt particles will flow with the interstitial water, causing the drainage board to be blocked and unable to drain. How to solve the problem of drainage system blockage has become the key to the rapid drainage of silt. The rapid drainage technology of the field silt is to lay multiple layers and rows of horizontal drainage ditches in the silt, with the layer spacing and row spacing of about 60-80cm, forming a high-density silt drainage network. The network is connected to the horizontal drainage pipe sealed on the ground, and then connected to the jet drainage device. Pumping air and water can accelerate the drainage of silt. At present, the development and key issues of this technology are still in the early exploration stage.

4. Conclusion

The river dredging project of small and medium-sized rivers and rural rivers not only has the traditional dredging purpose, namely drainage, flood control, irrigation, etc., but also has the deep-level purpose of improving river water quality, promoting ecosystem health, and enhancing river landscape. Therefore, it is necessary to ensure the characteristics of this multi-purpose river dredging from the preliminary work of dredging, plan formulation, process selection, project implementation and other links. The river dredging project of small and medium-sized rivers and rural rivers is different from the river dredging project of port channels and large rivers. It has the characteristics of small engineering volume, difficulty for large ships to pass, dredging objects containing various garbage, and complex nature. Based on this characteristic, the simpler river dredging technology is often easy to use. Regardless of the technology used, the main goal of river management-improvement of water quality should be a problem that must be considered in the selection of dredging methods and processes. It is realistic to choose drainage dredging according to local conditions and take into account the various conditions of the river. If conditions permit, it is realistic to choose drainage dredging. If drainage is not possible, it is also a good choice to install a small sand suction pump on the improved cutter suction dredger, which can also be used as the development direction of small river dredging technology. Unlike coastal cities, rural areas have advantages such as low terrain and good land turnover. Therefore, it should be adapted to local conditions, and the idea of adapting to local conditions should be the focus of river dredging.

Sludge treatment should also be taken seriously. The most suitable practice for rural areas is to restore land use after stacking in low-lying areas, return sludge without heavy metal pollution, and use it as filler for river bank reinforcement and paving after simple treatment. At present, some new technologies and methods of river dredging projects are developed for large-scale projects and urban areas. The applicability of river dredging engineering equipment in small and medium-sized rivers and rural areas needs further research and development.

OCEAN PUMP is a manufacturer and supplier specializing in the production and supply of high-quality sand pumps, slurry pumps, gravel pumps, and sand pumping ships. The OCEAN PUMP technical team has been engaged in the research and development, matching and application of sand pumps for river dredging, land reclamation, tailings treatment, etc. since the 1990s. With rich experience, excellent technology and thoughtful service, OCEAN PUMP has won the trust of users. The company has been focusing on sand pumps for more than 20 years. Not only does it provide high-quality products and overall solutions for the industry, but it also provides on-site installation services. Please feel free to contact us for any questions about river dredging, sand suction scheme design and equipment.