New Comparison of Slags and Gravels as Substrates in Horizontal … JWS-A-15... · 2015. 7. 8. · Comparison of Slags and Gravels as Substrates in Horizontal Sub-surface Flow Constructed

Comparison of Slags and Gravels as Substrates in Horizontal Sub-

surface Flow Constructed Wetlands for Polluted River Water

Treatment

Yuan Ge1, Xiaochang C. Wang

1*, Yucong Zheng

1, Mawuli Dzakpasu

1,2,

Jiaqing Xiong1

, Yaqian Zhao2

1 Key Laboratory of Northwest Water Resources, Environment and Ecology, Ministry of Education, School of En-

vironmental and Municipal Engineering, Xi’an University of Architecture and Technology, Xi’an 710055, China 2 Centre for Water Resources Research, School of Civil, Structural and Environmental Engineering, University

College Dublin, Newstead, Belfield, Dublin 4, Ireland

ABSTRACT

Comparative studies were conducted on the performance of two identical horizontal subsurface flow(HSSF) wet-

lands (surface area 340 m2, depth 0.6 m, HLR 0.2 m/d), but using different substrates (slags and gravels) for treat-

ing polluted river water over a one-year period. Water quality analyses were conducted for suspended and dissolved

pollutants, which were partitioned using a 0.45 μm membrane filter. Average removals of 89.5%, 74.1%, 82.2%,

58.2% and 89.0%, respectively, were achieved for SS and suspended COD, BOD, TN and TP in the wetland with

slags as substrates, and 81.7%, 70.8%, 75.1%, 56.0% and 64.4%, respectively, for that with gravels. The advantage

of using slags as substrates over gravels was more obvious regarding dissolved COD, BOD, TN and TP removals

(65.5%, 84.9%, 23.4% and 18.0%, respectively, for slags versus 41.8%, 62.7%, 19.3% and 6.5%, respectively, for

gravels). The much higher dissolved TP removal by the slags were mainly due to the much higher affinity of phos-

phates to the more porous slag particles than gravels, as shown by the adsorption capacities of slags and gravels

measured as 3.15 g/kg and 0.81 g/kg, respectively. Slags were proven to be the preferred substrate for subsur-

face-flow CWs for enhancing pollutants removal, especially dissolved phosphorus removal.

Keywords: Substrates; slags; gravels; subsurface-flow; phosphorus removal

1. INTRODUCTION

In China, the provision of urban sewerage in-

frastructure cannot always accommodate the

rapid development of industrialization and

urbanization (Zhang et al., 2012). The majori-

ty of urban sewage and industrial wastewater

are discharged into the environment without

effective treatment. Over the years, uncon-

trolled discharge of such wastewaters directly

into urban rivers has led not only to the se-

rious pollution of urban streams, but also, the

destruction of the ecological environment

along the river basins (Huang et al., 2011).

According to the latest statistical data (MEP,

2012), 36.5% of the rivers in China were pol-

luted and unsuitable as source water for

drinking water production. The main conta-

mination indexes were COD, total phosphorus

and ammonia-nitrogen. Therefore, considering

the environmental and ecological threats,

finding economical, esthetic and ecologically

sustainable treatment approaches to improve

urban river water quality is highly desirable in

Journal of Water Sustainability, Volume 4, Issue 4, December 2014, 247-258

© University of Technology Sydney & Xi’an University of Architecture and Technology

*Corresponding to: [email protected]

DOI: 10.11912/jws.2014.4.4.247-258

248 Y. Ge et al. / Journal of Water Sustainability 4 (2014) 247-258

many cities.

Since the late 1980s, as an effective eco-

logical technology for the treatment of

wastewater, constructed wetlands (CWs) have

drawn wide attention (Vymazal and

Kropfelova, 2009). Due to their low cost,

simple operation, maintenance and little sec-

ondary pollution and favorable environment

appearance, CWs are used for treating several

types of wastewater, including industrial and

agricultural wastewater, landfill leachate and

storm water runoff (Arroyo et al., 2013;

Babatunde et al., 2008; Białowiec et al., 2012;

Chan et al., 2008). The treatment of pollutants

in CWs is accomplished through a combina-

tion of biological, physical and chemical

processes and interactions among wetland

components (Saeed and Sun, 2012). Based on

the water flow type, CWs can be categorized

as free water surface flow (FWS), horizontal

subsurface flow (HSSF), and vertical flow

(VF) wetlands. HSSF CWs, which were typi-

cally more effective than the FWS are useful

for removal of organics and suspended solids

but are less effective for nitrogen, unless a

longer hydraulic retention time and enough

oxygenation are provided. In recent years,

HSSF CWs have been successfully tested for

domestic or municipal, milking wastewater,

polluted rivers and lakes treatment in different

part of the world (Abou-Elela et al., 2013; Cui

et al., 2011; Wu et al., 2011). As an effectively

ecological treatment system, constructed wet-

lands were installed on the river bank for pol-

luted river water treatment. A two-stage baf-

fled surface-flow constructed wetland which

was constructed along Jialu River floodplain

for the river water treatment achieved better

TN, TP, NH3-N, COD, and SS removal effi-

ciency in summer (Wang et al., 2012). A

FWS-SSF system was used to ammonia ni-

trogen in Erh-Ren River in Taiwan (Jing and

Lin, 2004).

In addition, wetland plants, as an important

component of CWs, are considered to remove

nutrient from CWs (Liang et al., 2011). Plant

tissue in water favors a number of physical

effects, such as filtering, increased rate of se-

dimentation and reduced risk of re-suspension

(Vymazal, 2011). The common wetland plants

often employed in HSSF CWs are emergent

macrophytes, such as Phragmitesaustralis and

Typhalatifolia. Oxygen is transferred from

their roots into the surrounding rhizosphere,

which facilitates aerobic degradation of pol-

lutants (Weaver et al., 2012). Many previous

studies showed that the aboveground and be-

lowground parts of Phragmitesaustralis pro-

vide large surface for the growth of microbes,

so it was used in CWs widely (Lee and Scholz,

2007). In CWs, the choice of substrate is of

major importance. On the one hand, the sub-

strates provide support for organisms and also,

storage for many contaminants (Name and

Sheridan, 2014). On the other hand, the dif-

ferent adsorption properties of many sub-

strates can be harnessed for the removal of

phosphorus and ammonia nitrogen (Calheiros

et al., 2008). The substrates used are mostly

natural, such as gravel, slag, sand and some

organic wastes, including building waste,

broken pottery, and cinder (Barca et al., 2014;

Wang et al., 2013). Among the substrate tested,

slag has shown a high capacity for phosphorus

adsorption and a suitable environment for mi-

crobial growth (Korkusuz et al., 2005).

Xi’an is the biggest megacity in the north-

west of China. Dry climate and insufficient

rainfall brings about limited base flow in river

channels. Zaohe River, which is near to Xi’an

is severely impacted by discharge of munici-

pal and industrial wastewater, where it appears

black and malodorous. Thus, implementing

low-technology systems like CWs is thought

to provide an appropriate solution for im-

proving the quality of the highly polluted river

water in Xi’an, China (Jin et al., 2008). How-

ever, the diverse sources of wastewater re-

ceived by the Zaohe River presents a high

level of variability with regard to the river

Y. Ge et al. / Journal of Water Sustainability 4 (2014) 247-258 249

water quality. Thus, the design and operation

of CWs have to deal with circumstances much

different from that of several previous wetland

studies. In this research, two on-site experi-

mental HSSF CWs with different substrates

(slag and gravel) were installed at the Zaohe

River deltas for the treatment of the river wa-

ter. The main objective was to compare the

pollutants removal efficiencies of the two

HSSF CWs with different substrates. Fur-

thermore, the study evaluated the nutrient up-

take and assimilation capacity of a local wet-

land plant and the phosphorus adsorption

characteristic of the substrates to select the

most appropriate wetland substrates for the

treatment of highly polluted river water in

northwest of China. It is hoped that this study

will provide important opportunities to gain

knowledge and experience for the design and

operation of a full-scale CW system to im-

prove the water quality of the Zaohe River.

2. MATERIALS AND METHODS

2.1 Description of pilot platform

In 2010, the two groups of pilot-scale HSSF

CW were constructed in the floodplain near the

confluence of the Zaohe River to the Weihe

River, in the northwest of Xi’an, China

(34o22’54”N, 108

o51’05”E) (Fig. 1). The area

has a warm temperate semi-humid continental

monsoon climate, with annual precipitation

between 584.9-732.9 mm and with half of the

precipitation concentration during autumn. The

annual temperature is 13.0-13.4˚C, while the

mean temperature during the growing season

(May-September) is 14-30˚C. As illustrated in

the Table 1, the area of each HSSF CW was

340 m2, with the same lengths and widths. The

two wetland beds were fill with locally avail-

able substrates, which were slag and gravel

with the same thickness, porosity and size,

respectively. The water depth in the two wet-

lands was controlled at 55 cm (5 cm beneath

the top of the substrates) and the bottom slope

was 0.5%.

The wetlands were planted with shoots of

Phragmitesaustralis (common reed), which

were obtained from the field near the river

bank, transplanted at a density of 9 roots/m2.

Even though the plants were not fertilized, they

grew up very fast in the first month as they

started to receive the highly polluted river

water.

Figure 1 Layout of the HSSF-slag and HSSF-gravel


Table 1 Details of size, filter material and surface vegetation of hybrid sub-surface CWs

CWs

Area

of bed

(m2)

Flow

rate

(m3/d)

Surface vegetation Substrate

Type

planting

density

(roots/m2)

Type Thickness

(cm)

Porosity

%

Size

(mm)

HSSF-slag 340 68 Phragmites

australis 9 Slag 0.6 50 1-70

HSSF-gravel 340 68 Phragmites

australis 9 Gravel 0.6 50 1-70

2.2 Operation of the wetlands

The construction of the two HSSF CWs was

completed in August 2010. After 3 months trial

operation, they were turned to continuous op-

eration from November 2010. Using a pump

with coarse screen, the highly polluted river

water was diverted from Zaohe River to an

elevated tank. With a hydraulic retention time

for about 4 h, the elevated tank also, performed

the function of a pre-settler for the removal of

solid substances. Then the settled polluted

river water was diverted from elevated tank to

the two wetlands by gravity via PVC pipes

with valves and flow meters for adjusting and

monitoring the flow rate. The two wetlands

received polluted river water at a flow rate of

68 m3/d. The theoretical hydraulic retention

time in both wetlands was calculated as 1.25 d.

2.3 Sampling, physical and chemical

analysis

The experimental period was lasted for about

323 days, the influent and effluent water sam-

ples from the two wetlands were collected

weekly to evaluate their treatment perfor-

mances. Water temperature, pH, dissolved

oxygen (DO) and Oxidation-Reduction Poten-

tial (ORP) were measured in-situ using a

potable meter (HQ30d53LEDTM, HACH,

USA). Water samples were sent to the chemi-

cal laboratory for analyses within 24 h. The

parameters analyzed include suspended solids

(SS), CODcr, BOD5, total nitrogen (TN), am-

monia-nitrogen (NH3-N), nitrate-nitrogen

(NO3--N), total phosphorus (TP) and ortho-

phosphates (PO43-

-P). In order to investigate

the dissolved substances, 0.45 μm membrane

filters were used to fractionate the pollutants to

suspended and dissolved parts.

2.4 Plant biomass quantification and

analysis

Plant growth analysis was carried out by de-

termining plant height every month during the

experimental period. At the end of experi-

mental period, the aboveground vegetation in

the two wetlands was harvested. The fresh and

dry weight of the aboveground plant parts

(stems, leaves and flower) were determined.

The biomass dry weight was calculated by

drying the selected harvested plants, which

were washed with distilled water first and

oven-dried at 80˚C for 48 h to a constant dry

weight. All of the dry matters were milled to

pass a 25 mm screen. The total nitrogen and

phosphorus contents were analyzed by the

routine analysis method for soil

agro-chemistry (Bao, 2000). Plant uptake of

nitrogen and phosphorus was estimated by

multiplying the total dry biomass of the sys-

tem by the specific ratio of nutrients per dry

biomass.

2.5 Physicochemical and adsorption

properties of the substrates

Physicochemical properties. The surface

morphology and microstructure of the two

substrates were examined by scanning electron


microscopy (SEM, JEOL, JSM-5800) to vi-

sualize inner porosity, surface properties, and

the potential environment for biofilm bacterial

attachment and growth. The energy-dispersive

X-ray (EDX) was combined with SEM to de-

termine the major elemental chemical compo-

sition mass percentage of each substrate.

Adsorption study. In this study, the phos-

phorus adsorption capacity of the two sub-

strates, including slag and gravel were esti-

mated. For the batch sorption capacity valua-

tion, potassium phosphate monobasic salt

(KH2PO4), which is a strong electrolyte and

dissociates in solution easily, was dissolved in

0.02 M KCL to prepare stock solutions. The

experiments were performed by adding 3 g of

substrate material (particle, 0.5-1 mm) into a

150 mL Erlenmeyer flask containing 50 mL of

standard phosphorus stock solution at nine

initial different concentrations (2, 5, 10, 20, 50,

100, 150, 200, 400 mg/L). Parallel treatments

were performed for individual P-concentrations. The Erlenmeyer flasks were

shaken for 48 h at 25˚C and 150 rpm. The su-

pernatants in the Erlenmeyer flask were fil-

trated with 0.45 μm membrane filters and

ready for phosphorus measurement. Separate

equilibrium adsorption studies were conducted

in order to determine the isotherm constants

and regression coefficients were obtained us-

ing the different isotherm models, which are

Langmuir and Freundlich isotherm models.

2.6 Statistical analyses

Statistically significant differences were de-

termined at α=0.05, unless otherwise stated.

Comparisons of means were by paired samples

t-test and one-factor analysis of variance. All

statistical analyses were performed by IBM

SPSS Statistics 20 (IBM Corporation, Armonk,

NY, USA) and Microsoft Excel.

3. RESULTS AND DISCUSSION

3.1 Characterization of the influent river

water

The layout of the two system was showed in

Fig. 1, the pollute river water was pumped

from Zaohe River located in the west suburb

of Xi’an. The water quality monitoring results

in 323 days study period at the inflow of the

receiving tank are presented in Table 2. The

influent in Table 2 means the effluent from the

elevated tank. The annual average (±SE) con-

centrations of SS, COD, BOD, NH3-N, TN

and TP were 305.16±20.7, 325.6±13.1,

102.5±5.9, 29.92±1.11, 39.0±1.0 and 3.4±0.1

mg/L, respectively, indicating that the river

water quality was unexpectedly dirty, with

pollutant concentrations similar to that of

sewer water. The cause of this extreme pollu-

tion situation of the Zaohe River, as stated

above in the introduction is its current func-

tion as an urban drainage channel, which re-

ceived urban runoff, treated domestic effluent

and untreated industrial wastewater.

3.2 Pilot-scale CWs performance

The Fig. 2 illustrated that the HSSF with slag

as the substrate apparently achieved better SS,

COD, BOD and TP removals (p≤0.05), prob-

ably due to the aggregation of microbes on the

coarser surface of the slag, which assisted the

removal of particulate and colloidal sub-

stances, such as SS and part of the COD and

BOD, by aerobic and/or anaerobic hetero-

trophic bacteria in the wetlands (Dong and

Reddy, 2010). It can be seen that for nitrogen

removal (NH3-N and TN) there was almost no

difference (p>0.05) between the two cells,

because the most important nitrogen removal

process in wetlands would be nitrification and

denitrification, and the identical subsurface

flow conditions might have provided similar

anaerobic circumstance. The removal rates

observed in both systems were similar to that


recorded in other large-scale studies (Comino

et al., 2011), but slightly lower than that pub-

lished in small scale or lab-scale experiments

(Ávilaet al., 2013). At the beginning of this

experiment, the systems were less stable, after

about month, the two systems became more

stable. Compare to conventional activated

sludge process, the construction cost of HSSF

was low which was reported in previous study

(Chan et al., 2008).

3.3 Reduction of dissolved pollutants

Because both HSSF CWs performed highly

for the removal of SS from the influent, most

of the suspended pollutants were effectively

removed as well. The dissolved substances,

however, proved difficult to remove.

Carbonaceous matter. Fig. 3a, b shows the

organic components in the influent and the

effluent water from HSSF-slag and

FSSF-gravel. The suspended COD and BOD5,

which account for 62% and 53% of total COD

and BOD5, respectively, were higher than the

dissolved COD and BOD5. The suspended

carbonaceous matter was effectively removed

by the filtration and sedimentation action of

the wetland beds with both substrates. About

73.5% and 70.1% suspended COD and 82.23%

and 75.05% suspended BOD was effectively

removed in the two HSSF. The advantage of

using slags as substrates over gravels was more

obvious regarding dissolved COD and BOD.

The reason was that HSS-slag can provide a

beneficial environment for microbial growth.

Because the influent was diverted from Zaohe

River, which received industrial discharge

along the way of the flow, the influent con-

tained high concentrations of dissolved COD,

which was difficult to be degraded.

Table 2 Average concentrations (±SE) of water quality parameters of influent and effluent

during treatment in two CWs

Parameter Unit Influent

concentration

HSSF-slag HSSF-gravel

Effluent

concentration

R.E.%* Effluent

concentration

R.E.%

pH - 7.39±0.04 8±0.09 - 7.23±0.17 -

DO mg/L 0.23±0.04 2.57±0.5 - 2.05±0.57 -

ORP mV -209.48±9.05 -146.75±35.9 - -102.4±19.14 -

SS mg/L 305.16±20.7 38.95±8.03 87.23 45.33±9.7 85.15

T ˚C 18.85±1.05 17.18±1.61 - 17.26±1.69 -

TN mg/L 39.04±1.02 26.79±0.8 31.4 28.2±1.14 27.8

NH3-N mg/L 29.92±1.11 22.29±0.8 25.5 22.58±1.04 24.5

NO3-N mg/L 0.55±0.05 0.55±0.8 - 0.44±0.4 -

Org-N mg/L 8.47±1.12 3.25±0.74 61.58 4.07±0.86 51.91

TP mg/L 3.4±0.13 1.56±0.1 53.9 2.18±0.11 35.7

PO4-P mg/L 1.65±0.09 1.31±0.1 20.74 1.51±0.12 8.84

COD mg/L 325.6±13.1 97.2±4.08 70.16 134.0±10.86 58.85

BOD mg/L 102.5±5.9 16.94±2.28 83.47 31.53±4.83 69.25

TOC mg/L 94.3±27.2 12.5±2.3 86.8 17.15±2.5 81.82

Note: *R.E.%: Removal efficiency


Figure 2 The concentrations of water quality parameters of influent and effluent in two CWs

during one-period

Figure 3 Distribution and removal of dissolved and suspended pollutants by the CWs

0

100

200

300

400

Influent HSSF-slag HSSF-gravel

CO

D (

mg

/L)

Suspened Dissolved

0

30

60

90

120

150


BO

D (

mg

/L)

Suspended Dissolved

0

10

20

30

40

50


TN

(m

g/L

)

Suspended Dissolved

c.

0

1

2

3

4


TP

(m

g/L

)

Suspended Dissolved

d.

a. b.


Nitrogen. TN in the influent water was

composed of 77% of dissolved TN, with the

main constituent being NH3-N, which account

for 99% of dissolved TN (Fig. 3c). Organic

nitrogen was the main component of sus-

pended TN. 58.3 % and 56.0% suspended TN

was effectively removed in the two HSSF

with SS removal. A part of the dissolved TN

was absorbed by wetland plants; the other part

was mainly removed by nitrification and deni-

trification. The lower DO concentration

(0.23±0.04) of influent led to the lack of oxy-

gen for nitrification of ammonia to nitrate or

nitrite. As such the removal rates of dissolved

TN were lower, at only 23.4% and 1.3% in

HSSF-slag and HSSF-gravel, separately.

Phosphorus. As illustrated in Fig. 3c., ob-

vious differences were observed between the

two HSSF wetlands for removal of suspended

TP in the influent (p≤0.5). HSSF-slag

achieved higher removal efficiency, which

was 89% and that of HSSF-gravel was only

64.4%. The main reason was the different ad-

sorption capacities of the two substrates for

suspended TP removal. Compared to gravel,

slag has bigger specific surface area and its

adsorption ability is much higher, so it

achieved higher removal rate for suspended

TP from influent. The dissolved TP was hard-

er to remove that removal efficiencies less

than 7% were achieved in the two HSSF CWs.

A part of dissolved TP was removed by wet-

land plants uptake and assimilation, and the

microbial action played a limited role in dis-

solved TP removal.

3.4 The nutrients uptake by plants

The two CW systems did not show much dif-

ference in the growth of plants. Table 3 shows

the dry matter production for the aboveground

parts of the plants in the two HSSF was 1.47

and 1.41 kg/m2, respectively for HSSF-slag

and HSSF-gravel. Furthermore, the concentra-

tion of TN and TP were 29.9 and 2.9 mg/g in

HSSF-slag and 29.2 and 2.8 mg/g in

HSSF-gravel. There were no significant dif-

ferences between the two HSSF CWs. Table 4

shows that the proportion of dissolved TN and

TP removals contributed by plants uptake and

harvesting were only 2.0-2.1% and 3.4-3.6%,

respectively. Thus in two substrate HSSF

wetlands, the effect of plants accumulation of

nutrients was limited; the removal by sub-

strates was the main removal pathway.

3.5 Phosphorus removal by substrates

Substrate characterization. As it is well

known, adsorption and precipitation by sub-

strate constitute the main sink for phosphorus

in CWs (Arroyo et al., 2013). Previous re-

search have pointed out that phosphorus easily

reacts with calcium, iron, and aluminum ions

released by substrates and precipitate as inso-

luble compounds in the interstitial water of

constructed wetlands (Zhao, 2009). Table 5

shows the major elemental chemical composi-

tion mass percentage of substrates used in this

study, the percentages of aluminum and cal-

cium was higher in slag than those in gravel.

Particularly for calcium, it was 19.57% in slag,

but only 0.73% in gravel, which could also

bring about effective removal of phosphorus

by physicochemical actions.

Phosphorus adsorption. Results from batch

tests of phosphorous adsorption to determine

the adsorption isotherms are presented in Fig.

4. The phosphorus adsorbed to the substrates

drastically increased with the increases of ini-

tial phosphorus concentration (0-400). Under

the same P-concentration, slag performed bet-

ter, with higher adsorption than gravel.

Langmuir and Freundlich isotherm models

were used to describe the adsorption equili-

brium in this study. From the correlation coef-

ficients in Table 6, it is shown that the expe-

rimental data of slag could be described by the

Langmuir isotherm equation with higher coef-

ficients of correlation than that of gravel.


Therefore, it can be seen that surface adsorp-

tion and diffusion into particles functioned

together for phosphorous adsorption. The slag

showed a Qe of 3.15 mg/g, which was nearly 4

times that of gravel (0.81 mg/g). The higher

calcium and aluminum content of slag was a

good reason to explain this (Li, 2013). The

constant b·Qe is always used to assess adsorp-

tion buffer capacity of adsorbents. As showed

in Table 6, the constant b·Qe of slag was high-

er than that of gravel, which illustrated that

the phosphorus adsorption of slag is much

more stable than grave.

Table 3 Biomass and nutrient concentration in plants at the end the research period

CWs Roots/m2 Biomass concentration

Dry matter(kg/m2) N(mg/g) P(mg/g)

HSSF-slag 160 1.47±0.07 29.9±2.1 2.88±0.21

HSSF-gravel 152 1.41±0.01 29.16±1.8 2.8±0.2

Table 4 Nutrient mass balance and proportion of the plant uptake in the removal of nutrient in

the pilot CW system during the research period

CWs Parameter Influent Effluent Plants CWs bed Plants uptake

(%)

HSSF-slag TDN(g/m2) 2055.98 1575.89 43.05 437.04 2.0

TDP(g/m2) 115.03 94.33 4.15 16.55 3.4

HSSF-gravel TDN(g/m2) 2055.98 1658.70 41.22 356.06 2.1

TDP(g/m2) 115.03 107.55 3.96 3.52 3.6

Table 5 Major elemental chemical composition mass percentage of substrates

Element C O Na Mg Al Si K Ca Ti Fe Total

Slag 4.38 48.29 - 2.39 6.40 12.00 - 19.45 0.49 6.60 100.00

Gravel - 51.61 1.11 1.01 8.79 27.58 4.05 0.73 - 5.12 100.00

Figure 4 Adsorption isotherms for phosphorus by slag and gravel

0

0.5

1

1.5

2

2.5

3

3.5

0 50 100 150 200 250 300 350 400

q(m

g/g

)

Ce mg/L

Gravel

Slag


Table 6 Adsorption isotherms equations of slag and gravel and coefficient of determination

Substrates Langmuir Freundlich

b Qe/(mg/g) b·Qe R2 k n R2

Slag 0.074 3.15 0.233 0.956 0.674 3.406 0.974

Gravel 0.006 0.81 0.005 0.993 0.019 1.718 0.966

CONCLUSIONS

In order to improve the water quality of the

Zaohe River, an urban river polluted with mu-

nicipal and industrial wastewater, two HSSF

CWs with different substrates, slags and gra-

vel were evaluated to compare removal effi-

ciencies and determine the most appropriate

for potential use in a full-scale system. For the

one year study period, the following conclu-

sions were made:

Firstly, the HSSF-slag performed better in

SS, COD, BOD, NH3-N, TN and TP removal

than the HSSF-gravel.

Secondly, the suspended contaminants were

efficiently removed with SS removal in the

HSSF wetlands.

Thirdly, the substrates did not have any

considerable effect on plant growth, and

2.0-2.1% dissolved TN and 3.4-3.6% dis-

solved TP were accumulated in the plants.

Slags were proven to be preferable as sub-

strates for HSSF CWs for enhancing pollu-

tants removal from the polluted river water,

especially total phosphorus.

Overall, this study showed that high pollu-

tants removal rates can be achieved in HSSF

CWs with gravel or slag as substrate. The dif-

ferences in the removal performances of

phosphorus have resulted from the physical

structures and the chemical compositions of

the individual substrates. Based on 323 days

operating, the experimental results indicated

that the properly designed and operated con-

structed wetlands could also be used for highly

polluted river water treatment in Xi’an, China.

ACKNOWLEDGEMENTS

This research has been funded by the National

Program of Water Pollution Control (Grant No.

2014ZX07323-001-02, 05), Social develop-

ment key project of Shaanxi Province (Grant

No. 2011KTZB03-03-03) and the Program for

Innovative Research Team in Shaanxi (Grant

No. IRT 2013KCT-13).

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