Tài liệu Organic matter distribution of the root zone in a constructed subsuface flow wetland pptx

============================================================================ 1ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE FLOW WETLAND Le Anh Tuan1,2 and

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ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE

IN A CONSTRUCTED SUBSURFACE FLOW WETLAND

Le Anh Tuan1,2) and Guido Wyseure2) 1) Department of Environmental and Water Resources Engineering, College of Technology

Can Tho University, Campus II, Street 3/2, Can Tho City, Vietnam

E-mail: latuan@ctu.edu.vn 2) Division for Land and Water Management, Faculty of Bioscience Engineering

Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium

E-mail: guido.wyseure@biw.kuleuven.be

Abstract

Constructed wetlands are known widely by their characteristic properties like utilization of natural processes, simple and easy of construction, operation and maintain as well The constructed subsurface flow wetland is designed as a tank with an impervious boundary to prevent seepage and contain a suitable porous media in which emergent plants grow The water remains below the surface of the gravel/stone/rock media Soil in constructed subsurface flow wetland absorbs and stores organic matter several years This accumulation potentially leads to a decline of the filter ability of the constructed wetland

A survey on the vertical and horizontal distribution of the organic matter in sand bed was done in the experimental constructed subsurface flow wetland in Can Tho University’s campus, Vietnam The linear decreasing organic matter distribution to the increasing vertical and horizontal flow direction is confirmed as the hypothesis in highly deposition of suspended solids and organic matters in the head section of the root zone It also proves a homogeneous flow pattern in the system

Keywords: Constructed wetland, wastewater, organic matter, distribution, root zone

1 Introduction

Constructed wetlands (CW) are mainly built for wastewater treatment purposes CW are widely used in the USA, Europe and some Asia countries They are easy in construction, operation and maintain as well (Watson and Hobson, 1989, Kadlec and Knight, 1996, Mitsch and Gosselink, 2000) They form one possible promising and feasible approach for a small scale decentralized domestic wastewater treatment The constructed subsurface flow wetland (CSFW) is designed as a tank with an impervious boundary to prevent seepage and contain a suitable porous media in which emergent plants grow The water remains below the surface of the gravel/stone/rock media All the complicated physicochemical and biological interactions among vegetation, microorganisms, soil and pollutants occur below the surface in the wetland

root zone (Jing et al., 2001) The wastewater is treated by the physical-chemical and

bio-chemical com-plex processes of filtration, sorption and precipitation processes in the soil and

by microbiological degradation Finally, the treated wastewater flows out in the bed The wastewater is therefore not causing any odour or mosquito breeding opportunities

Soil organic matter (OM) is the organic fraction of soil, including wastewater pollutants, plant roots, animal and plant residues, and microbial biomass OM influences the chemical and physical properties of soils even at the relatively low amount usually found in soils The macrophyte plants transport approximately 90% of the oxygen available in the root zone (Lee, 2007) Such the oxygen in the root zone supports the aerobic decomposition process of OM

and the growth of nitrifying bacteria (Reddy et al., 1989; Brix, 1997; Scholz, 2006) However, Stottmiester et al (2003) proved that OM in the wastewater is degraded mainly by the

existing of micro-organisms in the wetland system Composed organic matters synthesize dark, amorphous, colloidal mass, called humus Humus is the active component of soil organic matter and is responsible for water retention, nutrient retention and cohesion Soil in CSFW absorbs and stores OM several years This accumulation potentially leads to a decline

of the filter ability of the constructed wetland

The objective of this study is to survey the vertical and horizontal distribution of the OM in sand bed of the experimental constructed subsurface flow wetland in Can Tho University’s campus, Vietnam This treatment system are operating since 2003 The hypothesis is the OM distribution in sand bed descending linearly to the flow direction

2 Materials and methods

Sand sampling was done during January 2007 in the experimental constructed subsurface flow wetland (CSFW) located at Campus I of Can Tho University (Figure 1) The main part

of the system is a sand treatment tank (12.0 x 1.6 x 1.1 m) In this tank, river sand (average porosity of 47%) is filled up with a thickness of 1.1 m The emergent plant chosen to plant in

this tank is common reed (Phragmites spp.) as a very common and easy growing plant in the

MD The reed is planted with an initial density of 25 plants per square meter Since 2003 the CSFW treats domestic wastewater from the surrounding dormitories

System water quality data was monitored since 2003 until to 2006 The data showed that constructed subsurface flow wetland removes pollutants significantly and satisfy Vietnamese standards for wastewater discharge to water body

The positions for sand sampling explore both vertical and horizontal direction in the CSFW

In vertical direction, three depths were taken: 20 cm, 50 cm and 80 cm from the surface In horizontal direction, there are five cross-sections of system for the sampling with the distances from the inflow sand bed cross-section 0.5 m, 1.0 m, 2.0 m, 4.0 m and 8.0 m The purpose is to have a more detailed sampling in the start of the CSFW In each cross-section, five positions for sand sampling from the right site are 20 cm, 50 cm, 80 cm, 100 cm and 140

cm to study the homogeneity along the cross-section Figure 1 shows the positions and the coordinate system with the origin So, the Ox direction gives the length along the flow direction, Oy horizontal direction orthogonal to flow the side and Oz direction is the depth of the sand bed as compared to the surface

Organic matters in sand are analyzed by using combustion method and the results are reported

on a dry weight basis as:

% 100 M -M (M (%)

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where

OM - organic matters in present (%);

Mc - weight of cup (gr);

Ms - weight of dried sand sample (gr) by drying at 110 °C in 3 hours;

Msc - weight of sand and cup after combusting at 550 °C in 3 hours (gr)













×100%

M

M

-M

s

c

sc is the percentage of ash from the combusted organic matters

Fig 1: A systematic longitudinal cross-section of the CSFW in Campus I, Can Tho

Fig 2: Coordinates xyz for 3-dimensional sand sampling

In analysis, data were compared graphically and by an ANOVA analysis at the significant

level α = 0.05 to test for differences (Neter et al., 1996) The visual appearance of the sand in

CSFW was also observed during the survey

3 Results and discussion

From the surface to the depth 10 cm, the originally yellow sand is mixed with the OM due to decomposing plants From 10 cm to 40 cm, the sand is still yellow and clean with numerous roots From 40 cm to 60 cm, the sand color changes from yellow to dark grey and brown Lower then 60 cm up to 100 cm, which is the bottom, the sand color returns to the original yellow Figure 3 shows the average OM contents in the sand bed are linear reducing to the Ox direction In the Oz direction, the average value of OM in each cross-section is highest at the depth 50 cm and lowest at the depth 80 cm This result is in line with the visual observation of the sand color and the root system distribution The most root density was found at the depth

30 - 50 cm

50 cm

y = -0.0022x + 3.3117

R 2 = 0.9331

20 cm

y = -0.0021x + 3.1719

y = -0.0021x + 3.2176

R2 = 0.9396

0.000

0.500

1.000

1.500

2.000

2.500

x (cm)

20 cm

50 cm

80 cm

Fig 3: OM trend lines in the sand bed Figure 3 gives the distribution of OM in terms of interpolated contour graphics at three depths

20 cm, 50 cm and 80 cm The differences in distribution of OM contents among side cross sections are low

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The output ANOVA test gives estimated difference of 9 pairs in length statistically significant differences at the 95.0% confidence level (Table 1) A group of mean (50 cm and 100 cm) is not statistically significant difference

Table 1: The multiple range tests for the OM by the length (Ox direction)

Method 95.0 percent least significant (LS) difference

Length Count LS Mean Homogeneous groups

50 - 100 - 0.0366667 0.12887

50 - 200 * 0.374 0.12887

50 - 400 * 0.627333 0.12887

50 - 800 * 1.00267 0.12887

100 - 200 * 0.410667 0.12887

100 - 400 * 0.664 0.12887

100 - 800 * 1.03933 0.12887

200 - 400 * 0.253333 0.12887

200 - 800 * 0.628667 0.12887

400 - 800 * 0.375333 0.12887

* denotes a statistically significant difference

4 Conclusions

Firstly, the survey is to achieve a better understanding of the inner of a CSFW in Can Tho University Secondly, the survey can be translated a new insight to adjusted design parameter

of constructed wetland in tropical countries for domestic wastewater treatment

The linear decreasing OM distribution to the increasing vertical and horizontal flow direction

is confirmed as the hypothesis in highly deposition of suspended solids and organic matters in the section It also proves a homogeneous flow pattern in the system This conclusion is a useful for constructed wetland management and design As more and more OM deposit, in particular in the head of the root zone, the sand of the CSFW should be clean or replace after certain operating years It is recommendation that the depth of CSFW, with the common reed plant, should not exceed 80 cm in design

5 Acknowledgement

Authors would like to thank sincerely the Belgium - Can Tho University VLIR-E2 project (through the Institutional University Co-operation Programme between the Flemish

Inter-University Council (Vlaamse Interuniversitaire Raad) and Can Tho Inter-University) We

acknowledge staff members of the Department of Environmental and Water Resources Engineering, College of Technology, Can Tho University for their supports to our research

6 References

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Jing, S.R., Y.F Lin, D.Y Lee, T.W Wang (2001) Using constructed wetland systems to

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Lee, B.H., M Scholz (2006) What is the role of Phragmites australis in experimental

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