Phosphorus removal ability of three inexpensive substrates

e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581
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Phosphorus removal ability of three inexpensive substrates:
Physicochemical properties and application
Baohua Guan a,b , Xin Yao b , Jinhui Jiang a , Ziqiang Tian c , Shuqing An a,∗ ,
Binhe Gu d , Ying Cai a
a
The State Key Laboratory of Pollution Control and Resource Reuse, School of Life Sciences, Nanjing University, Nanjing 210093, PR China
Key Laboratory of Lake Science & Environment, Nanjing Institute of Geography and Limnology,
Chinese Academy of Sciences, Nanjing 210008, PR China
c The Institute of Water Science, Chinese Academy of Environmental Science, Beijing, PR China
d University of South Florida, St. Petersburg, FL, USA
b
a r t i c l e
i n f o
a b s t r a c t
Article history:
Eutrophication of shallow freshwater lakes is a severe ecological problem all over the world.
Received 4 October 2007
Phosphorus is one of the main triggering nutrients responsible for eutrophication of shallow
Received in revised form
freshwater lakes. Constructed wetlands provide an effective means of phosphorus removal
9 April 2008
from enriched waters. The cost of a constructed wetland depends mainly on the sub-
Accepted 15 April 2008
strates filled within it. Here we used three inexpensive substrates including loess, cinder
and limestone to construct a wetland along the shore of Lake Taihu, and then tested the
total phosphorus (TP) removal ability of the wetland, the chemical and physical character-
Keywords:
istics, and the abilities of phosphorus adsorption and inhibition of substrates. We found
Constructed wetland
that the artificial wetland constructed with layered loess, cinder, and limestone but without
Substrates selection
hydrophytes and mesophytes had high phosphorus removal ability during the 44-day test,
Physicochemical properties
especially in the first 10 days. The average removal rate for TP was 41% for the overall testing
Phosphorus adsorption ability
time. Chemical properties of the substrates had stronger impacts on phosphorus removal
Phosphorus inhibition ability
than physical properties did. Among the three substrates, limestone had the highest phosphorus removal and inhibition ability due to its highest calcium content. This suggests that
more attention should be paid to chemical composition during the selection and assembly
of substrates for both constructed wetland study and its engineering applications.
© 2008 Elsevier B.V. All rights reserved.
1.
Introduction
Phosphorus is a key nutrient element responsible for the
eutrophication of freshwaters (Schindler, 1974; Baldy et al.,
2007). Sufficient phosphorus removal from enriched lake
waters is an effective method for overcoming the eutrophication problem (Schindler, 1974). The constructed wetland
(CW) is a common technique for removing phosphorus from
eutrophic water bodies instead of the natural wetland in
areas where natural wetlands are in short supply or have
∗
been destroyed (Vymazal, 2006). Compared to natural wetlands, the CW needs a smaller surface to treats the same
wastewaters, so it is easily applied in specific cases (Prochaska
and Zouboulis, 2006).
CWs have significant efficiencies in removing total phosphorus (TP), on average at least 40%, from the wastewater
(Verhoeven and Meuleman, 1999; Vymazal, 2006). Furthermore, more and more scientists have found that the major
mechanisms for removing phosphorus from eutrophic water
by CWs are chemical adsorption and sedimentation by sub-
Corresponding author. Tel.: +86 25 83594560; fax: +86 25 83594560.
E-mail addresses: [email protected] (B. Guan), [email protected] (S. An).
0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved.
doi:10.1016/j.ecoleng.2008.04.015
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581
strates, rather than plant uptake and microbe removal (Tanner
et al., 1999; Mitsch and Gosselink, 2000). Thus, the selection
and assembly of substrates are very important for wetland
construction.
For cost concerns, basic substrates such as loess, cinder, and limestone are widely used in CWs (Dinges, 1982).
In order to construct a high efficiency wetland with relatively inexpensive substrates, scientists try to correlate the
substrates’ physical and chemical characteristics with the
efficiency of phosphorus removal (Gray et al., 2000). Studies
have shown that the physical characteristics of substrates,
including granule size, specific surface area, consistency and
porosity, significantly impact phosphorus removal effectiveness; at the same time, the chemical composition of the
substrates has a great impact on the adsorption and inhibition efficiency of phosphorus in constructed wetlands (Dinges,
1982; McEldowney et al., 1993; Garcia et al., 2004). Thus,
it is necessity to test which one, the physical or chemical
characteristics of the applied substrates, mainly decides the
phosphorus removal efficiency of CW.
Lake Taihu is a large shallow lake in eastern China, which is
now troubled by eutrophication (Chen and Mynett, 2003; Qin
et al., 2007). It is urgent to seek an inexpensive and highly
efficient CW as one of the main means to abate influent
phosphorus loadings and purify the enriched lake water. In
order to establish a stable and effective constructed wetland,
we used three facile substrates along Lake Taihu, including
loess, cinder and limestone, to construct an experimental
and demonstration wetland. The physicochemical properties,
adsorption and inhibition ability of different substrate were
studied after analyzing the phosphorus removal efficiency of
the CW. The aims of this paper are to propose useful information on choice and application of substrates in constructed
wetlands for which the goal is phosphorus removal, and to
provide some helpful information for loading abatement and
wastewater treatment of Lake Taihu.
2.
Materials and methods
2.1.
Research site and basic circumstance
Lake Taihu is one of the five largest freshwater lakes in China.
It is located in the centre of the Yangtze River Delta, Eastern
China (30◦ 55 40 N to 31◦ 32 58 N, 119◦ 52 32 E to 120◦ 36 10 E).
With optimum climate (Li et al., 2008), there are approximately
32 million inhabitants (867.33 capitals per km2 ) and about 75%
of the arable land is used for rice cultivation in the densely
populated and intensively cropped Lake Taihu drainage basin.
This area is also one of the most economically developed and
rapidly urbanized areas in China (Zhang et al., 2003; Niu et
al., 2004). The eutrophication of the lake is accelerating with
economic growth and urbanization (Chen and Mynett, 2003).
The mean TP of the whole lake surface water was generally
more than 0.14 ± 0.03 mg L−1 after the year 2000 (Niu et al.,
2004).
2.2.
Setup of a constructed wetland
A subsurface flow CW with 140 m × 10 m area was constructed
as an experimental and demonstration wetland along the
577
Fig. 1 – Flow chart of water cycle across the constructed
wetland and Lake Taihu.
shore of Lake Taihu. Eutrophic lake water was pumped via
polyvinyl chloride (PVC) pipe into an artificial trench along the
CW and then flowed into the CW. There was another parallel
artificial trench dug along the opposite side of the CW to discharge the purified water (Fig. 1). The three layer substrates
filled in the CW from bottom to surface were limestone, cinder,
and loess with the thickness of 20, 20 and 10 cm, respectively.
In order to avoid loss of loess along the interspaces of limestone and cinder, a layer of fabric was laid under the layers
loess.
A pump working about 10 h a day (from 7:00 h to 17:00 h)
with a hydraulic loading rate of 11 m3 s−1 was used to pump
enriched lake water to the CW. Total phosphorus concentration of water pumped from Lake Taihu was 0.63 ± 0.22 mg L−1 .
2.3.
CW
Measurement of phosphorus removal rate for the
After pumping 2 h each day, samples were collected from the
influent and effluent trenches of the wetland, and the sample
days were the 1st, 2nd, 7th, 11th, 20th, 43rd and 44th day. Both
the influent and effluent trenches had six sample sites, each
of which was twin-sampled four times with 2-h intervals from
9:00 h to 15:00 h for each day. These samples were transported
to the laboratory in polyethylene (PE) bottles and analyzed
within 24 h. The total phosphorus was determined by molybdate reagent colorimetry after HClO4 –H2 SO4 digestion (Marr
and Cresser, 1983).
2.4.
Measurement of physicochemical properties for
the substrates
The particle size of these three substrates was measured following the method of Drizo et al. (1999). A pycnometer was
used to determine the true and bulk densities (Lee et al., 1997).
The porosity of substrates was determined by the Standard Soil
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e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581
Fig. 2 – Daily dynamic (left) and longtime TP removal rate (right) by the wetland constructed with loess, cinder and
limestone.
Science Procedure based on estimations of bulk density and true
density (Klute, 1986). Specific surface areas were determined
through the N2 gas adsorption method (−195.698 ◦ C) (Davis
and Kent, 1990).
The collected samples for loess, cinder and limestone from
the CW were ground and then sieved using a 0.25 mm sieve
mesh. The powders of these three substrates were washed two
times with dilute hydrochloric acid, and then dried 48 h in an
oven at 80 ◦ C. The presences of chemical composition of the
substrates was determined with X-ray fluorescence diffraction
analysis. Measured contents were expressed as per unit dry
weight. These were performed by the Modern Analysis Centre
of Nanjing University. Measured contents were expressed as
ratio of mass.
(BG), 2.000 g of the ground sediment powder was put into a
taper bottle, and then was covered with 100 mL of distilled
water. Three other treatments were to put 2.000 g sediment
into each bottle, and then covered by 1 g loess, cinder or
limestone, respectively before filled with distilled water. Each
treatment had three replications. A period of 48 h allowed
phosphate to be released from the substrates and further
transferred into the added distilled water under 25 ◦ C. The
total phosphorus concentration of the covering water was
then measured. Higher phosphorus concentration in the covering water means lower phosphorus inhibition ability.
The data were statistically analyzed by using SPSS (Ver.
12.0) if needed, with the significant level at 0.05 by using post
hoc with Duncan-test to indicate the differences.
2.5.
Measurement of adsorption isotherms for the
substrates
3.
Results
The dry and sieved substrates (2.000 g for each) were put into
taper bottles (200 mL). Then, 100 mL of potassium phosphate
monobasic (KH2 PO4 ), with a series of different concentrations
from 2.5 to 40 mg L−1 , were put into taper bottles. A few drops
of chloroform were put into the bottles to inhibit microbial
bioactivity. Bottles were shaken on a rotating shaker (100 rpm)
at constant temperature (25 ◦ C) for 48 h. Suspensions were
then centrifuged and the covering waters were determined for
phosphorus concentration. Phosphorus adsorption was calculated from the slightly modified Langmuir equation (Kuschk et
al., 2003):
3.1.
TP removal efficiency by the CW
(C0 × V) − (Ct × V)
M
3.2.
Padsorption (mg kg−1 ) =
The average TP concentration for all the 44 test days of the
effluent samples was 0.37 ± 0.13 mg L−1 , while the influent was
0.63 ± 0.22 mg L−1 ; thus, the total TP removal rate by the constructed wetland was 41%. The longtime average TP removal
rate decreased from the 1st day to the 43rd day, and then had a
stabilization (Fig. 2, right); the daily TP removal rate increased
from 9:00 to 15:00 O’clock, and then decreased a little (Fig. 2,
left).
Physicochemical properties of substrates
(1)
where C0 is the original concentration of TP in the solution;
Ct is the concentration at equilibrium; V is the volume of the
solution; M is the mass of the substrate put into the bottle.
2.6.
Measurement of phosphorus inhibition ability of
the substrates
Samples of hyper-enrichment sediment collected from
Yangtze River were ground after it was air-dried, and then
passed through a 0.25 mm mesh sieve. As the background
The measured values of the physical parameters including
true density, bulk density, porosity, diameter, and specific surface area are given in Table 1. Limestone had the highest true
density, while cinder ashes had the lowest value. As for the
bulk density, loess had the highest value, while cinder had
the lowest one. Cinder had higher porosity, whereas limestone
particles were lower in porosity than those of the other substrates. Except for cinder particles, limestone had the largest
particle size among the other substrates in terms of the diameter. The specific surface area of the loess was much higher
than those of cinder and limestone.
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e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581
Table 1 – Major physical properties of the three substrates used in the constructed wetland
Substrates
True density (g mL−1 )
Bulk density (g mL−1 )
Porosity (%)
Diameter (mm)
Specific surface
area (m2 g−1 )
Loess
Cinder
Limestone
1.983
1.813
2.687
1.174
0.761
1.620
40.8
58.1
39.7
0.5–1.0
1.5–2.5
1.0–2.0
34.692
7.414
1.8912
Mean
2.129
1.224
43.4
0.8–1.6
11.979
Table 2 – Main chemical composition of the substrates used in the constructed wetland (mass %)
SiO2
Al2 O3
Fe2 O3
CaO
Ti2 O3
SO3
K2 O
MgO
Na2 O
P2 O5
SrO
Others
Loess
Cinder
Limestone
37.2
46.4
1.36
10.9
30.9
0.38
5.2
8.96
0.13
28.5
2.2
35.5
0.77
1.3
0.02
1.5
1.0
44.3
2.5
1.3
0.08
2.9
0.48
0.20
0.36
0.37
<0.01
0.28
0.14
<0.01
0.13
0.08
0.36
<0.01
<0.01
<0.01
Mean
28.32
14.06
4.76
22.07
0.7
15.6
1.29
1.19
0.24
0.14
0.19
<0.01
The mean values for true density, bulk density, porosity,
diameter and specific surface of the three substrates in constructing the wetland were 2.129 g mL−1 , 1.224 g mL−1 , 43.4%,
0.8–1.6 mm, 11.979 m2 g−1 , respectively.
As for the main chemical composition of the substrates,
loess had higher content of K2 O, MgO and P2 O5 , cinder occupied greater content of SiO2 , Al2 O3 , Fe2 O3 , Ti2 O3 , Na2 O, while
limestone had more of CaO, SO3 , SrO than the other substrates
(Table 2). The CW filled with loess, cinder and limestone had
more than 10% of SiO2 , Al2 O3 , CaO and SO3 .
3.3.
Adsorption isotherms of the substrates
Limestone showed the greatest P adsorption among the three
substrates, followed by loess and cinder (Table 3). The bonding
capacity of cinder was the highest among the three substrates,
following by the loess and limestone. P adsorption maxima
varied between 2.002 and 0.890 mg g−1 , while bonding capacities varied between 0.254 and 0.100. The mean P adsorption
and the bonding capacity of the wetland constructed by loess,
cinder and limestone were 1.256 mg g−1 and 0.180.
3.4.
Phosphorus inhibition ability of substrates
There was significant difference of the water TP concentration
between background (BG) and the other treatments covering
with the substrates (p < 0.05, Fig. 3). Furthermore, The TP concentrations in the water column were significantly different
(p < 0.05) between different substrates adding to cover the sed-
iment. As for the TP inhibited rate, limestone had the maximal
value, and cinder had the minimal one.
4.
Discussion
4.1.
Phosphorus removal ability of the constructed
wetlands
Constructed wetlands are engineered systems that have been
designed and constructed to utilize natural processes to
remove pollutants from wastewater (Sakadevan and Bavor,
1998; Vymazal, 2006). There are many kinds of constructed
wetlands in terms of the waterways flowing within the wetland beds (Mitsch and Gosselink, 2000). Removal rate of
total nitrogen in most types of constructed wetlands with
hydrophytes varies between 40 and 55%, while total phosphorus is around 40–60%. Without hydrophytes in the cold season,
the removal rate for TP of the CW would be about 10% (Tanner,
1996; Peng et al., 2005; Vymazal, 2006).
The constructed wetland we created with three inexpensive substrates along Lake Taihu had a mean TP removal rate
of 41% during the 44 tested days with the maximum rate of
48% on the 1st day. Since there were no hydrophytes or wetland plants rooted in the constructed wetland, the efficiency
was relatively higher in removing total phosphorus comparing
Table 3 – P adsorption maxima, bonding capacities and
correlation coefficient, derived from the Langmuir
equation
Substrate
P adsorption
maximum (mg g−1 )
Bonding
capacity
Correlation
coefficient (r)
Loess
Cinder
Limestone
1.082
0.890
2.002
0.204
0.254
0.163
0.658
0.472
0.523
Mean
1.256
0.180
0.437
Fig. 3 – Total phosphorus (TP) concentration in the covering
water, and the TP inhibition rate of different substrates.
580
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 576–581
with other CW without hydrophytes (Peng et al., 2005). With
some wetland lawn grass growing on it, the constructed wetland costed about 400 yuan RMB per cubic meter (7.5 yuan = 1
USD as of December, 2007). Compared to other CWs in China,
the costs is lower (Liu et al., in press).
4.2.
Phosphorus removal ability and the
physicochemical properties of substrates
The physical and chemical properties determine the P adsorption capacity of substrates (Tanner et al., 1999). Consequently,
the P removal efficiency of the constructed wetland depends
mostly on the physic-chemical properties of applied substrates (Drizo et al., 1999; Burley et al., 2001). At the same time,
since the constructed wetland was established above the high
eutrophic sediment of Lake Taihu, the inhibition ability of substrates was thus important too. To advance and maintain high
phosphorus removal rate of the constructed wetlands, more
attention should be paid to chemical composition in future
engineering applications.
Otherwise, selection of suitable substrates is useful in
improving the performance of constructed wetland in removing pollutants, especially for the phosphorus (Sakadevan and
Bavor, 1998). Thus, some less expensive substrates need to
be selected and used to enhance the P adsorption capacity of
the substrates, and to maintain the activity of the constructed
wetlands.
Substrates with high total calcium content can remove
phosphorus effectively (UNEP, 2002). Among the three substrates, limestone has the highest calcium content (35.5% of
mass), while cinders have the lowest. Cinders had highest
mass content of iron (30.9%) and aluminum (8.96%). Since
calcium has a higher adsorption ability than the iron and aluminum, the more the limestone was used in the constructed
wetlands, the more TP would be removed. Among the three
substrates, limestone would be the best substrate to construct
a wetland due to the highest P-adsorption and P-inhibition
ability.
We found that the artificial wetland constructed with the
three inexpensive substrates in our studies had high phosphorus removal ability, especially during the first 10 days.
Phosphorus removal rates of constructed wetlands generally
decline after an initial equilibration period unless special substrates with high sorption capacity are used (Tanner et al.,
1999). Phosphorus transformations during wastewater treatment in CWs include adsorption, de-adsorption, precipitation,
dissolution, plant and microbial uptake, fragmentation, leaching, mineralization, sedimentation (peat accretion) and burial
(Drizo et al., 1999). The major phosphorus removal processes
are sorption, precipitation, plant uptake (with subsequent harvest) and peat/soil accretion. However, the first two processes
are saturable (Sakadevan and Bavor, 1998). Thus, wetland
plants and macrophytes would be necessary to maintain a
long lifespan CW (Vymazal, 2006). That is not only because of
nutrients absorption by wetland plants, but also because of the
supporting to microorganisms and wetland animals, which in
turn benefit the CW in removal of plant remains and compose
a wetland ecosystem similar to a natural wetland (Brix, 1994;
Vymazal, 2006).
Acknowledgements
The authors express their gratitude to Dr. Juan Yang in
Louisiana State University, Dr. Junyan Liu and Dr. Fenmeng
Zhu in David University for their assistance in emendation the
MS. This research was supported by the National High Technology Research and Development Plan (2003AA06011000-04
and 2002AA601012-06).
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