Development of Constructed Wetlands

8/16/2019 Development of Constructed Wetlands

1/16

Review

Development of constructed wetlands

in performance intensifications for wastewater

treatment: A nitrogen and organic matter targeted

review

Shubiao Wua,

*, Peter Kuschkb

, Hans Brixc

, Jan Vymazald

, Renjie Donga

aCollege of Engineering, China Agricultural University, Qinghua Donglu 17, Haidian District, 100083 Beijing,

PR Chinab Department of Environmental Biotechnology, Helmholtz Centre for Environmental Research e UFZ,

Permoserstrasse 15, Leipzig D-04318, GermanycDepartment of Bioscience, Aarhus University, Ole Worms Allé 1, 8000 Aarhus C., DenmarkdFaculty of Environmental Sciences, Czech University of Life Sciences Prague, Kymýcká 129, 165 21 Praha 6,

Czech Republic

a r t i c l e i n f o

Article history:

Received 23 December 2013

Received in revised form

19 February 2014

Accepted 9 March 2014

Available online 19 March 2014

Keywords:

Constructed wetlands

Wastewater treatment

Performance enhancement

Operation strategy

a b s t r a c t

The knowledge on the performance enhancement of nitrogen and organic matter in the

expanded constructed wetlands (CWs) with various new designs, configurations, and

technology combinations are still not sufficiently summarized. A comprehensive review is

accordingly necessary for better understanding of this state-of-the-art-technology for op-

timum design and new ideas. Considering that the prevailing redox conditions in CWs

have a strong effect on removal mechanisms and highly depend on wetland designs and

operations, this paper reviews different operation strategies (recirculation, aeration, tidal

operation, flow direction reciprocation, and earthworm integration), innovative designs,

and configurations (circular-flow corridor wetlands, towery hybrid CWs, baffled subsurface

CWs) for the intensifications of the performance. Some new combinations of CWs with

technologies in other field for wastewater treatment, such as microbial fuel cell, are also

discussed. To improve biofilm development, the selection and utilization of some specific

substrates are summarized. Finally, we review the advances in electron donor supply to

enhance low C/N wastewater treatment and in thermal insulation against low temperature

to maintain CWs running in the cold areas. This paper aims to provide and inspire some

new ideas in the development of intensified CWs mainly for the removal of nitrogen and

organic matter. The stability and sustainability of these technologies should be further

qualified.

ª 2014 Elsevier Ltd. All rights reserved.

* Corresponding author. Tel.: þ86 10 62737852; fax: þ86 10 62736067.E-mail addresses: [email protected], [email protected] (S. Wu).

Available online at www.sciencedirect.com

ScienceDirect

j o u r n a l h o m e p a g e : w w w . e l s e v i e r . c om / l o c a t e / w a tr e s

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5

http://dx.doi.org/10.1016/j.watres.2014.03.020

0043-1354/ª 2014 Elsevier Ltd. All rights reserved.

mailto:[email protected]:[email protected]://www.sciencedirect.com/science/journal/00431354http://www.elsevier.com/locate/watreshttp://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://www.elsevier.com/locate/watreshttp://www.sciencedirect.com/science/journal/00431354http://crossmark.crossref.org/dialog/?doi=10.1016/j.watres.2014.03.020&domain=pdfmailto:[email protected]:[email protected]


2/16

Contents

1. Introduction . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 41

2. Operation strategies for performance intensification . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.1. Effluent recirculation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 42

2.2. Artificial aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 432.3. Tidal operation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 44

2.4. Drop aeration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.5. Flow direction reciprocation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.6. Earthworm integration . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

2.7. Bioaugmentation . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 46

3. Configuration innovations to enhance performance . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.1. Circular-flow corridor CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.2. Towery hybrid CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.3. Baffled subsurface-flow CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

3.4. Microbial fuel cell CWs . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 47

4. Supply of electron donors to enhance the removal of selected inorganic oxygenated anions . . . . . . . . . . . . . . . . . . . . . . . . 48

4.1. Organic carbon added CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.2. Organic filtration media CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 484.3. Episediment layer-integrated CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 48

4.4. Step-feeding CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

4.5. Autotrophic denitrification-driven CW . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

5. Specific soil material selection for microbial biofilms establishment . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

6. Thermal insulation in cold climate . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 49

7. Conclusions . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

Acknowledgments . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

References . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . . 51

1. Introduction

The constructed wetlands (CWs) for wastewater treatment,

also known as treatment wetlands, are engineered systems

designed and constructed to utilize natural processes and

remove pollutants from contaminated water within a more

controlled environment (Faulwetter et al., 2009; Vymazal,

2011a). These systems have developed rapidly over the last

three decades, and CWs have been established worldwide as

an alternative to conventional more technically equipped

treatment systems for the sanitation of small communities

(Garcia et al., 2010). These systems are robust, have lowexternal energy requirements, and are easy to operate and

maintain, which makes them suitable for decentralized

wastewater treatment in the areas that do not have public

sewage systems or that are economically underdeveloped

(Brix, 1999; Vymazal, 2009).

The technology of wastewater treatment by CWs was

especially spurred on by Ka ¨ the Seidel in the 1960s (Seidel,

1961) and by Reinhold Kickuth in the 1970s (Kickuth, 1978;

Brix, 1987). At the early stage of CW development, the appli-

cation of CWswas mainlyused forthe treatment of traditional

tertiary and secondary domestic/municipal wastewater

(Kivaisi, 2001) and was often dominated by free-water-surface

CWs in North America and horizontal subsurface-flow (HSSF)

CWs in Europe and Australia (Brix, 1994b; Vymazal, 2011a).

Aiming at inexpensive and effective ecological wastewater

purification, CW development has received great attention

from both scientists and engineers in the last decades. The

application of CWs has also been significantly expanded to

purify agricultural effluents (Zhao et al., 2004b; Wood et al.,

2007), tile drainage waters (Borin and Toccheto, 2007;

Kynka ¨ a ¨ nniemi et al., 2013), acid mine drainage (Wieder,

1989), industrial effluents (Mbuligwe, 2005; Calheiros et al.,

2012), landfill leachates ( Justin and Zupancic, 2009), aquacul-

ture waters (Trang and Brix, 2014), and urban and highway

runoff (Scholes et al., 1999; Istenic et al., 2012).

The removal of contaminants in CWs is complex and de-pends on a variety of removal mechanisms, including sedi-

mentation, filtration, precipitation, volatilization, adsorption,

plant uptake, and various microbial processes (Vymazal, 2007;

Kadlec and Wallace, 2009; Faulwetter et al., 2009). These pro-

cesses are generally directly and/or indirectly influenced by

the different loading rates, temperatures, soil types, operation

strategies and redox conditions in the wetland bed

(Biederman et al., 2002; Stein et al., 2003; Stein and Hook, 2005;

Yang et al., 2011). Given the fast urbanization and the land

protection for crop production, natural passive CWs cannot be

fully promoted because of the large area requirement. The

number of research groups that study how these factors

perform in the contaminant removal in CWs has dramatically

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5 41

http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020


3/16

increased in recent years. Similarly, the volume of knowledge

and information published in international journals and

books on minimizing the influences of these factors and

possible solutions suggested to improve the treatment per-

formance has increased considerably. Better understanding of

the intensified removal processes responsible for water

treatment has expanded concurrently with CW usage and has

led to a great variety of designs and configurations, such asaerated subsurface-flow CWs (Nivala et al., 2007, 2013b),

baffled subsurface-flow CWs (Tee et al., 2012), and combina-

tions of either various types of CWs (Vymazal, 2013) and/or

with other technologies, to enhance the performance of CWs

for wastewater treatment [e.g., microbial fuel cell (MFC) and

electrochemical oxidation] (Grafias et al., 2010; Yadav et al.,

2012) (Fig. 1).

The main objective of this paper is to review and discuss

the recent developments in CW technology considering a wide

range of expanded designs, configurations, and combinations

with other technologies for the enhancement of wastewater

treatment, mainly targeted on the removal of nitrogen and

organic matter. By this study, new ideas should be inspired.

2. Operation strategies for performanceintensification

2.1. Effluent recirculation

Effluent recirculation has been proposed by various authors

(Sun et al., 2003; Arias et al., 2005; He et al., 2006a,b ) as an

operational modification to improve the effluent quality of

CWs (Table 1). The concept of this method consists of

extracting a part of the effluent and transferring it back to

the inflow of the system. The main goal of effluent recircu-

lation is to enhance aerobic microbial activity through the

intense interactions between pollutants and micro-

organisms, which are close to the plant roots and onto the

substrate surface, without significant alterations in the

Fig. 1 e Intensified constructed wetlands (a, artificial aerated CW modified with graphical components from Wallace and

Knight (2006); b, drop aerated CW modified with graphical components from Wallace and Knight (2006) and from Zou et al.,

2012; c, baffled flow CW modified with graphical components from Wallace and Knight (2006); d, step feeding CW modified

with graphical components from Wallace and Knight (2006); e, hybrid towery CW modified from Ye and Li, 2009; f, circular-

flow corridor CW modified from Peng et al., 2012 ).

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 542



4/16

system operation (Zhao et al., 2004b). As shown in Table 1,

the application of recirculation mostly occurs in subsurface

flow CWs including horizontal subsurface flow CWs, vertical

flow CWs and tidal flow CWs. Moreover, the recirculation

ratio varies from 0.5 to 2.5. Prost-Boucle and Molle (2012)

investigated the use of recirculation on a single French

vertical-flow CW to replace the classical French vertical-flow

CWs, which generally comprise two stages of treatment.

Considering a total surface of 1.1 m2 /p.e. to 1.6 m2 /p.e. on

the studied recirculated single-stage vertical-flow CW, thetreatment performance is similar to that obtained on a

classical French system with two successive stages for a

total surface of 2 m2 /p.e.; this result indicates the positive

effect of recirculation on the performance enhancement in

CWs for wastewater treatment (Prost-Boucle and Molle,

2012). The application of recirculation in hybrid CWs in

serially operated with horizontal and vertical-subsurface-

flow CWs has also been proved to be effective in total N

(TN) removal enhancement (Arias et al., 2005; Ayaz et al.,

2012). Lavrova and Koumanova (2010) recommend that the

recirculation ratio should be considered for the proper

design of CWs by investigating the influence of recirculation

in a lab-scale vertical-flow CW on the treatment of landfillleachate. However, effluent recirculation may cause prob-

lems in horizontal-flow CWs given the increased hydraulic

load, whereas it is suggested as an easily applicable and

effective method in the vertical-flow systems with high

hydraulic conductivity values (Laber et al., 1997; Brix and

Arias, 2005). Stefanakis and Tsihrintzis (2009) studied the

effect of effluent recirculation on the removal efficiency of

pilot-scale HSSF CWs. Their results obtained do not support

the idea that effluent recirculation can improve the removal

rates. The effluent recirculation negatively affected wetland

performance, which resulted in a reduction of all pollutant

removal rates. However, Arias et al. (2005) clearly docu-

mented that the recirculation of treated and nitrified

effluents from a vertical-flow CW enhanced TN removal by

denitrification when the nitrified re-circulated water was

mixed with untreated organic C-rich wastewater in the

inflow. This recirculation also removed other wastewater

constituents. The use of recirculation to enhance the per-

formance in CWs depends on many factors, including the

CW types and influent loads. Moreover, in full-scale oper-

ating facilities, this modification may increase operation

costs given additional energy consumption for pumping.

2.2. Artificial aeration

The poor oxygen transfer rates in traditional HSSF CWs often

restrict treatment efficiency. The energy inputs to CWs can

overcome oxygen transfer limitations to meet advanced

treatment standards (Austin and Nivala, 2009; Nivala et al.,

2013a). The aeration of CWs with compressed air ( Fig. 1 and

Table 2) (Nivala et al., 2007;Tang et al., 2009; Zhang et al., 2010)

requires about half of the power of an equally performing and

sized-activated sludge system for N removal (Austin and

Nivala, 2009). Even though the use of aeration was found in

both horizontal subsurface flow CWs and vertical flow CWs,

but still mostly in vertical flow CWs (Table 1). A significantimprovement of organic matter, ammonium as well as fecal

coliform bacteria (Escherichia coli) removal by using artificial

aeration has been indicated (Headley et al., 2013). However,

the effect of artificialaeration on the removal of phosphorus is

still not clear. Tang et al. (2009) applying aeration cycles of 8 h

daily, showed that the artificial aeration (dissolved oxygen

concentrations above 2 mg/L, and ORP of þ300 mV), increased

P removal to 50% in vertical flow CW. Moreover, Vera et al.

(2014) found a significant effect of aeration in the gravel me-

dium mesocosm-scale CW with an increase in up to 30% for

PO34eP removal. However, Tao et al. (2010) and Zhang et al.

(2010) found that artificial aeration did not have significant

influence ( p > .05) on P removal.

Table 1 e The application of recirculation in subsurface flow CWs treating various wastewaters.

CWtype

Scale WT Area(m2)

HLR(L/m2d)

Recycleratio

COD NH4eN Remarks Reference

In (mg/L) Out(mg/L)

% In (mg/L) Out(mg/L)

%

VF Pilot D 2.25 168 44 0.6 438 88 68 36 85 58 9 16 5 72 1

VF Pilot P 1 40 1 613e1193 43 529e1005 81 Zeolite 2

VF Pilot P 1 40 2.5 613e

1193 48 529e

1005 92 Zeolite 2VF Pilot P 1 40 5 613e1193 47 529e1005 95 Zeolite 2

VF Full D 0.4 m/d 1 736 240 73 7 92 48 5 15 2 77 3

VF Full D 0.4 m/d 0.5 867 127 146 11 90 70 5 33 10 57 3

HF Pilot O 45.5 69 1 6684 685 90 16.2 7.3 55 4

HF Pilot S 2.25 0.5 458.4 63.6 85 25.1 14.9 38 5

VF Pilot P 4 100 0.25 440.5 190.3 56.8 111.6 64.4 42.3 6

VF Pilot P 4 100 0.5 410.6 136.8 66.7 101.5 56.9 43.9 6

VF Pilot P 4 100 1 360.6 93.4 74.1 94.5 40.6 57 6

VF Pilot P 4 100 1.5 330.5 61.8 81.3 85.9 32.9 61.7 6

TF Lab P 0.028 420 1 1359 337 75.2 121 63 47.9 7

TF Lab L 0.028 430 1 2464 77.3 121 61.8 8

Recycle ratio was defined as the recirculated volume/influent volume. The COD data shown in the table referred from He et al., 2006a,b and Sun

et al., 2005 is provided in BOD. WT means wastewater type including domestic (D), piggery (P), olive mill (O), synthetic (S), artificial leachate (L)

wastewater.

Reference: 1, (Foladori et al., 2013); 2, (Huang et al., 2013); 3, (Prost-Boucle and Molle, 2012); 4, (Kapellakis et al., 2012); 5, (Stefanakis and

Tsihrintzis, 2009); 6, (He et al., 2006a,b); 7, (Sun et al., 2005); 8, (Zhao et al., 2004b).

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5 43



5/16

Although the oxygen input from the plant roots is quite

limited compared to the artificial aeration, the role of plants

cannot be replaced (Brix, 1994a; Vymazal, 2011b). Ouellet-

Plamondon et al. (2006) investigated the effects of vegetation

and artificial aeration on the pollutant removal performance

of CWs. The results indicate that the artificial aerationimproved TKN removal for the unplanted units in both sum-

mer and winter. However, the additional aeration did not fully

compensate the absence of plants, which suggests that the

role of macrophytes is beyond the sole addition of oxygen in

the rhizosphere (Ouellet-Plamondon et al., 2006).

The artificial aeration in subsurface flow CWs performedin

continuous mode (i.e., 24 h per day) can lead to the contra-

diction between the removal of ammonium nitrogen (NH4eN)

and TN because of the lack of favorable anaerobic conditions

for denitrification. Moreover, the operation costs also remain

questionable. Intermittent aeration appears to be an effective

method to achieve high TN removal by providing alternate

aerobic/anaerobic conditions for the simultaneously occur-ring nitrification and denitrification. The intermittent aeration

is also much energy-economic than the continuous mode. Fan

et al.(2013a,b,c) reported an intermittent aeration SSFCW with

a removal efficiency of about 90% of ammonium (3.5 g/m2 d)

and 80% of TN (3.3 g/m2 d). Moreover, an extraordinary ni-

trogen removal performance with mean total nitrogen

removal efficiency of 90% under N loading rate of 46.7 gN/m2d

was demonstrated in a laboratory scale alum sludge-based

intermittent aeration CW (Hu et al., 2012a,b).

The decision to aerate SSFCWs leads to the additional costs

for operation and maintenance of the facility. Aeration is only

justified when its lifecycle cost is sufficiently offset by the

reduction in the capital cost by the net savings of reduced

wetland area size (Kadlec and Wallace, 2009). Wetland de-

signers should also consider the fouling of air diffusers within

CWs and the provisions for the cleaning or replacement of the

diffuser assemblies (Kadlec and Wallace, 2009)

Aside from improving the pollutant removal efficiencies,

artificial aeration also influences the solid accumulation inCWs (Chazarenc et al., 2009). Artificial aeration may have both

positive and negative effects. Aeration (gas bubbling) reduces

the settling of suspended solids, such that they can be better

flushed out fromthe system. Nevertheless,aeration alsocauses

higher microbial biomass yield. Artificial aeration also in-

creases microbial activities, leading to a change of both mi-

crobial community structure and diversity. Furthermore, this

method affects other processes inside the CW bed. Hence, the

long-termeffects of artificial aeration on CWs, such as clogging

etc., should be further investigated (Chazarenc et al., 2009).

2.3. Tidal operation

A method for solving the oxygen transfer limitations in

traditional CWs is the tidal-flow operation, which is charac-

terized by multiple periodical flood and drain cycles per day.

As wastewater fills and drains, air drawn into the soil pores

and rapidly oxygenates the bio- and remaining waterfilms

(Sun et al., 2007; Chan et al., 2008; Wu et al., 2011a,b ). Inten-

sified nitrification mainly occurs when the wetland bed

drains; thus, oxidizing ammonium ions that are adsorbed to

biofilms/soil particles dissolved in the remaining water on the

soil particle and root surface. Nitrate ions desorb into the bulk

water in subsequent flooded phase and are reduced to N gas

by denitrifiers with organic C as electron donor ( Austin, 2006).

The N removal is enhanced by the alternate aerobic and

Table 2 e The application of aeration in subsurface flow CWs treating various wastewaters.

CWtype

Scale WT Area(m2)

HLR(L/m2d)

COD NH4eN Aerationtype

Reference

In (mg/L) Out(mg/L)


%

VF Pilot D 2.25 158 17 438 88 52 17 86 58 9 20 4 69 Intermittent 1

VF Pilot D 6.2 95 233 76 5.0 4.4 54.9 16.6 0.5 0.3 2

VF Lab S 0.03 70 113 6 10 13 40 0.4 0.4 0.9 3

VF Lab S 0.03 70 217 13 11 7 40 0.9 0.3 0.5 3

VF Lab S 0.03 70 429 14 17 13 40 0.4 0.3 0.5 3

VF Lab S 0.03 70 836 17 22 13 40 0.4 1.7 1.0 3

VF Full D 2495 1600 53 29 31 19 50 5.14 3.10 85 4

VF Lab S 0.03 70 352 12 10 4 97 46.1 1.2 0.6 0.2 99 Continuous 5

VF Lab S 0.03 70 352 12 13 6 96 46.1 1.2 1.3 0.3 97 Intermittent 5

VF Lab R 0.018 190 65e158 20 80 3.5e10.6 1 87 Continuous 6

VF Lab R 0.018 190 65e158 25 78 3.5e10.6 1.9 78 Intermittent 6

VF Lab R 0.018 380 65e158 20 75 3.5e10.6 0.9 80 Continuous 6


VF Lab R 0.018 760 65e158 25 73 3.5e10.6 2.5 65 Continuous 6


HF Pilot D 2.1 65 570 72 94 0.9 35.7 9.7 89 7 Limited

aeration

7

HF Pilot D 2.1 65 570 72 87 4.4 35.7 9.7 72 11 Limited,

unplanted

7

WT means wastewater type including domestic (D), synthetic (S), and polluted river (R). The COD data shown in the table referred from Nivala

et al., 2013 is provided in BOD.

Reference: 1, (Foladori et al., 2013); 2, (Nivala et al., 2013b); 3, (Fan et al., 2013b); 4, (Pan et al., 2012); 5, (Fan et al., 2013a); 6, (Dong et al., 2012); 7,

(Zhang et al., 2010).

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 544



6/16

anaerobic environments. This technology has been demon-

strated in multiple studies and projects (Sun et al., 1999; Zhao

et al., 2004a; Chan et al., 2008; Abou-Elela and Hellal, 2012) and

requires about half of the power of aerated wetlands (Austin

and Nivala, 2009).

The literature reported application of tidal operational

strategy in CWs was summarized in Table 3. For this tech-nology, most investigations are conducted in laboratory scale,

and thus more pilot and even full scale measurements should

be further demonstrated for better understanding of the

mechanisms of pollutants removal. The performance of tidal-

flow CWs depends on many factors, such as flood drain ratios,

oxygen transfer, and substrate characteristics. Zhao et al.

(2004c) optimized five-stage identical tidal-flow CWs with

three different flood drain ratios in treating high-strength

agricultural wastewaters. The experimental results demon-

strate that the system produced the highest pollutant removal

efficiency with relatively short saturated period and long un-

saturated period, highlighting the importance of oxygen

transfer into reed-bed matrices (Zhao et al., 2004c). Moreover,

a pilot field-scale alum sludge-based CW operated in this tidal

flow mode showed significant enhanced capacity for phos-

phorus and organic matter removal from animal farm

wastewater (Zhao et al., 2011). However, with the filtration of

suspended solids and the accumulation of biomass, the reed-

bed matrices gradually clogged that affected the long-term

efficiency of the current tidal-flow reed-bed system (Zhaoet al., 2004c; Wu et al., 2011a,b).

The cation-exchange capacity (CEC) of aggregates (or

media) is proved to affect the treatment performance in tidal-

flow wetland-treatment systems. Higher CEC could stimulate

more ammonium adsorption during the flooded phase and

increase N removal. In a column study, an electrostatically

neutral, high-density polyethylene has been compared to

lightweight expanded shale aggregate with a CEC of approxi-

mately 4.0 meq/100 g. The results show that the CEC of ag-

gregates or media in flood and drain wetlands should be a

critical design criterion (Austin, 2006). Therefore, the selection

of substrates with high CEC should be emphasized in the

future. The longevity of the substrate and the influence of

Table 3 e The application of tidal operational strategy in subsurface flow CWs treating various wastewaters.

No. Scale WT Area(m2)

HLR(L/m2 d)

Fill anddrain timeratio (h:h)

COD NHþ4 eN Reference

In (mg/L) Out(mg/L)


%

1 Lab S 0.025 900 3:3 193 44 80 20 84 10 38 10 7 4 82 13 [1]

2 Lab S 0.025 900 3:3 193 44 28 15 82 8 75 6 13 3 74 13 [1]

3 Lab S 0.025 900 3:3 366 37 62 30 86 9 75 6 30 6 67 16 [1]4 Lab S 0.025 900 3:3 366 37 51 13 91 4 34 6 23 6 33 17 [1]

5 Lab P 0.112 210 1:3 2157 1716 20 104 98 6 [2]

6 Lab P 0.112 210 1:3 2157 1450 33 104 90 13 [2]

7 Lab P 0.112 210 1:3 2157 1142 47 104 81 22 [2]

8 Lab P 0.112 210 1:3 2157 918 57 104 76 27 [2]

9 Lab S 0.018 1200 1.5:0.5 200 26 40 80 20 3 1 941 [3]

10 Lab S 0.018 1200 1.5:0.5 200 26 40 80 20 3 1 951 [3]

11 Lab S 0.018 1200 1.5:0.5 200 26 100 50 20 3 3 872 [3]

12 Lab S 0.018 1200 1.5:0.5 200 26 190 5 20 3 e e [3]

13 Lab P 0.0071 430 1:3 2464 559 77.3 121 46 61.8 [4]

14 Lab P 0.008 1600 3:1 4254 1791 57.9 159.2 120.4 24.4 [5]

15 Lab P 0.008 1600 2:2 4254 1306 69.3 159.2 117.3 26.3 [5]

16 Lab P 0.008 1600 1:3 4254 617 85.5 159.2 81 39.0 [5]

17 Lab S 0.025 480 1.5:0.5 189.6 11.8 94 20.1 1.1 95 [6]

18 Lab S 0.025 480 1:3 246.7 50.1 79.7 27.2 20.4 24.9 [7]19 Lab S 0.025 480 2:3 246.7 23.9 90.3 27.2 10.5 61.4 [7]

20 Lab S 0.025 480 3:3 246.7 28.1 88.6 27.2 8.7 68.1 [7]

21 Lab S 0.025 480 4:3 246.7 36.0 85.4 27.2 11.5 57.9 [7]

22 Lab S 0.025 480 5:3 246.7 36.0 85.4 27.2 10.4 61.8 [7]

23 Lab W 0.328 22.5 e 30 22 23 24.4 1.0 95.5 [8]

24 Lab S 0.007 440 6.75:0.5 590 252 49 42 23.5 43 [9]

25 Lab S 0.007 440 5.75:1.5 436 133 65 46 13.9 70 [9]

26 Lab S 0.007 440 4.75:2.5 552 91 83 51 2.2 96 [9]

27 Lab S 0.007 440 4.75:2.5 207 78 62 55 3.3 94 [9]

28 Lab S 0.007 440 4.75:2.5 224 64 70 52 2.2 96 [9]

29 Lab S 0.007 440 4.75:2.5 464 81 82 50 2.5 95 [9]

30 Pilot P 40.03 120 e 2750 557 80 201 84 58 [10]

31 Pilot S 8.9 191 e 428 5.2 98.7 e e e [11]

32 Pilot D 13.2 0.15 1:0.5 206 84 3.4 3.8 98 49 18 4.3 10.5 91 12,13]

WT means wastewater type including piggery (P), demestic (D), synthetic (S), and secondary effluent from WWTP (W). The COD data shown in

the table referred from Wu et al., 2011a,b, Sun et al., 2005 and Nivala et al., 2013a and 2013b is provided in BOD.

Reference: 1, (Wu et al., 2011a,b); 2, (Sun et al., 2005); 3, (Lv et al., 2013a); 4, (Zhao et al., 2004a); 5, (Zhao et al., 2004c); 6, (Lv et al., 2013b); 7, (Wu

et al., 2010); 8, (Liu et al., 2012); 9, (Hu et al., 2014); 10, (Sun et al., 2006); 11, (Austin et al., 2003); 12, (Nivala et al., 2013a); 13, (Nivala et al., 2013b).

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5 45



7/16

biofilm on the surface of the selected substrate on the cation

exchange should also be investigated.

This tidal approach could also be used for partial nitrifi-

cation with following anaerobic ammonium oxidation

(anammox) in the case of ammonium-rich wastewaters with

low organic C content. Nevertheless, the process control

regarding limited ammonium oxidation to nitrite in solid filter

systems remains a challenge.

2.4. Drop aeration

Considering the low pollutant removal efficiency in conven-

tional CWs and limited oxygen transfer capability, a novel

vertical-flow CW system feed with drop-aerated influent has

been developed (Fig. 1) (Zou et al., 2012). The capacity of

enhanced oxygen transfer with a multilevel, two-layer drop

aeration and its corresponding pollutant treatment perfor-

mance has been investigated in two pilot-scale vertical-flow

CWs of 0.75 m2 each. The results demonstrate that compared

with the feed of direct drop aeration, the multilevel, two-layer

drop aeration supplied 2 mg/L to 6 mg/L higher dissolved ox-ygen in the influent per meter of drop height. After the

installation of the six-level, two-layer drop aeration, the five-

day biological oxygen demand (BOD5) removal load

increased from 8.1 g/m2 d to14.2 g/m2 d. As no any operational

problem occurred during the whole investigation period (Jan.

2009eMar. 2011), the vertical-flow CWs with drop aerated

influent seem to be an appropriate alternative for rural

wastewater treatment, with numerous advantages, such as

low capital and operation costs, easy maintenance, high hy-

draulic loading rate, high pollutant removal efficiency, and no

clogging. The drop aeration canwork well in subtropical zones

around the whole year and in moderate climatic zones during

the summer period. However, low temperature would freezethe influent dropping device in cold climates. Nuisance and

insect problems may occur because of the exposure of poorly

treated wastewater to the atmosphere.

2.5. Flow direction reciprocation

Horizontal subsurface CWs are widely used to treat waste-

water. However, their capacity is severely confined by clog-

ging problems, which are very common during the lifespan of

subsurface CWs. Shen et al. (2010) executed a new operation

mode by changing the flow direction periodically and studied

its performance on pollutant removal. The three year-

experimental results show that the CW with new operationmode achieved better pollutant removal efficiency than

traditional operation mode. The microorganism test shows

that the reciprocating flow direction had larger quantity

microorganism, which effectively prevented organic com-

pound accumulation. The readings of gauge glass in the

traditional SSFCW rose gradually, while the water level kept

stable in reciprocating one, which also reflected the severity of

the clogging problem in the two wetlands. During the whole

operation period, the SSFCW with reciprocating operation

mode did not have any infiltration problem, whereas the

SSFCW with traditional operation mode had visible clogging

problems as a result of the pollutant accumulation in the inlet

zone (Shen et al., 2010).

2.6. Earthworm integration

Owing to the high solid and organic matter contents in

wastewater, clogging potential is one of the major obstacles

for the efficient use of SSFCWs when treating high-strength

wastewaters. Kadlec and Wallace (2009) recommended that

cross-sectional BOD loading should be less than 250 g/m2 d for

the bed media with a d10 greater than 4 mm. Finer bed mediawould require an even lower cross-sectional loading, which is

still unable to be elucidated due to the limited data. As

earthworms play an important role in the ecological systems

because they can breakdown a wide range of organic mate-

rials, they are applied in a form of vermicomposting tech-

nology to treat swine manure and vermifiltration to purify

wastewater (Taylor et al., 2003). To solve the clogging problem

and help digest the solids associated with clogging within

CWs, they have also been integrated into SSFCWs in recent

years. Davison et al. (2005) state that the intentional intro-

duction of earthworms may offer a natural alternative for

cleaning clogged substrates in HSSF CWs. In lab- and pilot-

scale studies, this concept has been examined in terms of alleviating the clogging situation (Chiarawatchai et al., 2007;

Chiarawatchai and Nuengjamnong, 2009). The results show

that earthworms helped in reducing the sludge production on

the surface of the experimental vertical-flow CWs (40% by

volume), which resulted in lowering the operational costs

required to empty and treat sludge.

The introduction of earthworms in subsurface flow CWs

could also enhance the density and biomass of wetland

plants, resulting in higher N and P uptake (Xu et al., 2013).

However, given the limited nutrient content in plants, only a

minor difference has been reported in terms of removal effi-

ciency when comparing the unit with earthworms to the one

without earthworms (Li et al., 2011).

2.7. Bioaugmentation

The bioaugmentation in CWs is the supplementing of mi-

crobes that have certain favorable metabolic traits into

wetland beds to accelerate the biodegradation of pollutants

(Nurk et al., 2009; Merlin and Cottin, 2012). To achieve the

water purification efficiency that is typical of the mature CWs,

an adaptation period after the construction is generally

needed to develop the treatment capacity for N and C trans-

formations (Nurk et al., 2009). Bioaugmentation would be one

possibility for the shortening of the adaptation period to

accelerate the development of the necessary characteristics of the local microbial community. Bioaugmentation has also

been performed in CWs for intensifying the degradation of

some specific pollutants, such as pesticides (Runeset al., 2001)

and organic chemicals (Simon et al., 2004), and the removal of

heavy metals (Park et al., 2008), because the metabolic path-

ways of these functional bacteria are not highly present in the

environment. Adding a specially adapted microbial commu-

nity could generally yield positive results. Runes et al. (2001)

investigated the effect of bioaugmentation on small quanti-

ties of atrazine spill-site soil in CWs with a mineralization of

25e30% and compared with unbioaugmented CWs with an

atrazine mineralization rate of 2e3%. Zaytsev et al. (2011)

studied the effect of adding low concentrations of a

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 546



8/16

sediment/microbial community suspension into the wetland

bed to fasten the development of denitrification capacity in

the HSSF CWs during one year. The findings emphasize the

high variability of the bioaugmentation effect and its impor-

tance in a full-scale operation may be overshadowed by the

effect of other factors determining treatment performance.

3. Configuration innovations to enhance

performance

3.1. Circular-flow corridor CW

The application of CWs has been increased in the last decades

due to its cost-effectiveness and efficiency. However, some

operational problems arise if the conventional subsurface

flow wetlands were directly used for the treatment of high-

strength wastewaters, such as the inhibition of high influent

concentration ammonium on plants and deficiency of oxygen

for large amounts of organic matter degradation. Considering

the fact that the partial recirculation of treated wastewaters

within wetlands benefits the removal of TN, a circular-flow

corridor wetland has been developed in circular-flow opera-tional mode treating swine wastewater (Peng et al., 2012).

Several compartments connected in an annular corridor

(Fig. 1). An overflow weir used in the final compartment for

effluent collection could control certain amount of treated

water flow back to the inflow zone. For the treatment of high-

strength wastewater, such as swine wastewater, this circu-

lation can not only enhance TN removal but also dilute the

inflow water to avoid the negative effects for both plants and

microorganisms from high pollutant concentration. Interest-

ingly, the circular flow corridor CW was found to avoid the

adverse effect of low temperature on the removal perfor-

mance, possibly due to the internal circular-flow mode.

Moreover, this internal circular-flow mode delays the clogging of wetland porous media and increases the utility of released

Ca2þ and Mg 2þ from zeolite for P removal.

3.2. Towery hybrid CW

To enhance N removal, another novel CW configuration with

three stages, i.e., towery hybrid CW, has been designed (Ye

and Li, 2009). In this system, the first and third stages are

rectangular subsurface horizontal-flow CWs, and the second

stage is a circular three-layer free-water-flow CW (Fig. 1). The

increased dissolved oxygen concentration by the passive

aeration of a tower-type cascade overflow from the upper

layer into the lower layer in the second stage of the wetland

enhanced nitrification rates. Denitrification rates were also

improved by additional organic matter supplied as a result of

the bypass of influent directly into the second stage. The

average removal percentage was 89%, 85%, 83%, 83%, and 64%

for total suspended solid (TSS), chemical oxygen demand

(COD), NH4eN, TN, and TP, respectively. No significant dif-

ference was found at low and high hydraulic loads (16 and

32 cm/d) for performance. The nitrifying and denitrifying

bacteria as well as potential nitrification activity and potential

denitrification rates measurement show that nitrifica-tionedenitrification is the main mechanism for N removal in

wetlands.

3.3. Baffled subsurface-flow CW

A novel design for the horizontal subsurface flow CWs incor-

porating up and down flow sequentially has been developed

as baffled subsurface-flow CWs to enhance pollutant removal

(Tee et al., 2012; Wang et al., 2012). This design allows the

treatment of the pollutants under multiple aerobic, anoxic,

and anaerobic conditions sequentially in the same CW (Fig. 1).

This task is achieved by inserting vertical baffles along the

width of the wetland, which forces the wastewater to flow upand down instead of horizontally as it traveled from the inlet

to the outlet. The results show that the planted baffled unit

achieved 74%, 84%, and 99% NH4eN removal versus 55%, 70%,

and 96% for the conventional unit at hydraulic retention time

(HRT) of 2, 3, and 5 days, respectively (Tee et al., 2012). The

better performance of the baffled unit was explained by the

longer pathway because of the up-flow and down-flow con-

ditions sequentially, which allowed more contact of the

wastewater with the roots/rhizomes and micro-aerobic zones.

The changes in the total slope design because of the longer

water flow path must be considered.

3.4. Microbial fuel cell CWs

Microbial fuel cell consists of two chambers, anaerobic and

aerobic chambers, where oxidation and reduction reactions

occur. On the assumption that CWs also consist of aerobic and

anaerobic zones, this similarity in both technologies moti-

vated the combination of CWs with MFCs (i.e., CWeMFC)

(Yadav et al., 2012). A cathode electrode has been placed in the

upper near to the rooted zone of the wetland bed. This zone is

more aerobic than the deeper less rooted zone owing to the air

diffusion from the immediate outer atmosphere and the ox-

ygen leakage from the helophyte (emergent water plant) roots

(Schamphelaire et al., 2008). The anode has been placed near

to the bottom of microcosm CW with the idea that this zone

Table 4 e The performance of literature reported lab-scale microbial fuel cell CWs.

Wastewater Operation HRT (d) COD removal (%) Electricity peakproduction (mW/m2)

Reference

Dye Batch 4 65.0e75.0 9.95e15.73 Yadav et al., 2012

Swine Batch 10 73.6e75.1 0.013 Zhao et al., 2013

Batch 10 65.8e71.6 0.006

Continuous 1 76.5 9.40

Dye Continuous 3 85.7 30.20 Fang et al., 2013

Continuous-unplanted 3 82.7 19.10

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5 47



9/16

will be comparatively anaerobic and suitable for the anodic

reaction of the MFC. Experiments are limited only, and the

results of electricity production are quite variable (Table 4).

The role of plants has also been examined in two CWeMFC

systems for azo dye-wastewater treatment with/without the

vegetation of Ipomoea aquatica (Fang et al., 2013). The results

indicate that the plants around the cathode can foster the

output voltage of MFC given the enhanced oxygen concen-tration in the cathode. The effect of the artificial aeration of

cathode in CW-MFC has also been investigated and a signifi-

cant power density has been achieved (Zhao et al., 2013).

This integration of MFC with CWs bears the potential to

achieve the dual goals of power generation concomitantly and

advance wastewater treatment. Nevertheless, whether mini-

mal construction and operation costs in near future electricity

of an appropriate amount and economic relevance will be

produced by this integration remains a question. Although no

adverse effect of MFC on the ability of CW to fulfill its primary

objective of efficient wastewater treatment has been

observed, the responses of the structure and function of the

microbial community to the external circuit are also of sci-entific interest.

4. Supply of electron donors to enhance theremoval of selected inorganic oxygenated anions

The nitrite and nitrate in domestic sewage are easily reduced

by microorganism to N gas and leave the wastewater. How-

ever, oxygenated inorganic anions, such as sulfate (SO42), can

also be reduced, which can be technically applied for heavy

metal precipitation as the insoluble sulfides. Other industrial

chemicals, such as chlorate, perchlorate, chromate, and di-

chromate, that contaminate effluents, surface waters, andgroundwater can also be reduced and detoxified by micro-

organisms (Kosolapov et al., 2004). The nitrate nitrogen

ðNO3 eNÞ in CWs is removed mainly by plant uptake and

microbial denitrification, which is believed to be the domi-

nant and long-term mechanism, especially when nitrate-

loading rates are high (Lin et al., 2002). As the main mecha-

nism for removing nitrate in CWs, denitrification is an

anaerobic dissimilative pathway, in which an electron donor

is often needed, such as organic carbon. The carbon source in

the system of CWs usually comes from wastewater, soil, and

the rhizodeposition products of plants (Zhai et al., 2013). For

the CWs that receive poorly-treated secondary effluent, some

of the carbon required for denitrification is normally con-tained in the effluent. In contrast, nitrate-contaminated

groundwater would normally do not have labile carbon to

sustain denitrification, and 100% of the carbon required for

denitrification would have to come from the wetland. Wet-

lands could potentially use plant productivity, either from

biomass or root release, as the source of energy and C to

sustain denitrification (Zhai et al., 2013). To treat low C/N

ratio wastewaters, such as nitrate-rich agricultural runoff

and polluted groundwater, the carbon source only from the

root exudates of macrophytes is not sufficient to maintain a

high performance of nitrate removal (Davison et al., 2005; Lu

et al., 2009). This phenomenon derives the supply of the

electron donors externally.

4.1. Organic carbon added CW

The addition of various carbon sources, such as glucose, so-

dium acetate, methanol, starch, and cellulose, to enhance the

denitrification rate in wetlands has been investigated in the

last decades (Sirivedhin and Gray, 2006; Lu et al., 2009). Lin

et al. (2002) established several microcosm wetlands to

investigate the effects of vegetation and externally addedorganic matter on nitrate removal from groundwater in CWs.

The results showed that the planted wetlands exhibited

significantly greater nitrate removal than the unplanted wet-

lands, indicating that macrophytes fostered efficient nitrate

removal. Although adding external carbon to the influent

improved the nitrate removal, a significant fraction of the

added carbon was lost via other microbial processes (e.g.,

oxidation) in the wetlands and it obviously increased the

costs.

4.2. Organic filtration media CW

The limitation of costly external carbon addition fosters theexploration of employing low-cost alternatives in wetland

systems for the enhancement of denitrification. Solid organic

materials, rich in organic carbon, are one of the possible op-

tions to meet the demandof electron donors in denitrification.

Saeed and Sun (2011) conducted a comparative evaluation of

different materials (i.e., gravel, organic wood mulch, and

mixture of gravelewood mulch) on N removal in six lab-scale

CWs, including both vertical and horizontal units. Higher

removal efficiencies in the vertical-wetland columns with

organic mulch substrate was observed for both BOD5 and TN,

which was primarily caused by the enhanced oxygen transfer

for nitrification and the organic carbon from the wood mulch

substrate for heterotrophic denitrification. Among thehorizontal-flow CWs, conventional gravel substrate was the

most efficient for the removal of NH4eN and organic matter.

By contrast, the other two horizontal-flow CWs, which

employed wood mulch and gravelemulch media, caused net

increases in organics, phosphorus, and TSS in the synthetic

wastewater. Overall, the results demonstrate the potential of

using organic materials in vertical-flow CWs to enhance TN

removal, but the organic materials should not be used in

horizontal-flow systems.

4.3. Episediment layer-integrated CW

An episediment zone in surface flow wetland microcosms hasbeen designed to test whether the variations in the macro-

porous structure of the denitrification zone affect the overall

nitrate removal (Fleming-Singer and Horne, 2002). The epis-

ediment zone is a distinct layer of loosely aggregated litter

pieces placed at the top of the sediment matrix. The results

show that the average denitrification is 33% greater in the

episediment treatment than in the sedimentation treatment

only. The analysis of vertical nitrate profile data using diffu-

sive and turbulent mixing models indicates that about 40% of

the nitrate removal occurs in the episediment zone. The

establishment of an episediment layer can increase the

denitrification in treatment wetlands, which receive nitrate in

overlying water.

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 548



10/16

4.4. Step-feeding CW

For the targeting enhancement of denitrification in CW

treating wastewaters with high nitrate and low organic mat-

ter, a step-feeding strategy can be adopted to introduce the

gradational inflow of the wastewater into the wetland bed

(Fig.1). This term refers to thewastewater inflow at more than

one input point along the wetland flow length. Although thepublished literature on wastewater step-feeding in wetland

systems is lacking, this strategy has been proposed by some

researchers (Stefanakis et al., 2011; Hu et al., 2012a,b; Fan

et al., 2013a,b,c). In the pilot-scale systems, the concept of

step-feeding has been used to realize more effective utiliza-

tion of the whole wetland surface area and avoid rapid clog-

ging by distributing suspended solids and organic loading in

the influent along a greater portion of the wetland ( Stefanakis

et al., 2011). Aside from improving the effective utilization of

wetland bed, the intensified denitrification from C source via

step-feeding by distributing organic matter in the raw influent

wastewater to the later stage of wetlands could be more

important (Fan et al., 2013a,b,c). This design/operationparameter should be carefully investigated and optimized

avoiding the second pollution of the treated effluent from the

former wetland stages.

4.5. Autotrophic denitrification-driven CW

Bezbaruah and Zhang (2003) used elemental sulfur/limestone

autotrophic denitrification in nitrate removal enhancement in

a non-vegetated lab-scale SSF CW for wastewater treatment.

The experimentalwetland system had a nitrification zone and

a sulfur/limestone (S/L) autotrophic denitrification zone, fol-

lowed by an anaerobic polishing zone. The S/L autotrophic

denitrification contributed 21e

49% of the total NO3--N removalacross the whole wetland and 50e95% across the S/L column.

The position of the S/L column was changed (1.78, 2.24, and

2.69 m from the inlet), and no remarkable difference in N

removal was observed (Bezbaruahand Zhang, 2003). However,

without the S/L column, the total inorganic N removal

decreased from approximately 88e92e74% and the effluent

NO3 eN increased about two times (from approximately

3.56 mg/L to 4.09 mg/L to 9.13 mg/L). A concurrent sharp

decrease in NO3 eN concentration and a sharp increase in

SO24 concentration immediately after the S/L column confirm

the occurrence of autotrophic denitrification in the S/L col-

umn. An insufficient supply of organic carbon may result in

high levels of nitrate or nitrites, whereas an overloading willprobably result in high concentrations of residual carbon in

the treated water. Moreover, the N2O emission in this system

would be higher than other traditional CWs, whereas no any

data reported on this issue is available. By contrast, the use of

an S/L section in a CW would promote autotrophic denitrifi-

cation and does not need an organic C source. In addition, the

S/L autotrophic denitrification produces a very low amount of

biomass (Zhang, 2002); hence, the system will not be clogged

easily. Although further studies are needed, the actual loca-

tion of the S/L section should be toward the end of the

wetland. Considering the production of SO24 after the S/L

section and the negative effect of high concentration of SO24in the receiving water bodies, a gravel-filled anaerobic

SO24 -reducing bed should follow the S/L section. However,

how the gravel-filled anaerobic SO24 -reducing bed works

without sufficient organic carbon as electron donor for SO24reduction poses another challenge.

5. Specific soil material selection for

microbial biofilms establishment

Different substrates also influence the establishment of mi-

crobial biofilms and the microbial community structure

within complex wetland ecosystems, as well as the treatment

performance. A porous matrix, such as expanded clay, pro-

vides a greater surface area for treatment contact and biofilm

development. Calheiros et al. (2009) investigated the bacterial

communities in the CWs with different soil materials, i.e., two

types of expanded clay aggregates (FiltraliteMR3-8-FMR and

Filtralite NR3-8-FNR) and fine gravel. Higher pollutant re-

movals in terms of COD and BOD5 were achieved in the

expanded clay planted units after a long-term operation (31

months). The similar behavior of the expanded clay systemsconcerning the pollutantremoval may be attributed to the fact

that they may have similar functional group of microorgan-

isms (Calheiros et al., 2009). Li et al. (2008) examined the in-

fluence of soil material type on the removal and

transformation of DOM in experimental CWs with gravel,

zeolite, and slag. However, these materials did not show any

significant influence on the mean removal efficiency in this

study. Both, bacterial species richness and diversities

retrievedfrom the DGGE profiles provedthat hybrid substrates

(gravel, zeolite, and slag) were suitable to bacterial survival

provided protective and favorable habitats for microorgan-

isms through the pore size exclusion of predators.

Based on the conception of using ponds with artificialfloating plant islands, plant root mats, and wetlands for the

treatment of different contaminated waters (Van de Moortel

et al., 2010; Tanner and Headley, 2011; Chen et al., 2012 ),

emergent plants are used to grow not in a soil but only as a

hydroponic root mat. Densely interwoven roots provide

anchoring and stability of the plant stems against tilting (Chen

et al., 2012). Such a hydroponic root mat may either float at

elevated water level or rest on the bottom of a basin at low

water level. In the latter case, water is forced to flow through

the “root filter,” and the roots, such as the soil particles in

CWs, can provide a huge solid surface for the attachment of

microorganism and stimulate the formation of biofilms on

them (Seeger et al., 2013). However, only few examples exist inthe scientific literature, so that a profound comparison of this

technology variant with the commonly used soil-based CWs

must be conducted. Further research focusing on specific root

surface area for biofilms and its secretion on biofilm devel-

opment should be addressed.

6. Thermal insulation in cold climate

Although a variety of removal mechanisms, including filtra-

tion, precipitation, volatilization, adsorption, and plant up-

take, have been well documented (Vymazal, 2007; Kadlec and

Wallace, 2009), the removal of most pollutants in CWs

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5 49



11/16

primarily caused by microbial activity has been a cornerstone

of the technology (Faulwetter et al., 2009). The processes, such

as sedimentation and decantation, important in particulate

organic matter removal are mostly unaffected by low tem-

perature conditions (Ouellet-Plamondon et al., 2006). Howev-

er, biological processes are highly dependent on the variation

of temperatures and influence the overall performance of

wetlands on pollutant removal. Therefore, the operation atcold climate has been perceived as a problem associated with

wetland technology (Werker et al., 2002).

From the perspective view of economic and landscape,

some scientists and engineers tried to screen and/or select

cold-resistant plants. However, from the existing research

and engineering practice, it would be quite hard for herba-

ceous wetland plants to stand aboveground and active in cold

winter. Finally, various woody terrestrial plants were intro-

duced with advantages of deeper roots, stronger oxygen

transfer capacity and longer growing season, such as Salix sp.,

Alnus sp. and Ligustrum obtusifolium and so on (Wu et al.,

2011a,b; Gonzlez et al., 2001). Moreover, the psychrotrophic

bacterial populations in natural systems can acclimate tocolder temperatures. In principle, the apparent adaptation of

psychrotrophs to a wide range of temperatures indicates a

valuable potential success with wetland treatment year round

(Gow and Mills, 1984; Ying et al., 2010). However, in practice,

the pollutant removal in CWs is influenced by a complex array

of factors that are sensitive to climate (Kadlec and Wallace,

2009).

SSFCWs, as one of the main traditional types of wetlands,

have the primary advantage in colder climates, because the

water is not directly exposed to the cold atmosphere during

the treatment process. The microbial community is protected

from the frigid air, and the energy losses through evaporation

and convection are minimized (Wallace et al., 2001; Werkeret al., 2002). These features make SSFCWs more suitable for

winter and/or cold area applications. Nevertheless, the sole

use of this wetland type is inefficient because the treatment

performance in cold conditions is often not satisfactory. Thus,

varied adaptations of CW technology to sub-freezing envi-

ronments have been initiated through specific design (larger

and deeper bed), natural or artificial insulation (snow, ice,

straw, rock wool, polystyrene, greenhouse, etc.), and

enhanced operation strategy (artificial aeration) (Wallace

et al., 2001; Kadlec and Wallace, 2009).

Without a fully fundamental understanding of tempera-

ture dependence, winter performance can only be accom-

modated by effectively applying large factors of safetymeasures in design (Buchberger and Shaw, 1995). Tempera-

ture effects can be partially compensated by a higher HRT in

designed CWs, which has been reported to reduce the differ-

ences in efficiency between cold and warm periods to be less

than 10% for all parameters. Hence, the wetland system is

underutilized for a large part of the year. Moreover, the safety

factors make the design more land and investment intensive,

which often limits the scope for the wastewater treatment

applications using CWs (Werker et al., 2002). Further in-

vestigations should be enhanced first in the understanding of

pollutant removal mechanisms to seasonal and temperature

changes and subsequently in the development of CW design

models, which might help to reduce the safety factors through

the compensation of seasonal and temperature

considerations.

The design approach using SSFCWs covered with an

insulating mulch layer has been demonstrated to prevent

freezing (Wallace et al., 2001; Mæhlum and Jenssen, 2002). The

added insulation material may be supported by the soil bed

material or standing dead plants but it should be kept out of

water. A wide variety of mulch materials, including bark, pinestraw, and wood chips, have been referred for use in CWs. A

good mulch material should have the characteristics of a

fluffy structure with high-fiber content, balanced nutrient

composition, and circumneutral pH and should be substan-

tially decomposed without any secondary organic loading of

the treatment system. Leaf litter is often suggested as one

source of insulation but its spotty in distribution often allows

heat to escape (Wallace et al., 2001). Even small breaches in

the insulation of CWsare reported to result in substantial heat

losses in flowing water. Straw and blankets can be used to

supplement the standing dead plant material. To be effective,

Wallace et al. (2001) also suggested that insulation must be

uniform in coverage, which requires it to be designed as anintegral part of the wetland system. Wu et al. (2011a,b)

developed an integrated household CW with an integral

insulating layer of 15 cm wood chips. This insulating layer

kept the temperature of the household wetland bed at above

6 C from freezing at air temperatures to 8 C, which guar-

anteed a good performance of pollutant removal in winter as

in summer.

The wetland configurations that allow greater air move-

ment within the bed have also been reported to intensity the

removal of contaminants in cold climates (Kadlec and

Wallace, 2009). This characteristic might be attributed to the

adequate oxygen supply and resultant higher microbial ac-

tivity under greater air flux. N removal is believed to betemperature-dependent in CWs, which often stops at a tem-

perature of below 6 C. Moreover, the contribution of artificial

aeration on pollutant removal in winter has been tested, with

a combination of planted, unplanted, aerated, and non-

aerated mesocosms for treating a reconstituted fish-farm

effluent (Ouellet-Plamondon et al., 2006). Artificial aeration

improves the TKN removal in both unplanted and planted

units in winter, but the additional aeration does not fully

compensate the absence of plants. This result suggests that

the role of plants is beyond the sole addition ofoxygen into the

rhizosphere. However, the aeration has been demonstrated to

positively influence the root morphology of wetland vegeta-

tion and the resultant changes in redox potential (Ouellet-Plamondon et al., 2006).

Considering the special challenges presented in CWs for

wastewater treatment in extreme frigid climate, a full-scale

greenhouse-structured wetland system has been investi-

gated for the evaluation of the contaminant removal effi-

ciency and its economic and social values (Gao and Hu, 2012).

The temperature of wastewater in the wetland bed was al-

ways 8 C or higher, even the minimum ambient air temper-

ature decreased to 30 C. The construction of greenhouse for

the insulation in winter increased the investment costs.

However, some ornamental plants grown in this greenhouse

wetland and compensated certain amount of the costs by

selling the flowers.

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 550



12/16

7. Conclusions

The intensified removal of organic matter and nitrogen in

CWsis generallydirectly and/orindirectly influenced by many

factors, including the temperature, soil material types, oper-

ation strategies, and redox conditions in the wetland bed. The

present knowledge can be summarized as follows:

(1) The use of recirculation to enhance the removal per-

formance in CWs depends on many factors, including

CW types and influent loads. In most vertical-flow and

integrated CWs, the effluent recirculation enhances the

interactions between pollutants and microorganisms,

which results in positive effects on the treatment per-

formance, particularly on the effective removal of TN.

However, more energy for pumping is needed. In

horizontal-flow CWs with saturated conditions, effluent

recirculation may cause problems given the increased

hydraulic loading rates.

(2) To overcome oxygen transfer limitations in traditionalCWs, some additional measures that involve energy in-

puts to CWs have been developed, such as artificial

aeration and tidal operation. These technologies can

certainly increase the oxygenation capacity of CWs and

obtain a better treatment performance but also increase

the operation and maintenance costs. These in-

novations are only justified when the lifecycle cost is

sufficientlyoffsetby thereduction in thecapitalcost,i.e.,

thenet savings of reduced wetland size are less than the

costs of the aeration equipment and maintenance.

(3) Clogging is a common problem during the lifespan of

subsurface CWs, and proper pre-treatments have

already been regulated. The flow direction reciprocationand earthworm integration have been shown effective

to help to decrease the accumulation of solids in CWs.

However, earthworms are soil-based living creatures.

Gravel, as the most used soil material in horizontal flow

CWs, may not be suitable for earthworm movements.

The addition of organic substrates in bed has been

proposed, but the relationship between the degradation

of organic substrates and additional organic compound

production and the transfer into the effluents should be

carefully considered.

(4) Bioaugmentation can be used to accelerate the devel-

opment of necessary microbial community and shorten

the adaptation period. However, aiming at intensifying the degradation of some specific recalcitrant pollutants

of industrial wastewaters in CWs, bioaugmentation can

be a strategy similar to traditional wastewater treat-

ment technologies.

(5) The innovation of the configurations in CWs for per-

formance intensifications, including the circular-flow

corridor, towery hybrid, and baffled subsurface flow

CWs, are versatile. Regarding the energy input for a

gravity-driven water flow, pumping the water on a

necessary higher level should be considered. Electrical

power generation has been initiated in an integrated

MFC CW, but its full-scale application remains facing

many challenges or may not be expected in near future.

(6) To treat low C/N ratio wastewaters, such as nitrate-rich

agricultural runoff and polluted groundwater, the car-

bon source only from the root exudatesof macrophytes is

not sufficient to maintain a high performance of nitrate

removal. Denitrification can be enhanced by the external

supply of electron donors via direct organic carbon addi-

tion using organic filtration media and/or step feeding

operation. However, the potential secondary pollutionshould be considered. The promotion of autotrophic

denitrification, especially via the pathway of microbial

anammox, could be a potential promising strategy.

(7) The operation of CWs at cold climate is a challenge.

Various adaptations are initiated through specific

design (larger and deeper bed), natural or artificial

thermal insulation (snow, ice, straw, rock wool, poly-

styrene, greenhouse, etc.), and enhanced operation

strategy (artificial aeration). In extreme frigid climate,

greenhouse-structured wetland systems can be further

discussed but increased investment costs have to be

considered.

(8) The multidisciplinary collaboration between engineersand natural scientists will certainly inspire further

innovative ideas in the development of intensified CWs,

but the resilience and sustainability of these new

technologies have to be thoroughly evaluated.

Acknowledgments

This work was supported by the grants from “ The China

National Natural Science Fund (51308536),” “ Chinese Univer-

sities Scientific Fund (2013XJ003),” and the Sino-Danish CenterforEducation andResearch.” We greatly appreciate the critical

and constructive comments from the anonymous reviewers,

which helped to improve this manuscript.

r e f e r e n c e s

Abou-Elela, S.I., Hellal, M.S., 2012. Municipal wastewatertreatment using vertical flow constructed wetlands plantedwith Canna, Phragmites and Cyprus. Ecol. Eng. 47, 209e213.

Arias, C.A., Brix, H., Marti, E., 2005. Recycling of treated effluentsenhances removal of total nitrogen in vertical flow

constructed wetlands. J. Environ. Sci. Health Part A Toxic/Hazard. Subst. Environ. Eng. 40 (6e7), 1431e1443.

Austin, D., 2006. Influence of cation exchange capacity (CEC) in atidal flow, flood and drain wastewater treatment wetland.Ecol. Eng. 28, 35e43.

Austin, D., Nivala, J., 2009. Energy requirements for nitrificationand biological nitrogen removal in engineered wetlands. Ecol.Eng. 35, 184e192.

Austin, D., Lohan, E., Verson, E., 2003. Nitrification anddenitrification in a tidal vertical flow wetland pilot. Proc.Water Environ. Fed. 2003, 333e357.

Ayaz, S.C., Aktas, O., Findik, N., Akca, L., Kinaci, C., 2012. Effect of recirculation on nitrogen removal in a hybrid constructedwetland system. Ecol. Eng. 40, 1e5.

Bezbaruah, A.N., Zhang, T.C., 2003. Performance of a constructed

wetland with a sulfur/limestone denitrification section for

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 5 51

http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://dx.doi.org/10.1016/j.watres.2014.03.020http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref8http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref7http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref6http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref5http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref4http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1http://refhub.elsevier.com/S0043-1354(14)00210-3/sref1


13/16

wastewater nitrogen removal. Environ. Sci. Technol. 37,1690e1697.

Biederman, J.A., Allen, W.C., Stein, O.R., Hook, P.B., 2002.Temperature and wetland plant species effects on wastewatertreatment and root zone oxidation. J. Environ. Qual. 31,1010e1016.

Borin, M., Toccheto, D., 2007. Five years water and nitrogenbalance for a constructed surface flow wetland treating

agricultural drainage waters. Sci. Total Environ. 380, 38e

47.Brix, H., 1987. Treatment of wastewater in the rhizosphere of

wetland plants e the root-zone method. Water Sci. Technol.19 (1e2), 107e118.

Brix, H., 1994a. Functions of macrophytes in constructedwetlands. Water Sci. Technol. 29 (4), 71e78.

Brix, H., 1994b. Use of constructed wetlands in water pollutioncontrol: historical development, present status, and futureperspectives. Water Sci. Technol. 30 (8), 209e223.

Brix, H., 1999. How ‘green’ are aquaculture, constructed wetlandsand conventional wastewater treatment systems? Water Sci.Technol. 40 (3), 45e50.

Brix, H., Arias, C.A., 2005. The use of vertical flow constructedwetlands for on-site treatment of domestic wastewater: newDanish guidelines. Ecol. Eng. 25, 491e500.

Buchberger, S.G., Shaw, G.B., 1995. An approach toward rationaldesign of constructed wetlands for wastewater treatment.Ecol. Eng. 4, 249e275.

Calheiros, C.S., Duque, A.F., Moura, A., Henriques, I.S., Correia, A.,Rangel, A.O., Castro, P.M., 2009. Substrate effect on bacterialcommunities from constructed wetlands planted with Typhalatifolia treating industrial wastewater. Ecol. Eng. 35, 744e753.

Calheiros, C.S.C., Quitério, P.V.B., Silva, G., Crispim, L.F.C., Brix, H.,Moura, S.C., Castro, P.M.L., 2012. Use of constructed wetlandsystems with Arundo and Sarcocornia for polishing high salinitytannery wastewater. J. Environ. Manag. 95 (1), 66e77.

Chan, S.Y., Tsang, Y.F., Chua, H., 2008. Domestic wastewatertreatment using tidal-flow cinder bed with Cyperusalternifolius. Aqua. Ecosyst. Health Manag. 11, 206e211.

Chazarenc, F., Gagnon, V., Comeau, Y., Brisson, J., 2009. Effect of

plant and artificial aeration on solids accumulation andbiological activities in constructed wetlands. Ecol. Eng. 35 (6),1005e1010.

Chen, Z., Kuschk, P., Reiche, N., Borsdorf, H., Ka ¨ stner, M.,Ko ¨ ser, H., 2012. Comparative evaluation of pilot scalehorizontal subsurface-flow constructed wetlands and plantroot mats for treating groundwater contaminated withbenzene and MTBE. J. Hazard. Mater. 209-210, 510e515.

Chiarawatchai, N., Nuengjamnong, C., 2009. The use of earthworms in lab-scale constructed wetlands to treat swinewastewater. Thai J. Vet. Med. 39, 157e162.

Chiarawatchai, N., Nuengjamnong, C., Rachdawong, P.,Otterpohl, R., 2007. Potential study of using earthworms as anenhancement to treat high strength wastewater. Thai J. Vet.Med. 37, 25e32.

Davison, L., Headley, T.R., Pratt, K., 2005. Aspects of design,structure, performance and operation of reed beds-eightyears’ experience in northeastern New South Wales,Australia. Water Sci. Technol. 51 (10), 129e138.

Dong, H.Y., Qiang, Z.M., Li, T.G., Jin, H., Chen, W.D., 2012. Effect of artificial aeration on the performance of vertical-flowconstructed wetland treating heavily polluted river water. J.Environ. Sci. 24, 596e601.

Fan, J., Zhang, B., Zhang, J., Ngo, H.H., Guo, W., Liu, F., Guo, Y.,Wu, H., 2013a. Intermittent aeration strategy to enhanceorganics and nitrogen removal in subsurface flow constructedwetlands. Bioresour. Technol. 141, 122e171.

Fan, J.L., Wang, W.G., Zhang, B., Guo, Y.Y., Ngo, H.H., Guo, W.S.,Zhang, J., Wu, H.M., 2013b. Nitrogen removal inintermittently aerated vertical flow constructed wetlands:

Impact of influent COD/N ratios. Bioresour. Technol. 143,461e466.

Fan, J., Liang, S., Zhang, B., Zhang, J., 2013c. Enhanced organicsand nitrogen removal in batch-operated vertical flowconstructed wetlands by combination of intermittent aerationand step feeding strategy. Environ. Sci. Pollut. Res. 20,2448e2455.

Fang, Z., Song, H., Cang, N., Li, X., 2013. Performance of microbial

fuel cell coupled constructed wetland system fordecolorization of Azo dye and bioelectricity generation.Bioresour. Technol. 144, 165e171.

Faulwetter, J.L., Gagnon, V., Sundberg, C., Chazarenc, F.,Burr, M.D., Brisson, J., Camper, A.K., Stein, O.R., 2009. Microbialprocesses influencing performance of treatment wetlands: areview. Ecol. Eng. 35, 987e1004.

Fleming-Singer, M.S., Horne, A.J., 2002. Enhanced nitrate removalefficiency in wetland microcosms using an episediment layerfor denitrification. Environ. Sci. Technol. 36, 1231e1237.

Foladori, P., Ruaben, J., Ortigara, A.R.C., 2013. Recirculation orartificial aeration in vertical flow constructed wetlands: acomparative study for treating high load wastewater.Bioresour. Technol. 149, 398e405.

Gao, D.W., Hu, Q., 2012. Bio-contact oxidation and greenhouse-

structured wetland system for rural sewage recycling in coldregions: a full-scale study. Ecol. Eng. 49, 249e253.

Garcia, J., Rousseau, D.P.L., Morato, J., Lesage, E., Matamoros, V.,Bayona, J.M., 2010. Contaminant removal processes insubsurface-flow constructed wetlands: a review. Crit. Rev.Environ. Sci. Technol. 40, 561e661.

Gonzlez, J., Ansola, G., Luis, E., 2001. Experimental results onconstructed wetland pilot system. Water Sci. Technol. 44(11e12), 387e392.

Gow, J.A., Mills, F., 1984. Pragmatic criteria to distinguishpsychrophiles and psychrotrophs in ecological systems. Appl.Environ. Microbiol. 47, 213e215.

Grafias, P., Xekoukoulotakis, N.P., Mantzavinos, D.,Diamadopoulos, E., 2010. Pilot treatment of olive pomaceleachate by vertical-flow constructed wetland and

electrochemical oxidation: an efficient hybrid process. WaterRes. 44, 2773e2780.

Headley, T., Nivala, J., Kassa, K., Olsson, L., Wallace, S., Brix, H.,van Afferden, M., Mu ¨ ller, R.A., 2013. Escherichia coli removaland internal dynamics in subsurface flow ecotechnologies:effects of design and plants. Ecol. Eng. 61, 564e574.

He, L., Liu, H., Xi, B., Zhu, Y., 2006a. Effects of effluentrecirculation in vertical-flow constructed wetland ontreatment efficiency of livestock wastewater. Water Sci.Technol. 54 (11e12), 137e146.

He, L., Liu, H., Xi, B., Zhu, Y., 2006b. Enhancing treatmentefficiency of swine wastewater by effluent recirculation invertical-flow constructed wetland. J. Environ. Sci. 18,221e226.

Hu, Y.S., Zhao, Y.Q., Zhao, X.H., Kumar, J.L.G., 2012a. High rate

nitrogen removal in an alum sludge-based intermittentaeration constructed wetland. Environ. Sci. Technol. 46 (8),4583e4590.

Hu, Y.S., Zhao, Y.Q., Zhao, X.H., Kumar, J.L.G., 2012b.Comprehensive analysis of step-feeding strategy to enhancebiological nitrogen removal in alum sludge-based tidal flowconstructed wetlands. Bioresour. Technol. 111, 27e35.

Hu, Y., Zhao, Y., Rymszewicz, A., 2014. Robust biological nitrogenremoval by creating multiple tides in a single bed tidal flowconstructed wetland. Sci. Total Environ. 470, 1197e1204.

Huang, X., Liu, C.X., Gao, C.F., Wang, Z., Zhu, G.F., Liu, L., Lin, G.X.,2013. Comparison of nutrient removal and bacterialcommunities between natural zeolite-based and volcanicrock-based vertical flow constructed wetlands treating piggerywastewater. Desal. Water Treat. 51, 4379e4389.

w a t e r r e s e a r c h 5 7 ( 2 0 1 4 ) 4 0 e5 552

http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://refhub.elsevier.com/S0043-1354(14)00210-3/sref9http://refhub.elsevier.com/S0043-1354(14)00210-3/sref10http://refhub.elsevier.com/S0043-1354(14)00210-3/sref10http://refhub.elsevier.com/S0043-1354(14)00210-3/sref10http://refhub.elsevier.com/S0043-1354(14)00210-3/sref10http://refhub.elsevier.com/S0043-1354(14)00210-3/sref10http://refhub.elsevier.com/S0043-1354(14)00210-3/sref11http://refhub.elsevier.com/S0043-1354(14)00210-3/sref11http://refhub.elsevier.com/S0043-1354(14)00210-3/sref11http://refhub.elsevier.com/S0043-1354(14)00210-3/sref11http://refhub.elsevier.com/S0043-1354(14)00210-3/sref12http://refhub.elsevier.com/S0043-1354(14)00210-

Documents

Development of Constructed Wetlands