River Sediment Treatment
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In general, the water content of river sediment is high and its strength is low, and there are potential environmental risks. At the same time, the cleared sludge lacks enough land or space for storage.
Therefore, how to predict the sludge yield, how to choose the right way of dredging, how to control the environmental risk of sludge and how to find the way out of sludge are the most important issues to be implemented in dredging.
Sludge disposal status
(1) Open water treatment
This method transports sediment to other rivers / lakes / oceans through pipelines, ships or vehicles.
It is required that the sediment is not polluted, and the receiving water body needs to be evaluated, so it is not suitable to dispose the sediment of the polluted river.
(2) Restricted disposal
Put the sediment into a depression isolated from the nearby water body and built a dike for sanitary landfill.
(3) Utilization and disposal
Sludge is used as construction material or subgrade material to replace clay.
In some areas of the United States, dredged materials are used to build docks, wetlands and cement additives.
However, this treatment may cause secondary pollution of pollutants in the sediment.
(4) In situ repair
In situ remediation includes the following types: direct absorption and degradation of pollutants in situ by bio ecological remediation technology.
By laying one or more layers of clean sediment and other natural minerals on the surface of the sediment, the polluted sediment is isolated from the upper water body, so as to prevent the pollutants in the sediment from migrating to the water body.
By adding chemicals to the water sediment, the surface sediment solidifies and stabilizes, forming a sediment covering layer, and preventing the pollutants in the deep sediment from migrating upward. The disadvantage of this method is that it can not improve the flood control and drainage capacity of the river.
A case study of heavy pollution lake treatment project
S lake began to receive industrial wastewater in April 1976, most of which was chemical wastewater. When the industrial waste water from the whole area can be discharged in the flood season, the sluice will be lifted and put into the river, which will be put into the sea together with the river water.
The total area of the sewage reservoir is about 256.67 hectares, the total amount of sewage in the reservoir is 2399900 cubic meters, and the total amount of polluted mud is 2993800 cubic meters.
The monitoring points are arranged in the lake basin. The key areas are mainly the siltation areas on the West Bank and the south bank. The flow axis from the sewage inlet to the outlet is arranged with 400m as the grid monitoring section, with 18 sites in total. At the same time, 1-3 sampling points are arranged in the key areas that may be polluted. In the sewage reservoir, with 100m as the interval, 217 monitoring sites are arranged.
The sediment of the lake basin was sampled.
In the first stage, there are 25 sampling points with a sampling depth of 4m. The main goal of monitoring is to carry out qualitative analysis on the pollution of sediment and find out the types and distribution of main pollutants in sediment.
The first batch of sediment analysis results show that the sludge pollution in the bottom of the sewage lake is mainly concentrated in the black mud layer about 80cm in the surface layer, and the undisturbed silty clay layer can be seen at each point up to 1.5m.
In the second stage, there are 192 detection points.
On the basis of the monitoring results in the first stage, the key pollution items detected in the first stage are monitored at all sites to comprehensively reflect the pollution status of the sewage lake.
Through the observation and analysis of 217 sediment boreholes, it is found that the bottom mud of sewage lake generally has a very obvious sequence structure.
The top is a black silty clay layer – a seriously polluted layer, with a thickness of 1.3-2.5m. It is generally black, mainly composed of clay and fine silty particles, with an average particle size of 5-11 μ m, between heavy loam and light clay.
It has a high content of organic matter and an obvious odor.
The brown yellow mud layer at the bottom is mainly brown yellow natural deposition with low water content of about 70%.
The average particle size of most sample points is between 6-9 μ m, without peculiar smell. The boundary between the two layers is distinct, and no obvious pollutant transition zone layer is found.
According to the different water content, the seriously polluted layer can be divided into three sub layers:
The top layer is hydrosol sublayer, which is the transition zone of solid-liquid phase.
The water content is more than 98% and the thickness is between 20-80cm. As a whole, it changes with the change of water depth. There is obvious overlap between the layer and the water layer, and it changes with the degree of water disturbance.
In the middle is a black flocculent sublayer, which is located below the hydrosol sublayer. It is a thick black flocculent layer with a thickness of 20-60cm and an average moisture content of about 90%.
The lowest layer is a sub layer of black clay, with an average water content of about 85% and a thickness of 0.3-1.2m, mainly composed of black clay, silt and fine sand deposits.
The first two layers of the three sublayers have certain fluidity. Under the condition of large disturbance, they can move with the flow and easily enter the upper water body by upward disturbance.
30 pollutants were detected in the sediment, including:
11 heavy metal and inorganic pollutants.
9 semi volatile organic compounds.
6 volatile organic compounds.
Mercury pollution is the most serious.
1. Pay attention to the investigation of bottom mud (mud quantity, mud quality).
It is suggested that the mud quality investigation should be divided into two stages: preliminary investigation and detailed investigation. Before the implementation of each stage, a detailed investigation plan should be formulated, and it is suggested to formulate the specifications for sediment pollution investigation.
2. The relationship between comprehensive balance of transportation cost and treatment cost.
3. At least one sludge landfill or stacking plant is necessary.
When the water content of sediment is more than 60%, the strength is very low, so it is necessary to pay attention to the geotechnical mechanics of landfill.
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