INTEGRATED METHOD FOR CLEARANCE, COLLECTION AND CAPTURE OF INTERNAL POLLUTANTS AND ALGAE IN A SURFACE LAYER OF THE LAKE BOTTOM

Abstract

An integrated method for clearance, collection and capture of internal pollutants and algae at the bottom of a lake include the following steps: selecting areas where the pollution level is high, and organic or inorganic particulate matter is prone to accumulation and carrying out trenching operations at the bottom of the lake to form a plurality of traps; and removing the sludge and algae inside the traps and clearing the sediment inside the traps, for subsequent internal pollution control when the surface-layer sludge on both sides of the traps almost fills up the traps. This method makes use of the hydrodynamic disturbances of waves formed by natural wind energy and lake currents to continuously transport sludge with a high pollution level and a small specific gravity and algae in the surface layer of the lake bottom, which are rich in organic debris, to artificially built traps.

Claims

1. An integrated method for clearance, collection and capture of internal pollutants and algae in a surface layer of the lake bottom, wherein the method comprises the following steps: (1) selecting areas where the pollution level is high, and organic or inorganic particulate matter is prone to accumulation and building deep concave traps at the bottom of the lake in the accumulation-prone areas to form a plurality of traps collecting pollutants and algae at the bottom of the lake; and (2) removing the sludge and algae inside traps and clearing the sediment inside traps for subsequent internal pollution control when the surface-layer sludge on both sides of the traps almost fills up the traps.

2. The method according to claim 1, wherein the accumulation-prone areas at the step (1) are the convergence areas of lake currents at the bottom or the peripheral areas of the estuary.

3. The method according to claim 1, wherein in each of the accumulation-prone areas at the step (1), there are a plurality of high-frequency convergence points and the trap is a straight line or a curve connecting the plurality of high-frequency convergence points.

4. The method according to claim 3, wherein the high-frequency convergence points at the step (1) are determined through the following steps: (1.1) collecting data including the wind speed, wind direction, river mouth positions entering or leaving the lake, discharge, water depth, current direction and shear stress at the lake bottom; (1.2) according to the collected data, using a three-dimensional hydraulic model to calculate the lake flow field and wind wave for typical year, analyzing calculation results of the daily average or hourly average lake current at lake bottom and determining the frequency and distribution of the convergence points, among which the convergence points with an annual frequency of more than 25% are high-frequency convergence points.

5. The method according to claim 1, wherein the depth of the traps is determined according to the wavelength in the accumulation-prone area, the water depth of the trap bottom is more than half of wavelength of highest one-tenth wave in the typical year and the height difference between the trap bottom and the lake bottom is more than 1 m.

6. The method according to claim 1, wherein the step (1) further comprises protecting the edges of the formed traps.

7. The method according to claim 6, wherein the edge protection material is concrete, metal plate or engineering plastic plate.

8. The method according to claim 1, wherein the lake is a shallow lake with a water depth of less than 6 m at a normal water level.

9. The method according to claim 2, wherein the length of the traps is 0.6 to 0.9 time the length of the corresponding convergence zones, and the width is 4 to 20 m.

10. The method according to claim 2, wherein the convergence zones are zones that conform to $\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}<0,$ where u and v are velocity components in an east-west direction and a south-north direction at the bottom of the lake respectively, and x and y are the east-west and south-north coordinates of the lake.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

[0025] FIG. 1 is schematic diagram of the present invention.

[0026] FIG. 2 is schematic diagram of the present invention.

DETAILED DESCRIPTION

Embodiment 1

[0027] In the Zhushan Lake area where sludge pollutants and algae of the Taihu Lake are prone to accumulation and precipitation, the zones that conform to

[00002]$\frac{\partial u}{\partial x}+\frac{\partial v}{\partial y}<0$

are selected as convergence zones, the wind velocity, wind direction, lake current flow direction and lake bottom shear stress of the zones are investigated, the lake flow field and wave value in a typical year are calculated using a three-dimensional hydrodynamic model according to the method in the prior art (reference: Hu, W., Jørgensen, S. E., Zhang, F., 2006. A vertical-compressed three-dimensional ecological model in Lake Taihu, China. Ecol. Model. 190(3), 367-398.) and the calculation results of daily average or hourly average bottom-layer flow field are analyzed to determine the frequency and distribution of convergence points of the bottom-layer flow field and determine that the convergence points with an annual frequency of above 25% are high-frequency convergence points.

[0028] Trenching operations are conducted at the bottom of the lake to form a plurality of traps at the bottom of the lake. The traps randomly connect a plurality of high-frequency convergence points and are 4 m wide, 2 m deep and 0.6-0.9 time as long as the convergence zones. The water depth of the trap bottom is 7 m (more than half of wavelength of highest one-tenth wave in the typical year).

[0029] Meanwhile, engineering plastic plates are used to protect the edges of the traps.

[0030] 12 months later, after the polluted sludge and algae in the water settle in the traps through water current action, the sludge and algae inside the traps are collected and removed using shipborne dredging equipment.

[0031] After this technology is used, the total nitrogen and total phosphorus contents in the surface layer of the sediment are reduced by 40% and the content of chlorophyll a is reduced by 50%.