development of constructed wetlands
TRANSCRIPT
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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
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http://dx.doi.org/10.1016/j.watres.2014.03.020
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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
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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 ).
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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).
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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)
% 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 0.018 380 65e158 27 65 3.5e10.6 2.0 65 Intermittent 6
VF Lab R 0.018 760 65e158 25 73 3.5e10.6 2.5 65 Continuous 6
VF Lab R 0.018 760 65e158 32 64 3.5e10.6 3.2 54 Intermittent 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).
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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)
% 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).
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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
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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
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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.
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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
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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
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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.
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