Investigation on Sediment Treatment in a Heavily-Polluted River,

China

Senjun Huang

, Jun Wei

, Heng Zheng

, Jianqiang Xu

, Wei Zhao

and Jiange Jiao

2,*

Power China Huadong Engineering Corporation Limited, Hangzhou, China

Department of Mechanical and Electrical Engineering, China Jiliang University, Hangzhou, China

Keywords: Sediment pollution level, dredging depth analysis, sediment treatment, Maozhou River

Abstract: Proper treatment of contaminated sediment in heavily-polluted rivers is of importance to water quality

improvement, especially in black-odor rivers. In the present study, 86 columnar samples are obtained by

drilling with depth of 3~4 m in the Maozhou River, known as black and odorous water body. Firstly,

potential ecological risk indexes at different depths in the study area is calculated according to heavy metal

concentration. Afterwards, the dredging depths are determined based on vertical pollutant level analysis.

Then, a harmless treatment system is established to achieve stabilization and solidification of dredged

sediment. The results of this study provide systematic scheme and valuable reference for sediment treatment

in black and odorous rivers.

1 INTRODUCTION

Sediment is one of the most important component of

an aquatic ecosystem which can provide a

production environment for microorganisms and

food for fish. It is also the main sink of various

extraneous chemicals. As a result, the sediment will

be polluted if chemical concentration exceeds a

threshold, called contaminated sediment, thus

causing permanent hazards to water bodies (Singh,

2005; Kara, 2017). What’s worse, accumulated

contaminants in sediment could result in secondary

pollution because they can be released back to the

water body under disturbance of hydrodynamic

force or human activities by effect of molecular

diffusion, particle resuspension, and bioturbation

(Wang, 2016). Therefore, contaminated sediment is

often regarded as the secondary pollution source in

deteriorating water quality and becomes the focus in

water treatment (Kwaterczak, 2009). Among many

chemicals in sediment, Heavy metal elements

(HME) should be given enough attention. HME are

generally defined as metals with relatively high

densities, atomic weights, or atomic numbers. HME

are accumulated in sediment under effect of

adsorption, complexation and precipitation and

usually attach to fine-grained components (Lin,

2016). They not only influence aquatic ecosystem

but also threaten human health.

Thus, harmless treatment of contaminated

sediment is of importance in river projects. A variety

of remediation options for polluted sediments is

available (Rulkens, 1998; Thomas, 2001; Saponaro,

2003; Rulkens, 2005), mainly including in-suit and

ex-suit treatment. It is better to choose ex-suit

treatment as the pollutant concentration greater than

2~3 times the local background value (Chen, 2011).

Maozhou River, located in Bao'an District of

Shenzhen, Guangdong Province, Southwestern

China, which has been experiencing rapid

urbanization and industrialization. More and more

HME are released to water body and then

accumulated in the sediment. As a result, the

sediment is characterized as to be black and odorous,

which is threatening the living environment.

According to studies from year 1996 to 2016 (Jia,

2001; Dai, 2010; Gong, 2016), it can be seen that

sediment has been polluted by HME seriously. In

order to grasp the situation of sediment pollution, a

detailed investigation has been conducted using

columnar samples in 2016, especially on vertical

pollution of heavy metals in different depths.

Therefore, the objectives of the present study are:

(1) to understand the sediment pollution risk; (2) to

determine the dredging depths at different observed

points; (3) to establish a sediment treatment system

Huang, S., Wei, J., Zheng, H., Xu, J., Zhao, W. and Jiao, J.

Investigation on Sediment Treatment in a Heavily-Polluted River, China.

DOI: 10.5220/0008650600050010

In Proceedings of the International Conference on Future Environment Pollution and Prevention (ICFEPP 2019), pages 5-10

ISBN: 978-989-758-394-0

for solidification and dehydration of dredged

sediment.

2 SEDIMENT POLLUTION

CONDITIONS

2.1 Study Area

Maozhou River springs from the Yangtai Mountain,

running through Dongguan City and Shenzhen City

and into the Pearl River Estuary, as shown in Figure

1. There are about forty one rivers in the basin area,

including 1 trunk stream, 23 second-order tributaries

and 17 third-order tributaries.

Figure 1. Maozhou River Basin.

Figure 2. Sediment pollution conditions: (a) black and

odorous water body; (b)~(d) black and odorous sediment.

Due to the urbanization and industrialization in

recent decades, Maozhou River has been disturbed

by human activities significantly. As a result,

increasing domestic, municipal and industrial

wastewater have been discharged into rivers which

have caused black and odorous sediment as well as

water body, as shown in Figure 2.

2.2 Sample Collection

During the summer months of July and August in

2016, 86 borehole columnar samples were collected

with depth of 3~4 m in Maozhou River (reach A and

C) and Shajing River (reach B), as shown in Figure

3. Each point was located by GPS precisely. Each

borehole columnar sample was divided into four

layers, marked as I (0~1 m), II (1~2 m), III (2~3 m),

IV (>3 m) and layer I represents the surface. Each

layer was divided into two parts on average. Then,

the HME of Cu, Zn, Cr, Cd, Pb and Ni were tested

by plasma direct reading spectroscopy. Therefore,

for a certain HME, there were eight values for each

points.

Figure 3. Distribution of observed sections and points.

3 SEDIMENT POLLUTION RISK

LEVEL

3.1 Analysis Method

The potential ecological risk index (RI) is applied to

evaluate pollution risk level of HME. It was

proposed by Hakanson in 1980, which taken into

account the effects of pollutant concentration and

toxicity on the environment, and can

comprehensively reflect the impact of heavy metals

on the ecological environment (Hakanson, 1980; Xu,

2008). RI can be calculated as follows:

r,i

RI E

(1)

r,i r,i i

E T P

(2)

(3)

ICFEPP 2019 - International conference on Future Environment Pollution and Prevention

Where RI is the potential ecological risk index; E

r,i

the potential ecological risk coefficient of a certain

heavy metal element; P

is pollution index of a

certain heavy metal element; T

r,i

is biotoxicity

weighting coefficient of a certain heavy metal

element, according to Table 1; C

is the measured

concentration of pollutants of a certain heavy metal

element; C

is the soil background values of

Shenzhen City, according to Table 2.

Once the RI is calculated, pollution risk level can

be determined according to Table 3.

Table 1. Biotoxicity of heavy metals.

Element species

Biotoxicity

Table 2. Background values of heavy metals.

Element species

Background values

(mg/kg)

11.10

78.70

30.97

17.80

40.90

0.09

Table 3. Potential ecological risk coefficient and

comprehensive potential ecological risk index.

Er,i

Pollution risk level

<40

<150

Light ecological risk

40~80

150~300

Medium ecological risk

80~160

300~600

Strong ecological risk

160~320

≥600

Very strong ecological risk

≥320

≥600

Extremely strong ecological risk

3.2 Pollution Risk Analysis

According to the HME content, the sediment

pollution risk level at different observed points with

various depths are determined by potential

ecological risk index (RI).

Generally, along with the increase of depth from

riverbed surface, the RI decreases gradually for all

reaches. That is, the surface layer (layer I) is the

most polluted.

In the present study, average sediment pollution

level corresponding to value of RI at different layers

for each reach are given, as shown in Figure 4. The

RI in layers I~III of reach B and C are greater than

300. That is, the pollution risk level has exceeded

‘very strong’; it evenly reaches ‘extremely strong’

for reach B in layer I and II. For reach A, RI in layer

I approaches to 300, thus the pollution risk level is

‘medium’; RI in layers II and III are all less than

150, of which the pollution risk levels are ‘light’.

Lastly, for reach A, B and C, RI in layer IV are all

less than 150, of which the pollution risk levels

appear to be ‘light’.

Therefore, it is appropriate to dredge the

contaminated sediment for high pollution risk level.

Specifically, taking observed points (A10, B10 and

C10) for example, if the pollution risk level is

‘light’, sediment will not be dredged; otherwise,

sediment will be dredged, as shown in Figure 5.

Figure 4. Potential ecological risk index at different layers

for each reach.

Figure 5. Potential ecological risk indexes for observed

points A, B and C at different depths.

4 SEDIMENT DREDGING AND

TREATMENT

4.1 Dredged Depths Analysis in Study

Area

According to the pollution risk level of observed

points at different layers, the dredged depths can be

determined. For each columnar sample has been

divided into eight sub-samples with depth of 0.5 m,

it can be need to distinguish the polluted and

unpolluted sediment by physical methods such as

colour and plasticity. Generally, sediment in vertical

direction can be divided into pollution area,

transition area and clear area. In the pollution area,

the colour of the sediment is black to dark black,

which is slurry or flowing plastic and smells bad. In

the transition area, the colour is mostly grey-black

and soft-plastic, of which the density is greater than

the polluted area. In the clear area, the colour of the

sediment keeps the normal colour similar to the

unpolluted local soil, which is generally odourless

and has the maximum density.

Accounting for the above principles, the

minimum and maximum dredging depths in reach A

are 1.05 m 3.31 m, respectively; the values for reach

Investigation on Sediment Treatment in a Heavily-Polluted River, China

B are 1.20 m and 3.43 m, respectively; the values for

reach C are 1.65 m and 3.65 m, respectively.

Afterwards, the dredging depths in the study area

can be determined by reverse distance interpolation

method, as shown in Figure 6. The dredging depths

range from 1 m to 4 m in the study area. The total

amount of dredging is above two million cubic

metres.

Figure 6. Dredging depth in study area.

4.2 Dredging Method

Therefore, in order to improve the situation of black

and odor water in Maozhou River, it is necessary to

scavenge the contaminated sediment known as

‘authigenic pollution treatment’. However, the

sediment can also secondary pollution under violent

disturbance. Accordingly, in order to avoid

secondary pollution as much as possible, selection of

silt dredging method is also critical.

Common equipment for sediment dredging

include cutter suction dredger, rake suction dredger,

grab dredger, water excavator, stirring suction pump

and suction pump, each of which has its advantages

and disadvantages, as shown in Table 4. Therefore,

the cutter suction dredger has been chosen for lowest

ecological impact and highest dredging efficiency,

what’s more, it is convenient for underwater

transportation.

Table 4. Sediment dredging equipment.

Cutter

suction

dredger

Rake

suction

dredger

Grab

dredger

Water

excavator

Stirring

suction

pump

Suction

pump

●

◎

○

◎

○

●

○

◎

○

●

○

●

○

●

◎

Note: ●, ◎ and ○ represent high, medium and low,

respectively.

DE is dredging efficiency; CE is conveying efficiency;

MC is moisture content; EI is ecological impact.

4.3 Sediment Treatment System

Faced with a huge amount of dredged sediment, it is

beneficial to adopting ecological treatment method,

realizing resource reuse. The key problem is how to

deal with pollutants in sediment properly.

Except for heavy metals, polluted sediments very

often comprise a mixture of strongly different

pollutants such as organic pollutants and PAHs. A

comprehensive system is needed for treatment of

multi-pollutant sediment. In the present study, a

treatment system is established to achieve

harmlessness, stabilization and solidification of

dredged sediment, as shown in Figure 7. This system

mainly consists of five subsystems: ①Classification

decrement system; ② Concentration system; ③

Mixing system; ④ Homogenization conditioning

system; ⑤Solidification system.

(1) Classification-decrement system

Dredged sediment with a large number of water

bodies can be classified into gravel, waste and slush

for the purpose of decrement. The gravel is reused

by cleaning and the waste is treated by harmless

landfill. The slush goes into the next subsystem.

(2) Concentration system

Moisture content of slush is reduced significantly

after concentration system, which will be classified

into mud and residual water. Residual water can also

be classified in standard water and mud through

water treatment. Standard water is discharged into

river and mud returns to concentration system.

(3) Mixing system

The characteristic of mud mixed fully with

solidification, flocculant and chelator will be

changed, which makes the mud easy to be stabilized

and solidified.

(4) Homogenization conditioning system

After treatment of mixing system, mud flow into the

next system by self. Thus, active ingredients in the

added materials can be released speedy. Further,

mud is concentrated along with residual water.

(5) Solidification system

Concentrated mud will be made into mud cake after

solidification system using plate and frame filter

press. Mud cake is harmless to environment, of

which moisture content is less than 40%, and then

can be put in stackyard for reuse. Besides, the

residual water is transported back to concentration

system for next circulation.

Mud cake appears to be a good eco-building

materials, which is used to make ceramic ecological

ICFEPP 2019 - International conference on Future Environment Pollution and Prevention

permeable brick in the present study, as shown in

Figure 8. Ceramic ecological permeable bricks have

many advantages, such as high strength, strong

water permeability, good skid and wear resistance,

low cost and so on. They have been widely used as

revetment material and permeable paving material in

Maozhou River projects.

Figure 7. Sediment treatment system.

Figure 8. Ceramic ecological permeable brick.

5 CONCLUSION

In the present study, the dredging depths in

Maozhou River are determined based on analysis of

sediment pollution risk level at different depths of

86 observed points. What’s more, a sediment

treatment system is established to stabilize and

solidify the dredged slush. From the previous

discussion, the following important conclusions can

be drawn.

Sediment in the surface layer is suffering strong

ecological risk even very strong ecological risk,

which is necessarily to be dredged. Sediment with

depth greater than 3 m appears to be light ecological

risk.

Based on the principal that it needs to be dredged as

sediment pollution level greater than ‘light’,

dredging depth ranges from 1 m to 4 m in the study

area.

Answering for lowest ecological impact and

highest dredging efficiency, the cutter suction

dredger is used to dredge the polluted sediment.

The sediment treatment system mainly consists

of five subsystems, which can be applied to stabilize

and solidify contaminated sediment. Afterwards,

mud cakes can be used to make ecological

permeable materials.

ACKNOWLEDGEMENTS

This study is supported by Zhejiang Key Reaearch

and Development Program (2018C03G3241167);

besides, it is also supported by Comprehensive

Harnessing of Ecological Corridors (KY2018-SHJ-

04) from Power China Huadong Engineering

Corporation Limited.

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