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

The 5
th
International Symposium on Southeast Asian Water Environment
7 - 9 November, 2007. Chiang Mai, Thailand
================================================================================
============================================================================
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE
FLOW WETLAND (Le Anh Tuan and Guido Wyseure)
1
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE
IN A CONSTRUCTED SUBSURFACE FLOW WETLAND


Le Anh Tuan
1,2)
and Guido Wyseure
2)

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:
2) Division for Land and Water Management, Faculty of Bioscience Engineering
Katholieke Universiteit Leuven, B-3001 Heverlee, Belgium
E-mail:


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.

The 5
th
International Symposium on Southeast Asian Water Environment
7 - 9 November, 2007. Chiang Mai, Thailand
================================================================================
============================================================================
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE
FLOW WETLAND (Le Anh Tuan and Guido Wyseure)
2
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 (M
(%) OM
s
scsc
×
+
=
(1)
The 5
th
International Symposium on Southeast Asian Water Environment
7 - 9 November, 2007. Chiang Mai, Thailand
================================================================================
============================================================================
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE
FLOW WETLAND (Le Anh Tuan and Guido Wyseure)
3
where
OM - organic matters in present (%);
M
c
- weight of cup (gr);
M
s
- weight of dried sand sample (gr) by drying at 110 °C in 3 hours;
M
sc
- weight of sand and cup after combusting at 550 °C in 3 hours (gr).









× %100
M
M- M
s
csc
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

The 5
th
International Symposium on Southeast Asian Water Environment
7 - 9 November, 2007. Chiang Mai, Thailand
================================================================================
============================================================================
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE
FLOW WETLAND (Le Anh Tuan and Guido Wyseure)

4
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
R
2
= 0.947 80 cm
y = -0.0021x + 3.2176
R
2

= 0.9396
0.000
0.500
1.000
1.500
2.000
2.500
0 200 400 600 800
x (cm)
OM (%)
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.



Fig. 3: Contour graphs of OM (%) at (a) 20 cm; (b) 50 cm; and (c) 80 cm
The 5
th
International Symposium on Southeast Asian Water Environment
7 - 9 November, 2007. Chiang Mai, Thailand
================================================================================
============================================================================
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE

FLOW WETLAND (Le Anh Tuan and Guido Wyseure)
5

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
800 15 0.998667 x
400 15 1.374000 x
200 15 1.627300 x
50 15 2.001330 x
100 15 2.038000 x
Contrast Difference +/- Limits
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 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.

The 5
th
International Symposium on Southeast Asian Water Environment
7 - 9 November, 2007. Chiang Mai, Thailand
================================================================================
============================================================================
ORGANIC MATTER DISTRIBUTION OF THE ROOT ZONE IN A CONSTRUCTED SUBSURFACE
FLOW WETLAND (Le Anh Tuan and Guido Wyseure)
6

6. References

Brix, H. (1997). Do macrophytes play a role in constructed treatment wetlands? Water Sci.
Technol., 35, 11-17.

Jing, S.R., Y.F. Lin, D.Y. Lee, T.W. Wang. (2001). Using constructed wetland systems to
remove solids from high polluted river water. Wat. Sci. Tech.: Water Supply, 1,89-96.

Kadlec, R.H., R.L. Knight. (1996). Treatment Wetlands. Lewis Publishers, Boca Raton,
Florida, USA. 893 pp.

Lee, B.H., M. Scholz. (2006). What is the role of Phragmites australis in experimental
constructed wetland filters treating urban runoff? Ecol. Eng., 29, 87-95.

Mitsch, W.J., J.G. Gosselink, 2000. Wetlands. John Wiley and Sons, New York. 936p.

Neter, J., M.H. Kutner, C.J. Nachsheim and W. Wasserman. (1996). Applied linear statistical
models. 4th Ed. WCB/McGraw-Hill. 1048p.

Reddy, K.R., W.H. Patrick, C.W. Lindau. (1989). Nitrification-denitrification at the plant
root-sediment interface in wetlands. Limmol. Oceanogr. 34, 1004-1013.

Scholz, M. (2006). Wetland systems to control urban runoff. Elservier, Amsterdam, the
Netherlands.

Stottmeister, U., A. Weisner, P. Kuschl, M. Kappelmeyer, M. Kaster. (2003). Effects of plants
and microorganisms in constructed wetlands for wastewater treatment. Bio-tech. Adv., 22, 93-
177.

Watson, J.T., J.A. Hobson. (1989). Hydraulic design considerations and control structures for
constructed wetlands for wastewater treatment. In Hammer, D.E. (ed.) Constructed wetlands
for wastewater treatment: Municipal, industrial and agriculture. Lewis Publishers, Chelsea,
MI., pp. 379-391.