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Journal of Environmental Management 141 (2014) 116e131

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Review

Application of constructed wetlands for wastewater treatment in developing countries e A review of recent developments (2000e2013) Dong Qing Zhang a, *, K.B.S.N. Jinadasa b, Richard M. Gersberg c, Yu Liu a, Wun Jern Ng a, Soon Keat Tan a, d a

Advanced Environmental Biotechnology Centre, Nanyang Environment and Water Research Institute, School of Civill and Environmental Engineering, Nanyang Technological University, 1 CleanTech Loop, #06-10, Singapore 637141, Singapore Department of Civil Engineering, University of Peradeniya, Peradeniya 20400, Sri Lanka c Graduate School of Public Health, San Diego State University, Hardy Tower 119, 5500 Campanile, San Diego, CA 92182-4162, USA d Maritime Research Centre, School of Civil and Environmental Engineering, Nanyang Technological University, 50 Nanyang Avenue, Singapore 639798, Singapore b

a r t i c l e i n f o

a b s t r a c t

Article history: Received 10 December 2013 Received in revised form 6 March 2014 Accepted 13 March 2014 Available online 7 May 2014

Inadequate access to clean water and sanitation has become one of the most pervasive problems afflicting people throughout the developing world. Replication of centralized water-, energy- and costintensive technologies has proved ineffective in resolving the complex water-related problems resulting from rapid urbanization in the developing countries. Instead constructed wetlands (CWs) have emerged and become a viable option for wastewater treatment, and are currently being recognized as attractive alternatives to conventional wastewater treatment methods. The primary objective of this review is to present a comprehensive overview of the diverse range of practice, applications and researches of CW systems for removing various contaminants from wastewater in developing countries, placing them in the overall context of the need for low-cost and sustainable wastewater treatment systems. Emphasis of this review is placed on the treatment performance of various types of CWs including: (i) free water surface flow CW; (ii) subsurface flow CW; (iii) hybrid systems; and, (iv) floating treatment wetland. The impacts of different wetland design and pertinent operational variables (e.g., hydraulic loading rate, vegetation species, physical configurations, and seasonal variation) on contaminant removal in CW systems are also summarized and highlighted. Finally, the cost and land requirements for CW systems are critically evaluated. Ó 2014 Elsevier Ltd. All rights reserved.

Keywords: Constructed wetlands Wastewater treatment Developing countries

1. Introduction Inadequate access to clean water and sanitation has become one of the most pervasive problems affecting human health in developing countries, and problems with water are expected to worsen in coming decades (Hutton et al., 2007; Shannon et al., 2008). Developing countries are defined according to their Gross National Income (GNI) per capita per year. Countries with a GNI of US$ 11,905 and less are defined as developing (World Bank, 2012). According to a recent report by World Health Organization (2012), more than one-tenth of the global population (780 million) still

* Corresponding author. Tel.: þ65 6790 6619; fax: þ65 6790 6620. E-mail addresses: [email protected] (D.Q. Zhang), [email protected] (S.K. Tan). http://dx.doi.org/10.1016/j.jenvman.2014.03.015 0301-4797/Ó 2014 Elsevier Ltd. All rights reserved.

relied on sub-standard drinking water sources in 2010. Lack of sanitation is an even larger concern. An estimated 2.5 billion people are still without improved sanitation, and sanitation coverage is below 50% in many countries of Sub-Saharan Africa and Southern Asia. Consequently, millions of people die annually (3900 children per day) from diseases transmitted through unsafe water or human excreta (Shannon et al., 2008). With insufficient water resources to meet rising water demand, many sources of water (e.g., groundwater) that are considered easy to be developed geographically and technologically have been overexploited in many developing countries (Llamas and MartínezSantos, 2005; Jiang, 2009). This short-term strategy is likely to have detrimental effects on the environment, such as ground subsidence, salinity intrusion, and ecosystem deterioration (Kivaisi, 2001). In addition, many cities in developing countries have also

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

generally fallen behind in constructing and managing sewage treatment facilities. Among the various developments, treatment of wastewater is always considered one of the lowest priorities (Konnerup et al., 2011). The consequence of this is the common practice of discharging large amounts of untreated wastewater directly into streams and lakes in many developing countries (Senzia et al., 2003; Shrestha et al., 2001). Ecological technologies such as constructed wetlands for wastewater treatment represent innovative and emerging solutions for environmental protection and restoration, placing them in the overall context of the need for low-cost and sustainable wastewater treatment systems in developing countries (Babatunde et al., 2008; Vymazal, 2011). Constructed wetlands (CWs) are engineered wastewater treatment systems that encompass a plurality of treatment modules including biological, chemical, and physical processes akin to processes occurring in natural treatment wetlands (Kadlec and Knight, 1996; Vymazal, 2005). CWs have been successfully used to mitigate environmental pollution by removing of a wide variety of pollutants from wastewater such as organic compounds, suspended solids, pathogens, metals, and nutrients (Haberl et al., 1995; Kadlec and Wallace, 2008; Ranieri et al., 2013a; Gikas et al., 2013), as well as pharmaceutical and personal care products (Matamoros and Bayona, 2006; Ranieri et al., 2011; Zhang et al., 2014). Because of high removal efficiency, low cost, simple operation, and great potential for water and nutrient reuse, CWs have become an increasingly popular option for wastewater treatment (Tanner et al., 2002; Vymazal, 2007; Kadlec, 2009). However, to date there has been little information about the application of CWs in developing countries, and the adoption of CW technology in these countries has been surprisingly slow(Haberl, 1999; Kivaisi, 2001; Bojcevska and Tonderski, 2007). Since the application of CWs has been rapidly expanding in the past decade, this study presents a comprehensive review of the current state of the practice, applications, and researches evaluating CW removal of various contaminants from wastewater in developing countries since 2000. Emphasis of this review is placed on the treatment performance of various types of CWs at a macroscopic level. The impacts of different wetland designs and pertinent operational variables on contaminant removal in CW systems are also summarized and highlighted. Finally, the cost and land space requirements for the application of CWs in developing countries are also discussed. 2. Centralized versus decentralized wastewater treatment systems The current concept of wastewater collection, treatment, and discharge is based on centralized sewer systems, which have been regarded as the optimal solution for water pollution control and have prevailed in many industrial countries (Gikas and Tchobanoglous, 2009). The basic idea behind the use of centralized water treatment is that wastewater is transported out of the city and far away from residential sites as quickly as possible in order to reduce public health risks (Otterpohl et al., 2003). To a large degree, this centralized sewage treatment approach can solve the problems of sanitation very efficiently (Libralato et al., 2012). However, centralized approaches are often plagued by high capital cost, improper operation, and an over reliance on treatment technologies that are unaffordably maintained in areas with low population densities and dispersed households (Montgomery and Elimelech, 2007; Massoud et al., 2009). Indeed, the increasing challengeof providing clean water and wastewater disposal implies that large investments are required, and far too

117

often economics is the most important criterion in decisionmaking in most developing countries (Libralato et al., 2012; Massoud et al., 2009). Approximately 80e90% of the capital costs are related to the collection systems associated with densely populated areas (Maurer et al., 2006). Thus, constructing a centralized treatment system for small rural communities or periurban areas in low income countries results in heavy debt burden for the population (Parkinson and Tayler, 2003; Seidenstat et al., 2003). Given the multitude and complexity of water-related problems resulting from urbanization, coupled with the need to achieve sustainable development in fast growing middle- and small-sized cities in the developing world, complete replication of centralized technology has proved to be extremely cost-prohibitive and not feasible (Kivaisi, 2001). Sustainable wastewater treatment systems in developing countries should therefore focus on meeting local needs requiring minimal investment and less-sophisticated operation (Klarkson et al., 2010). Decentralized wastewater treatment systems that employ a combination of onsite and/or cluster systems are increasingly recognised as a feasible approach towards resolving the water supply and sanitation issues in developing countries (Libralato et al., 2012; Paraskevas et al., 2002). Such approaches are recognised as a viable long-term solution for small communities, rural centres, and industrial, commercial and residential areas in developing countries, because they are more flexible, less resource intensive, and more ecologically sustainable (Otterpohl et al., 2003). 3. Constructed wetlands in developing countries In the past several decades, CWs have become a popular option for wastewater treatment and have been recognized as attractive alternatives to conventional wastewater treatment methods. This is due to their high pollutant removal efficiency, easy operation and maintenance, low energy requirements, high rates of water recycling, and potential for providing significant wildlife habitat (Kadlec and Wallace, 2008; Vymazal, 2011). In developing countries, although CWs have been used to treat domestic sewage (Yang et al., 2008a,b; Katsenovich et al., 2009; Mburu et al., 2013; Zhai et al., 2011), increasingly the application of CWs has also been extended to treat other types of wastewater such as industrial wastewater(Chen et al., 2006; Maine et al., 2007), agricultural wastewater (Lee et al., 2004; He et al., 2006), lake/river water (Li et al., 2008; X. Li et al., 2009; Chen et al., 2008; Tang et al., 2009), sludge effluent (Kaseva, 2004; Ahmed et al., 2008), oilproduced wastewater (Ji et al., 2007), storm water runoff (Sim et al., 2008; Ávila et al., 2013), sugar factory wastewater (Bojcevska and Tonderski, 2007), hospital wastewater (Shrestha et al., 2001), laboratory wastewater (Meutia, 2001), landfill leachate (Nahlik and Mitsch, 2006), and agricultural runoff (Yang et al., 2008a,b; He et al., 2006). In terms of performance efficiency, most of the developing countries have warm tropical and subtropical climates, and it is generally acknowledged that CWs are more suitable for wastewater treatment in tropical than in temperate regions (Kivaisi, 2001; Denny, 1997; Haberl, 1999). A warm climate is conducive to yearround plant growth and heightened microbiological activity, which in general have positive effects on treatment efficiency (Kaseva, 2004; Poh-Eng and Polprasert, 1998). Wetlands in the tropics, which are exposed to higher temperatures and direct sunlight throughout the year, have higher year-round plant productivity and a concomitant decrease in the time necessary for microbial biodegradation. This in turn results in more efficient treatment of pollutants (Zhang et al., 2012).

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4. Contaminant removal in different types of CWs CW treatment systems generally fall into three categories: free water subsurface (FWS) CWs, subsurface flow (SSF) CWs, and hybrid CWs. SSF CWs may be further classified according to flow direction into vertical subsurface flow (VSSF) and horizontal subsurface flow (HSSF) systems (Cooper et al., 1996; Kadlec and Knight, 1996). Selection of the flow regime depends primarily on the targeted constituents for treatment, geographic location, cost, available area, and treatment goals (Horner et al., 2012). 4.1. Free water surface (FWS) CWs FWS systems are shallow basins with water on the surface, the treatment processes occurring through complex interactions between the vegetation and the associated biofilms in the water phase (Kadlec and Wallace, 2008; Vymazal, 2005). Like natural marshes, FWSCWs exhibit a broad spectrum of biological characteristics that are capable of removing various constituents for water quality improvement (Vymazal, 2010). The near-surface layer of water is aerobic while the deeper waters and the substrate are usually anaerobic. FWS systems typically have water depths less than 0.4 m and hydraulic loading rates (HLR) between 0.7 and 5.0 cm d1 (Kadlec and Knight, 1996). Recently, tertiary treatment using FWS CWs is increasingly popular as the polishing step of the treated effluent from conventional wastewater treatment plants (Kadlec, 2003; Toet et al., 2005). The FWS CWs are effective in removal of organics through microbial degradation, and removal of suspended solids through filtration and sedimentation (Kadlec et al., 2000). Generally in FWS CWs, removal efficiencies above 70% can be achieved for total suspended solid (TSS), chemical oxygen demand (COD), biochemical oxygen demand (BOD), and pathogens (Kadlec and Wallace, 2008). Removal efficiency for nitrogen is typically 40e50% (Vymazal, 2007) depending on many factors including inflow concentration, chemical form of nitrogen, water temperature, season, organic carbon availability, and dissolved oxygen concentration (Kadlec, 2009). FWS CWs provide sustainable removal of phosphorus but at relatively slow rates ranging from 40 to 90% (Vymazal, 2007). However, in FWS systems, phosphorus removal is limited by little contact between the water column and the soil (Vymazal, 2011). A summary of the reported design/operational parameters and treatment efficiency of FWS systems is shown in Table 1. Based on the present review, the removal of TSS (66.1%) andBOD (65.34%) appears unsatisfactory. In particular, FWS CWs exhibited the poorest performance for the removal of COD (44.9%) in cited studies. However, FWS CWs were found to be efficient in removing nitrate (NO3eN) (51.63%), ammonium (NH4eN) (60.87%), and total nitrogen (TN) (43%). As Table 1 shows, studies suggest that the removal of total phosphorus (TP) (49.16%) is moderate and variable, ranging from 19.5% to 96%. 4.2. Subsurface flow (SSF) systems SSF systems are designed with horizontal or vertical subsurface flow through a permeable medium (typically sand, gravel, or crushed rock) (Vymazal, 2005, 2011). The most common systems are designed with an HSSF CW configuration. More operational data exists for HSSF wetlands than for VSSF systems, but VSSF CW systems are becoming more popular (Vymazal, 2005, 2007). In HSSF CWs, the wastewater flows horizontally through the granular media and comes into contact with a network of aerobic, anoxic, and anaerobic zones in the subsurface (Vymazal, 2011). The aerobic zones occur around plant roots and rhizomes that introduce oxygen

into the substrate. In the VSSF CW systems, the wastewater is fed through the whole surface area via a distribution system and passes through the media vertically. Bed depth for SSF CWs is generally less than 0.6 m. Typical HLR ranges from 2 to 20 cm d1, which correspond to a wetland of 0.5e5 ha per 1000 m3 d1 of flow (Kadlec and Knight, 1996). A summary of design/operational parameters and treatment efficiency of HSSF and VSSF CWs is shown in Tables 2 and 3, respectively. Based on the present review, the removal efficiencies are 75.10% (HSSF) and 89.29% (VSSF) for BOD5, and 66.02% (HSSF) and 64.41% (VSSF) for COD. In comparison, a worldwide survey by Puigagut et al. (2007) demonstrated that SSF CWs for municipal sewage treatment usually exhibit high treatment efficiency of BOD5 and COD. In their survey, organic removal efficiencies varied from 75 to 93% (for BOD5), and from 64 to 82% (for COD), respectively. Domestic wastewater does not usually contain elevated concentrations of recalcitrant organics and, therefore, most organics are labile and easily degradable (Vymazal and Kröpfelová, 2009). Generally, VSSF CWs are expected to perform better than the HFCW CWsfor the reduction of BOD, as VSSF CWs are intermittently loaded, and have unsaturated flow resulting in a higher transfer of oxygen to the filter medium compared to HSSF CWs (Kadlec and Wallace, 2008). Based on the present review, VSSF CWs exhibits better removal performance for TSS (85.25%), BOD5 (89.29%), and COD (66.14%), respectively, compared to HSSF CWs (79.93% for TSS, 75.1% for BOD5, and 66.02% for COD, respectively). The removal efficiencies of TSS, BOD5, and COD for the VSSF and HSSF CWs were statistically compared using Student’s t-test but the differences were not statistically significant (p > 0.05). This result is consistent with Konnerup et al. (2011) who used CWs consisting of HSSF CWs and VSSF CWs planted with Oreochromis niloticus and Cyprinus carpiofor the treatment of fishpond water in a recirculating aquaculture system in the Mekong Delta of Vietnam. This comparative study reported that outlet concentrations for both BOD5 and COD did not differ significantly (p > 0.05) between these two types of CWs at any of the three different hydraulic loading rates (HLRs) investigated (750, 1500 and 3000 mm d1). In general, HSSF CWs can provide good conditions for denitrification, but the ability to nitrify ammonia is typically limited in these systems. In contrast, VSSF CWs can successfully remove NH3e N, but denitrification hardly occurs in these systems (Vymazal, 2005, 2007). Despite the fact that some literature points to higher NH4eN removal efficiencies in VSSF as compared to HSSF CWs because of the higher oxygenation in VSSF beds which enhances nitrification (Li et al., 2008; Truong et al., 2011), based on the present statistical comparison of studies, there was no significant difference (p > 0.05) between NH4eN removal by VSSF CWs r (61.20%) as compared to HSSF CWs (53.52%).Singh et al. (2009) investigated the performance of a model for decentralized wastewater treatment using a hybrid system consisting of HSSF and VSSF CWs to treat high strength wastewater from 80 households (400 population equivalent) in Nepal. The VSSF bed exhibited higher removal efficiencies of NH3eN (70.9%) as compared to the HSSF CW (23.8%). Although Vymazal (2007) indicated that the potential to remove NO3eN is expected to be low in VSSF wetlands, based on the present review, VSSF CWs showed enhanced NO3eN removal (67.76%) as compared to HSSF CWs (44.08%), however, this difference was not statistically significant (p > 0.05). For TN removal, both HSSF (51.97%) and VSSF (50.55%) CW systems were reported as moderately efficient. In comparison, removal of TN in the various types of CWs investigated varied between 40and 55%, with removed load ranging between 250 and 630 g N m2 yr1 depending on CW type and influent loading (Vymazal, 2007). With respect to TP removal, HSSF CWs exhibited enhanced removal (65.96%) compared to VSSF CWs (59.61%), but the

Table 1 A summary of the wetland design/operational parameters and treatment efficiency of FWS systems in developing countries. Type of wastewater (WW)/stage of treatment

Nyanza, Kenya Effluent concentration (mg L1) Removal efficiencies (%)

Dimension (m  m  m) (L  W  D)

Wetland design and operation

HLR (m3 d1)

HRT (d)

References

TSS

BOD5

COD

NH4eN

NO3eN

TN

TP

Municipal WW/ Secondary

45.8 71.9

19.2 68.2

e e

3.4 74.4

0.9 50.0

e e

1.36 19.0

25.0  1.0  0.6

Scirpusgrossus Typha angustifolia

13

18 h

Jinadasa et al. (2006)

Sugar factory WW

11.0 76

e e

e e

2.9 36

e e

e e

4.1 29

3.0  20.0  0.4

Cyperus papyrus Echinochloa pyramidalis

75 mm d1

e

Bojcevska and Tonderski (2007)

e e

5.93 16.5

1.37 22.8

0.85 34.2

3.97 19.8

0.10 35.1

20.0  1.5  0.8

Typha angustifolia

0.64 m d1

e

Li et al. (2008)

Plant species

Taihu, China Effluent concentration (mg L1) Removal efficiencies (%)

Lake water

Putrajaya city, Malaysia Effluent concentration (mg L1) Removal efficiencies (%)

Stormwater

e e

e e

e e

e e

0.96 70.7

e e

0.06 84.3

1.5  0.7  0.8

Phragmites karka Lepironia articulata

0.17e0.63

e

Sim et al. (2008)

Shanghai, China Effluent concentration (mg L1) Removal efficiencies (%)

River water

30 70

7.7 15.4

32 17.9

e e

e e

6.15 83.4

0.32 96

800 m2  0.75 m

Phragmites australis

1800

10

X. Li et al. (2009); M. Li et al. (2009)

Municipal WW/ Secondary

e e

20.08 80.78

72.80 65.18

0.54 95.75

e e

6.08 58.59

1.86 66.5

48.9  15.0  0.6

Typha angustifolia

151.4

9.8

Katsenovich et al. (2009)

Oil-produced WW

e e

3.9 88

77 80

e e

e e

9.7 10.2

0.53 18.5

75.0  7.5  0.25

Phragmites australis

18.75 15

1.2

Ji et al. (2007)

Municipal WW/saline condition

40.4 46.5

12.7 74.3

e e

5.18 75.4

0.35 e

e e

2.2 44.9

4.0  1.0  1.5

Typha angustifolia

6e150 mm d1

2 5

Klomjek and Nitisoravut (2005)

e

32.7 66.1

12.72 65.34

46.93 44.90

2.68 60.87

0.77 51.63

6.48 43.00

1.32 49.16

e

e

e

e

e

EI, Salvador Effluent concentration (mg L1) Removal efficiencies (%) Liaohe, China Effluent concentration (mg L1) Removal efficiencies (%) Petchaburi, Tailand Effluent concentration (mg L1) Removal efficiencies (%) Average concentration (mg L1) Average removal (%)

e e

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Peradeniya, Sri Lanka Effluent concentration (mg L1) Removal efficiencies (%)

Removal performance

119

Type of wastewater

Removal performance

120

Table 2 A summary of the wetland design/operational parameters and treatment efficiency of HSSF systems in developing countries. Dimension (m  m  m) (L  W  D)

Wetland design and operation HLR (m3 d1)

1.1  1.0  0.4

Phragmites australis

e

5

1.1  1.0  0.4

Phragmites australis

e

10

7.5  3.0  0.6

Cyperus papyrus

e

e

7.5  3.0  0.6

Cyperus papyrus

e

e

HRT (d) References

TSS

BOD5 COD

NH4eN NO3eN TN

TP

8.9

29.1

58.0

e

e

4.6

1.7

82.2

70.3

65.9

e

e

36.0

32.4

9.0

25.0

67.0

e

e

39.6

9.3

89.0

86.4

83.5

e

e

69.3

56.2

Municipal WW/Secondary 25.5

28.9

91.0

19.0

1.1

e

0.8

26.36

e

e

42.86

18.8

0.9

e

0.6

72.91 52.98 43.89

17.13

22

e

57.14

27.8 71.8

23.8 70.4

91 62.2

11.3 40.6

e e

e e

2 29.6

150  30  0.5

Phragmites australis Typha orientalis

20,000

120

Song et al. (2009)

e

e

41.80

15.86

0.83

e

e

4.2  1.4  0.6

Typha latifolia

0.043

2

Kaseva (2004)

e

e

60.70

23.01

44.30

e

e

Phragmites mauritianus

Dongying, Shangong, China Effluent concentration (mg L1) Municipal WW Removal efficiencies (%) Industrial WW

8.53 88.2

4.61 90

41.6 75.8

7.12 67.31

e e

e e

2 35.2 ha  0.5 59.23

e

50,000

1.8

Wang et al. (2006a)

Mother Dairy Pilot Plant, India Effluent concentration (mg L1) Municipal sludge/Tertiary Removal efficiencies (%)

12.0 81

4.0 90

55.0 72

e e

e e

7.5 67

1.5 75

69  46  0.3

Phragmites australis

43.05 L m1 d1 5.15

Jiaonan, Shangong, China Effluent concentration (mg L1) Municipal WW Removal efficiencies (%)

30 57.1

11 66.7

125 60.9

e e

e e

63.8 11.1

2.98 e

e

e

31.000

e

Song et al. (2009)

e e

e e

13.5 5.0 3.6  0.9  0.3 52.78 40.24

Strelitzia reginae Anthurium andreanum

128 L d1

4

Zurita et al. (2011)

33.90

e

e

9.11

Canna indica

e

11.5 h

Shi et al. (2004)

Egypt Effluent concentration (mg L1) Removal efficiencies (%) Effluent concentration (mg L1) Removal efficiencies (%)

Black water/Secondary

75.27 60.73 42.76 Municipal WW/Secondary 27.9

Rongcheng, Shandong, China Effluent concentration (mg L1) Municipal WW Removal efficiencies (%) Industrial WW Dar es Salaam, Tanzania Effluent concentration (mg L1) Removal efficiencies (%)

Municipal sludge/Tertiary

34.6

89.5

Ocotlán, Jalisco Mexico Effluent concentration (mg L1) Municipal WW/Secondary 10.4 25.4 59.4 Removal efficiencies (%) 81.66 77.94 76.32 Shatian, Shenzhen, China Effluent concentration (mg L1) Removal efficiencies (%)

Municipal WW/Secondary 7.92

7.68

0.56

80  30  1.5

86.78 86.40 76.72

e

e

44.93 81.70 58  20  1.6

Thaliade albata

Dhaka, Bangladesh Effluent concentration (mg L1) Tannery WW/Secondary Removal efficiencies (%)

12.1 55

0.08 98

0.2 98

15 86

33 50

e e

3 87

Phragmites australis

Taihu, Zhejing, China Effluent concentration (mg L1) Lake water Removal efficiencies (%)

e e

e e

4.23 39.6

1.16 32.0

0.37 65.3

2.29 52.1

0.052 20.0  1.5  1.0 Typha angustifolia 65.7

1.3  1.0  0.8

Abdel-Shafy et al. (2009)

Mburu et al. (2013)

Ahmed et al. (2008)

8h 6 cm d1

4.8 12.5

Saeed et al. (2012)

0.64 m d1

e

Li et al. (2008)

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Juja, Nairobi city, Kenya Effluent concentration (mg L1) Removal efficiencies (%) e CW1 Effluent concentration (mg L1) Removal efficiencies (%) e CW2

Greywater/Secondary

Plant species

e

e

e

e 17.86 2.72 e 51.97 65.96 6.15 44.08 10.25 53.52 19.96 20.28 64.75 79.93 75.10 66.02 Average concentration (mg L1) e Average removal (%)

e 31 mm d1 62 mm d1 104 mm d1 146 mm d1 Can Tho University, Vietnam Removal efficiencies (%) Removal efficiencies (%) Removal efficiencies (%) Removal efficiencies (%)

Municipal WW/Secondary 93 94 95 86

83 65 81 76

84 68 57 63

99 98 85 72

Phragmites vallatoria 12  1.6  1.1

Trang et al. (2010) 84 61 62 16 91 69 65 e

e

Katsenovich et al. (2009)

4.3. Hybrid system

151.4 12.04 2.61 39.3 e e e EI, Salvador Effluent concentration (mg L1) Municipal WW/Secondary 32.13 62.80 147.13 e Removal efficiencies (%) 84.15 22.0 56.2 e

121

difference was not statistically significant (p > 0.05) (see Tables 2 and 3). In comparison, Vymazal (2007) surveyed different types of CWs and concluded that the removal of TP varied between 40 and 60%, with mass removal ranging between 45 and 75 g P m2 yr1 depending on CW type and influent loading. Although phosphorus removal mechanisms in wetlands include plant/microbial uptake, fragmentation, leaching, mineralization, and burial, soil adsorption along controls the long-term phosphorus sequestration and is considered most effective in systems where wastewater comes into contact with the filtration substrate (Richardson and Marshall, 1986; Richardson, 1985). CWs with subsurface flow therefore have greater potential for phosphorus removal. Among the various CW systems, HSSF CWs show the highest potential as the substrate is constantly flooded and there is not much fluctuation in redox potential in the bed. In contrast, in the case of VSSF CWs where wastewater is fed intermittently, phosphorous removal may not be effective because the oxygenation of the bed may cause desorption and subsequent release of phosphorus (Vymazal, 2007). A comparison of treatment performance among the FWS and SSF systems was reviewed to assess the capabilities in purifying eutrophic water of Taihu Lake, China (Li et al., 2008). Three parallel pilot-scale CWs consisting of a VSSF, HSSF and an FWS CW planted with Typha angustifolia were examined. The authors reported that the treatment efficacy for the main parameters in the FWS CW was low (16.5% for COD, 22.8% for NH4eN, 19.8% for TN, and 35.1% for TP), compared to VSSF and HSSF CWs which showed statistically higher potential for the removal of COD (39.6% for HSSF and 40.4% for VSSF CW), NH4eN (32.0% for HSSF and 45.9% for VSSF CW), TN (52.1% for HSSF and 51.6% for VSSF CW), and TP (65.7% for HSSF and 64.3% for VSSF CW). Ruan et al. (2006) used an integrated system consisting of FWS and VSSF CW planted with Typha latifolia and Scirpus lacustris to improve the water quality of polluted Xinyi River, Jiangsu Province, China. The authors found that under different HLR (0.2e1.3 g m2 d1), the VSSF CW exhibited better removal of COD (77.38%) and NH3-N (96.9%) compared to FWS CW (61.1% for COD and 85.5% for NH3-N).

18.3  7.3  0.6 Phragmites australis

Zhang et al. (2010) e e e

e 85 e 86 e e e e e 89.1 e 94.4 Wuhan, China Effluent concentration (mg L1) Municipal WW Removal efficiencies (%)

3.0  0.7  1.0

e

130 L d1

Yang et al. (2008a,b) 3 5 Kandelia candel Aegiceras corniculatum 33  3  0.5 8.27 46 e e 6.28 50 25.31 70 8.37 90 Futian, Shenzhen, China Effluent concentration (mg L1) Municipal WW/Secondary e Removal efficiencies (%) e

0.65 60

e e 0.71 38.8 4.08 74.8 105.9 40.8 Peradeniya, Sri Lanka Effluent concentration (mg L1) Municipal WW/Secondary 47.33 18.6 Removal efficiencies (%) 65.8 65.7

8.03 61.2

1  25  0.6

Scirpus grossus Hydrilla verticillata

e

18

Tanaka et al. (2013)

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Due to the inability to provide both aerobic and anaerobic conditions simultaneously, single-stage CWs cannot achieve high removal of TN (Vymazal, 2005, 2007). In this regard, various types of CWs may be combined to leverage the advantages of individual systems. As many wastewaters may be difficult to treat in a singlestage system, hybrid systems which consist of various types of constructed wetlands staged in series have been introduced. Most hybrid systems are comprised of VSSF and HSSF systems arranged in a staged manner (Vymazal, 2011, 2007). The VSSF CW is intended to remove organics and suspended solids and to provide nitrification, while denitrification and further removal of organics and suspend solids occur in HSSF CW. A summary of the design/operational parameters and treatment efficiency of hybrid systems is shown in Table 4. The review reveals that hybrid systems in developing countries are primarily used in Asia (Table 6). Out of 15 surveyed hybrid systems, 11 have been designed to treat municipal sewage while other hybrid systems were designed to treat various types of wastewaters including lake waters, hospital wastewaters, laboratory wastewaters, and upflow anaerobic sludge blanket reactor effluent. In the 11 hybrid systems reviewed, the removal performance of TSS (93.82%), BOD5 (84.06%), COD (85.65%) and NH4eN (80.11%) were very effective, while TP (54.75%), NO3eN (63.58%), and TN (66.88%) showed a moderate and reliable removal. However, the removal efficiencies varied

122

Table 3 A summary of the wetland design/operational parameters and treatment efficiency of VSSF in developing countries. Type of wastewater

Removal performance TSS

BOD5 COD

NH4eN NO3eN TN

TP

Dimension (m  m  m) (L  W  D)

Wetland design and operation HLR (m3 d1) HRT (d) References

Salix babylonica

Plant species

e e

30.7 90

e e

e e

5.0 88

1.5  0.8  1.0

Longdao, Beijing Effluent concentration (mg L1) River water Removal efficiencies (%)

10.9 92.6

6.9 90.5

38.3 73.5

18.5 10.5

e e

18.5 10.6

1.59 30.6

28  0.021.5  1.45 Phragmites australis Zizania aquatica

Jinhe River, Tianjin, China Effluent concentration (mg L1) River water Removal efficiencies (%)

e e

e e

68.9 35

1.65 71.25

e e

2.58 0.2 0.196 m2  1.3 m 64.85 61.24

Ocotlán, Jalisco Mexico Effluent concentration (mg L1) Municipal WW/Secondary 21.9 20.8 49.5 e Removal efficiencies (%) 61.56 81.94 80.32 e

e e

14.6 4.2 1.8  1.8  0.7 49.38 50.14

Taihu, China Effluent concentration (mg L1) Lake water Removal efficiencies (%)

0.12 m d1

18 h

Wu et al. (2011)

3000

e

Chen et al. (2008)

Typha latifolia

0.8 m d1

e

Tang et al. (2009)

Strelitzia reginae Anthurium andreanum

128 L d1

e

Zurita et al. (2011)

e e

e e

4.25 40.4

0.89 45.9

0.5 62.9

2.37 51.6

0.05 51.6

20  1.5  1.0

Typha angustifolia

0.64 m d1

e

Li et al. (2008)

Shanghai, China Effluent concentration (mg L1) Municipal WW/Secondary e Removal efficiencies (%) e

e e

13.8 67

2.7 62

e e

3.7 53

1.9 33

e

e

e

e

Wang et al. (2006b)

Kampala, Uganda Effluent concentration (mg L1) Municipal WW Tertiary Removal efficiencies (%)

e e

e e

e e

7.1 75.43

0.09 60.87

16.1 2.6 0.58 m2  0.82 m 72.48 83.23

Cyperus papyrus

0.064

5

Kyambadde et al. (2004)

Wuxi, China Effluent concentration (mg L1) Livestock WW Secondary Removal efficiencies (%)

96 77.1

61.8 81.3

e e

32.9 61.7

e e

41.3 66.6

e 48.9

2.0  2.0  1.0

Phragmites communis Phragmites typhia

0.4

e

He et al. (2006)

Guangzhou, China Effluent concentration (mg L1) Municipal WW/Secondary e Removal efficiencies (%) e

8.37 90

25.31 6.28 70 50

e e

8.27 46

0.65 60

5.0  3.0  1.8

Cyperus alternifolius

0.45

18

Chan et al. (2008)

Chiang Mai, Thailand Effluent concentration (mg L1) UASB effluent/Secondary Total removal (%)

15 96

92 91

e e

97 76

0.6 97

2.0  2.0  1.4

Scirpusgrossus Linn.

e

e

Kantawanichkul et al. (2003)

e e

115.5 22.59 59.9 e

0.34 79.52

25.6 15

1.418 1.0  1.0  1.0 52

Typha orientalis Canna indica

250 mm d1

1.2

Chang et al. (2012)

87.04 20.78 50.95 17.43 85.25 89.29 64.14 61.20

0.31 67.76

20.97 1.72 e 50.55 59.61

e

e

e

e

4 98

Wuhan, China Effluent concentration (mg L1) Municipal WW/Secondary e Total removal (%) e Average concentration (mg L1) Average removal (%)

51 84

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Beijing, China Effluent concentration (mg L1) Municipal WW/Secondary 302.4 11.8 Removal efficiencies (%) 97 96

Table 4 A summary of the wetland design/operational parameters and treatment efficiency of hybrid systems in developing countries. Type of waste water (WW) Removal performance

Yongding River, China Effluent concentration (mg L1) Total removal (%) Kathmandu Valley, Nepal Effluent concentration (mg L1) Total removal (%)

Texcoco, Mexico Effluent concentration (mg L1) Total removal (%) Wuhan, China Effluent concentration (mg L1) Total removal (%) Nepal Effluent concentration (mg L1) Total removal (%) Turkey Effluent concentration (mg L1) Total removal (%) Ningbo, China Effluent concentration (mg L1) Total removal (%) Bogotá Savannah, Columbia Effluent concentration (mg L1) Total removal (%) Jakarta, Indonesia Effluent concentration (mg L1) Total removal (%) Lugu Lake, China Effluent concentration (mg L1) Total removal (%)

Hospital WW/Secondary

Municipal WW/Secondary

Municipal WW/Secondary

Municipal WW/Secondary

Municipal WW/Secondary

Municipal WW/Secondary

Municipal WW/Secondary

7.3 h m2

e

0.58 m3 m2 d1 34.26

Liu et al. (2007)

7  20

Phargmites karka

20

Shrestha et al. (2001)

Cyperus alternifolius

1.5e8.5

Phragmites communis

2.88

2.3

Belmont et al. (2004)

Thpha orientalis

250 mm d1

1.2

Chang et al. (2012)

TSS

BOD5 COD

NH4eN NO3eN TN

TP

12.3

5.91

5.47

4.27

1.51

6.38

0.1

99.1

77

67.4

52.8

62.25

59.4

91.8

2.83

3.29

20.20 1.61

e

e

4.22

e

e

46.60 11  11

97.25 97.01 93.80 95.18 Municipal WW/Secondary

Wetland design and operation HLR (m3 d1)

1.6

e

27.5

6.2

e

13.7

0.67

0.5  0.5  1.0

99.0

e

83.6

71.4

e

64.5

68.1

2.0  2.0  0.4

56.6

e

223.3 22.9

5.2

44.6

e

8.8  1.8  0.6

85.98 e

85.83 65.46

81.7

72.62 e

e

59.9

22.56 0.37

26.4

1.51

e

e

62.8

e

12.8

51.1

e

37.8

173.3 318.6 45

e

e

17.1

97.49 89.12 89.07 68.30

e

e

29.91 10.0  7.5  0.6

Canna latifolia

e

e

e

3.24

0.26

4.59

e

1.5  3.5  0.4

Iris australis

e

e

e

91.20

88.79

91.33 e

1.5  3.5  0.32

Phragmitesaustralis

14.2

e

49.3

7.7

e

11.9

861

Taxodium ascendens

16 cm d1

5.4

Zizania aquatica

32 cm d1

2.7

e

40 cm d1

0.6

10 cm d1

4.5

250 L d1

1

80.72

2.23

1.0  1.0  1.0

8.0  9.5  0.5

e

82.85 64.15 7  5  3

10

e

9

e

15

62.5

e

63.41 40

17,416 m2  0.5 m

0.06

0.65

3.04

3.0 m2  0.4 m

3

4354 m2  0.6 m

Laboratory WW/Secondary e

e

1.23

e

e

97.72 97.21

85.96

65.66 37.33

3.2

e

21.0

2.2

e

e

0.45

0.5  0.5  1.0

96.6

e

84.1

79.6

e

e

84.5

2.0  2.0  0.4

2.6

5.2

29.1

0.5

e

e

2.0

7  20

0.6

Zhai et al. (2011)

Canna india

84.60 83.11

28

e

2.8  4.0

88.57 e

96.90 92.26 e

Municipal WW/Secondary

HRT (d) References

Plant species

Pharagmites karka

Typha sp.

0.13 m d1

e

Singh et al. (2009)

60 L m2 d1

e

Tunçsiper (2009)

Ye and Li (2009)

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Fairy Mountain, China Effluent concentration (mg L1) Total removal (%)

Lake water

Dimension (m  m  m) (L  W  D)

Arias and Brown (2009)

Meutia (2001)

Lemna sp. Cyperus alternifolius

1.5e8.5

e

Zhai et al. (2011)

Phargmites karka

20

e

Shrestha et al. (2001)

Kathmandu Valley, Nepal Greywater/Secondary

123

(continued on next page)

e e e e 21.97 4.18 e 66.88 54.74 15.01 38.18 72.94 8.47 93.82 84.09 85.65 80.11 Average concentration (mg L1) e Average removal (%)

5.69 63.58

Kantawanichkul et al. (2003) e e Scirpus grossus Linn. 1.5  2.5  0.6 2.0  2.0  0.6 0.3 99 77 79 10 98

57 95

7 98 3 99 Chiang Mai, Thailand Effluent concentration (mg L1) UASB effluent/Secondary Total removal (%)

e e

e 400 Canna, Heliconia Papyrus 33 4.5 2300 m2  0.7 m 38.89 46.43 750 m2  0.6 m 25 e 91.58 e

e e 16 90 Koh Phi Phi, Thailand Effluent concentration (mg L1) Municipal WW/Secondary Total removal (%)

0.1 50

1.5  1.5  0.6 15 14 31 e 33 52

100 68

e e 20 79 Santa Fe de la Laguna, Mexico Effluent concentration (mg L1) Municipal WW/Secondary Total removal (%)

e e

35 e e 97

93

96 97 Effluent concentration (mg L1) Total removal (%)

TP NH4eN NO3eN TN BOD5 COD TSS

11  11

Plant species

Typha latifolia Phragmites australis

e e

4.0 0.5

HRT (d) References Wetland design and operation HLR (m3 d1)

Dimension (m  m  m) (L  W  D) Type of waste water (WW) Removal performance Table 4 (continued )

Brix et al. (2011)

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Rivas et al. (2011)

124

widely for TP (14e99%), NO3eN (13e89%), and TN (31e91%) depending on the system configuration, HLR, and plant species. In Nepal, a two staged CW system, which consisted of a threechamber septic tank, an HSSF bed (140 m2, 0.75-m depth), and a VSSF bed (12 m2, 1-m depth) was designed and constructed as a full-scale hybrid system to treat hospital and grey water (Shrestha et al., 2001). After one year of intensive monitoring, Shrestha et al. (2001) reported that the system showed high treatment efficiencies throughout the investigated period. The mean elimination rates were 97e99% for TSS, 97e99% for BOD5, 94e97% for COD, 80e99% for NH4eN, and 5e69% for TP. Rivas et al. (2011) investigated a multi-stage municipal wastewater treatment system consisting of an HSSFCW followed by a VSSF CW in Santa Fe de la Laguna, Mexico. The author reported efficient removals for BOD5 (94e98%), COD (91e93%), TSS (93e97%), and TN (56e88%), while significant TP removal was not attained in this study (25e52%). To promote the wastewater treatment and reuse of domestic wastewater as a water management solution in the Rio Texcoco watershed in Mexico, Belmont et al. (2004) conducted a pilot-scale wetland system consisting of a stabilization pond, HSSF CWs, and VSSF CWs planted with T. angustifolia and Chrysanthemum cinerariaefolium in the small community of Santa María Nativitas. The treatment system was operated at a flow of 2 L min1 which corresponded to a hydraulic retention time (HRT) of 2.3 d in each SSF CW. The average volume of wastewater treated daily was 2.88 m3 d1. This integrated treatment system reduced COD, TSS, NH4eN, NO3eN, and TN from domestic wastewater by an average of 84.9%, 58.6%, 53.9%, 81.7%, and 71.7%, respectively. Singh et al. (2009) investigated the performance of a model for decentralized wastewater treatment using a hybrid CW system to treat high strength wastewater from 80 households (400 PE) in Nepal. After the primary treatment in an anaerobic baffled reactor, the wastewater was treated in two HSSF CWs first, and then in two VSSF CWs planted with Phragmites karka and Canna latifolia, which served as secondary wastewater treatment. The authors indicated that this hybrid system was very effective at the contaminant removal: 96% for TSS, 90% for BOD5, 90% for COD, 70% for NH4eN, and 26% for TP. Besides HSSFeVSSF and VSSFeHSSF systems, hybrid CWs may quite often include an FWS stage (Vymazal, 2007). Brix et al. (2011) presented the design and implementation as well as the performance of a CW system, surrounded by resorts, restaurants, and shops on the tourist island of Koh Phi Phi in Southern Thailand. The treatment process train consisted of VSSF, HSSF, and FWS units. The composition of influent resembled domestic sewage, but more concentrated than anticipated (due to the lack of pre-treatment). The effluent from the system met the Thai effluent standards for TSS and TKN, but BOD concentrations were slightly higher (average 25 mg L1) than the effluent standard (20 mg L1). However, the removal of nitrogen was not satisfactory due to the unexpected high concentration in the influent. In addition to municipal wastewaters, hybrid systems comprised of FWS CWs and SSF CWs have also been successfully demonstrated for the treatment of river/lake water. Xiong et al. (2011) explored the treatment capacity of an integrated CW to reduce nitrogen from secondary effluent at a wastewater treatment plant in Hangzhou City, Zhejiang, China. The system consisted of a VSSF CW planted with Vetiver zizanioides and an FWS CW planted with Coislacryma-jobi. The results showed that the integrated CW demonstrated good potential for removing N and displayed superior removal efficiency for NO3eN (98.83%), NH3eN (95.60%), NO2eN (98.05%), and TN (92.41%), respectively. 4.4. Floating treatment wetland (FTW) Floating treatment wetlands (FTWs) are a novel treatment concept that employ rooted, emergent macrophytes growing on a

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

floating mat rather than rooted in the sediments (Tanner and Headley, 2011; Fonder and Headley, 2010). Unlike HSSF CW systems with limited application for stormwater treatment when subjected to high flow velocities, one of the main advantages of FTWs over conventional sediment-rooted wetlands is their ability to cope with variable water depth that is typical of event-driven stormwater systems (Kerr-Upal et al., 2000). This feature also enables the FTW system to be designed as an extended detention basin so that large runoff events can be captured and released slowly over subsequent days. A summary of the design/operational parameters and treatment efficiency of FTWsis shown in Table 5. Based on the present review, FTW systems showed better removal efficiencies for BOD5 (79.31%), COD (55.20%), NO3eN (73.45%), TN (62.45%), and TP (49.58%), compared to FWS CW systems (65.34% for BOD5, 44.9% for COD, 51.63% for NO3eN, 43% for TN, and 49.16% for TP, respectively). It is conceivable that plant assimilation of nutrients and other elements (e.g., heavy metals), may be higher in an FTW system compared to a sediment-rooted wetland (Headley and Tanner, 2011). In an FTW, the plant roots are not in contact with the benthic sediments or soil and can only access nutrients contained within the floating mat and in the water column (Kadlec and Wallace, 2008). This is in contrast to a sediment-bound wetland, such as an FWS CW, where the plant roots acquire nutrition from the underlying soil. Beneath the floating mat, a network of roots, rhizomes, and the hanging rootbiofilm network provides a biologically active surface area for the biochemical transformation of contaminants and physical processes such as filtering and entrapment of particulates (Kyambadde et al., 2004; M. Li et al., 2009). FTWs have been used for water quality improvement, habitat enhancement, and aesthetic improvement at ornamental ponds and lakes to date (Tanner and Headley, 2011). Boonsong and Chansiri (2008) examined the efficiencies of V. ziznnioides (L.) Nash cultivated with a floating platform technique to treat domestic wastewater under three different HRTs of 3, 5, and 7 d in Bangkok, Thailand. The average TN and NH4eN removal efficiencies were 9.97e62.48% and 13.35e58.62%, while the average TP and phosphate-P removal efficiencies ranged from 6.3% to 35.87% and from 7.40% to 23.46%. The results indicated that the 7-d HRT showed the best treatment performance for BOD, TN, and TP, with average removal efficiencies of 90.5e91.5%, 61.0e62.5%, and 17.8e 35.9%, respectively. In China, the purification of nitrate-rich agricultural runoff with influent TN and TP concentrations of 3.8e 7.9 g m3 and 1.2e1.5 g m3 and loading rates of 0.68e 4.3 g m2 d1 and 0.22e0.83 g m2 d1 using FTW hydroponic system was investigated by Yang et al. (2008a,b). The authors reported an efficient nitrateenitriteenitrogen (NOx-N) removal, 91%, 97% and 71% on average at 3-, 2- and 1-d HRT. Removal efficiencies of 17e47% for COD and 8e15% for TP were achieved. Sun et al. (2009) investigated removal of TN from polluted river water (inlet concentration of 8.7 g m3) in Guangzhou, China using FTW mesocosms at an HLR of 120 L m2 d1and reported that the removal efficiencies were 72.1% for total nitrogen, 75.8% for NO3eN, 95.9% for NO2-N, and 94.6 for COD. 4.5. Overall performance Fig. 1 shows a comparison of the treatment performance of different types of CWs. In the present review, all types of hybrid systems appear more efficient in the removal of TSS (93.82%), COD (85.65%), NH4eN (80.11%), and TN (66.88%), compared to other types of CWs. Both HSSF (65.96%) and VSSF (59.61%) CWs showed superior TP removal compared to FWS CWs (49.16%), hybrid systems (54.75%), and FTWs (49.58%). VSSF systems showed the best BOD (89.29%) removal compared to other types of CWs (65.34% for

125

FWS CWs, 75.1% for HSSF CWs, 84.06% for hybrid systems, and 55.22% for FTW). Haberl et al. (1995) reported on the pollutant removal efficiencies of 268 operational treatment wetlands in Europe. Using CW performance in developing countries (see Tables 1e5), a comparison of removal efficiencies of FWS CWs, HSSF CWs, VSSF CWs, hybrid systems, and FTWscan be made between developing countries and the developed nations of Europe. In the present review, the mean removal efficiencies of BOD5 for developing countries were 65.34%, 75.10%, 89.29%, 84.06%, and 79.31% for FWS CWs, HSSF CWs, VSSF CWs, hybrid systems, and FTWs, respectively. In comparison, the value for treatment wetlands in Europe was 79.1% (Haberl et al., 1995). Apparently, most treatment wetlands in developing countries appear to perform in the same general range as those in Europe. In contrast, compared to the mean COD removal efficiency in Europe (69.5%), the performance of most CW systems in developing countries was less satisfactory, with removal efficiencies of 44.90%, 66.02%, 66.41%, 85.65% and 55.20% for FWS CWs, HSSF CWs, VSSF CWs, hybrid systems, and FTWs, respectively. With regards to NH4eN removal in developing countries, efficiencies of 60.87%, 53.52%, 61.20%, 80.11%, and 69.98% for HSSF CWs, VSSF CWs, hybrid systems, and FTWs, respectively, were significantly higher than that in Europe (30.30%) (Haberl et al., 1995). The mean TN removal efficiencies of 43%, 51.97%, 50.55%, 66.88%, and 62.45% for FWS CWs, HSSF CWs, VSSF CWs, hybrid systems, and FTWs, respectively, were significantly higher than the mean TN removal rate of 39.6% for European treatment wetlands (Haberl et al., 1995). In the same manner, the mean removal efficiencies for TP of 49.16%, 65.96%, 59.61%, 54.75%, and 49.58% were calculated for treatment wetlands in developing countries for FWS CWs, HSSF CWs, VSSF CWs, hybrid systems, and FTWs, respectively. In comparison, the value reported for European systems of 47.1% (Haberl et al., 1995) was generally lower than that above for developing countries. 5. Impacts of wetland design and operational variables Pollutant removal efficiency in CWs depends on a number of variables including pollutant loading, hydrologic regime, vegetation type, and temperature, all which may be highly variable among various systems (Bojcevska and Tonderski, 2007; Trang et al., 2010). Most commonly, pollutant removal is often accomplished by manipulating the system’s hydraulic conditions and by selecting the appropriate type of dominant vegetation (Vymazal, 2007; Kadlec and Wallace, 2008). 5.1. Hydraulic loading rate Hydraulic conditions strongly influence the biotic community composition, biogeochemical processes, and the fate of pollutants in CWs (Kadlec and Wallace, 2008; Ranieri et al., 2013c). HRT, the length of time during which the pollutants are in contact with the substrate and the plant rhizosphere, is well known to be a crucial controlling factor in determining the removal efficiency of contaminants (Stottmeister et al., 2003). A long HRT allows for extensive interaction between contaminants and wastewater. In contrast, at a high HLR (or low HRT), wastewater moves rapidly to the outlet reducing the contact time among the wastewater, the rhizosphere, and the microorganisms. However, longer HRTs typically require larger land space and higher capital cost, which are key factors in operation. Thus, studying on the effects of different HRTs (or HLR) on removal performance of pollutants in CW systems is important. Konnerup et al. (2009) assessed the suitability of using CWs for the treatment of wastewater using gravel-based HSSF CWs planted with Canna xgeneralis L. Bailey at the Asian Institute of Technology, Thailand. Four HLRs were tested in the CWs: 55, 110, 220, and

126

Table 5 A summary of the wetland design/operational parameters and treatment efficiency of floating treatment wetlands in developing countries. Type of wastewater (WW)

Taihu Lake, China Effluent concentration (mg L1) Total removal (%) Effluent concentration (mg L1) Total removal (%) Kampala, Uganda Effluent concentration (mg L1) Total removal (%) Effluent concentration (mg L1) Total removal (%) Pear River, China Effluent concentration (mg L1) Total removal (%) Bangkok, Thailand Effluent concentration (mg L1) Total removal (%) Effluent concentration (mg L1) Total removal (%) Average concentration (mg L1) Average removal (%)

Dimension (m  m  m) (L  W  D)

Wetland design and operation

HLR (m3 d1)

HRT (d)

References

Weragoda et al. (2012)

TSS

BOD5

COD

NH4eN

NO3eN

TN

TP

Municipal WW

e e

9.7 65.5

e e

4.6 81.6

3.9 50.0

e e

11.5 88.5

1.0  0.5  0.65

Canna iridiflora Typha angustifolia

e

14

Agricultural runoff

e e e e

e e e e

36.70 47 40.2 24

0.54 60 1.31 e

1.42 71 0.05 97

2.86 64 2.95 35

1.34 13 1.16 15

2.0  1.0  0.75

Oenanthe javanica

e

1

e

2

Municipal WW Tertiary Municipal WW Tertiary

e e e e

e e e e

e e e e

7.7 89.3 23.4 70.45

8.5 89.76 28.4 67.76

5.4 84.53 12.5 64.71

e e e e

Cyperus papyrus

River water

e e

e e

1.98 94.6

0 100

0.67 75.8

2.42 72.1

e e

1.2  0.8  1.2

Municipal WW/Secondary

e e e e

7.5 91.89 18 80.54

e e e e

15 50 18.5 38.54

e e e e

19.9 57.57 23 50.96

4.3 31.75 e e

e

e

11.73 79.31

26.29 55.2

8.88 69.98

1.51 73.45

12.58 62.45

6.03 49.58

e

e

Plant species

Yang et al. (2008a,b)

e e e e

e e e e

Kansiime et al. (2005)

Canna

e

e

Sun et al. (2009)

Polystyrene sheet

e

7

Polystyrene sheet

e

5

e

e

Colocasiae sculenta

e

Boonsong and Chansiri (2008)

e

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

Kandy, Sri Lanka Effluent concentration (mg L1) Total removal (%)

Removal performance

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

127

loaded with a mixture of domestic and pig farm wastewater at three HRTs of 80, 160, and 320 mm d1. The maximum mass removal rates at the highest HLR were: BOD16, COD 50, TN 178, NH4eN 80 and TP 36 g m2 d1 in the planted VSSF beds, while the values were BOD13, COD 37, TN 119, NH4eN 58, and TP 23 g m2 d1 in the HSSF beds. As expected, the removal efficiency of all parameters decreased with increasing HLR. 5.2. The presence of vegetation

Fig. 1. Average removal efficiencies of contaminants in various CW systems: (A) TSS and TP removal; (B) BOD and COD removal; and (C) nitrogen removal. Note: (1) HS: hybrid system; (2) EU: European countries; and (3) Error bar: standard deviation.

440 mm d1. The authors indicated that the outlet concentrations of pollutants were significantly affected by HRT and increased over time. The mean mass removal efficiencies of TSS (96%, 95%, 93%, and 88%), COD (83%, 73%, 59%, and 42%), TN (37%, 19%, 13%, and 6%), and TP (35%, 23%, 12%, and 10%) were achieved for the mesocosms with different HLRs of 55, 110, 220, and 440 mm d1, respectively. Bojcevska and Tonderski (2007) investigated the effects of wastewater HLR on the treatment of sugar factory effluent using a pilotscale FWS CW system in western Kenya. For 12 months, four CWs were operated at HLRs of 75 and 225 mm d1. The authors indicated that when using mean data for each CW separately, the mass removal rates of the tested parameters, i.e., TP, NH4eN, and TSS, were significantly affected by mass loading rate (p < 0.001), and there was a linear relationship between the mass removal and mass loading rate. Truong et al. (2011) studied the potential of using replicated HSSF and VSSF systems planted with Sesbania sesban for the treatment of high strength wastewater. The systems were

It is generally assumed that planted wetlands outperform unplanted controls mainly because the plant rhizosphere stimulates microbial community density and activity by providing root surface for microbial growth, and a source of carbon compounds through root exudates (Tanner, 2001; Vymazal and Kröpfelová, 2009). Aquatic macrophytes can affect the redox status of CW sediments by releasing oxygen from their roots into the rhizosphere and thereby stimulating aerobic decomposition (Tanner et al., 2002). The better oxidizing condition in planted beds appears to enhance the biodegradation process by transferring oxygen down to the root system, which in turns stimulates the density and diversity of the microbial community and enhances the microbial activities associated with the rhizosphere (Stottmeister et al., 2003; Caselles-Osorio and García, 2007). In tropical climates where the plants grow faster and throughout the year, the uptake of nutrients can contribute to significantly higher removals of nutrients as has been reported in several studies (e.g., Kyambadde et al., 2004; Kantawanichkul et al., 2003). In an investigation on the effect of plant presence and plant species on contaminant removal in a CW, Kyambadde et al. (2004) compared the wastewater treatment efficiencies of CWs planted with two local plant species, Cyperus papyrus and Miscanthidium violaceum, dominant in the Nakivubo wetland in Kampala, Uganda. The authors reported removal efficiencies in CWs planted with C. papyrus of 75.43% NH4eN, 72.47% TN, and 83.23% TP, while that of CWs planted with M. violaceum of 64.59%NH4eN, 69.40% TN, and 48.39% TP.Both vegetated wetlands showed much higher removal efficiencies than those in the unplanted control bed (28%, 25.6%, and 8% for NH4eN, TN, and total reactive P, respectively). Yang et al. (2007) conducted a comparative study of the efficiency of contaminant removal among five emergent plant species in a small-scale CW in Guangzhou, China. The authors demonstrated that there was a significant difference in the removal rate of TN and TP, but no significant difference in the removal of organic matter between vegetated and unvegetated wetlands. The average removal efficiencies for TN and NH4eN in the vegetated wetlands were 75% and 72%, respectively, while those of the unplanted wetlands were more than 10% lower. Removal of TP was considerably higher in both, averaging over 90% for the vegetated wetlands and approximately80% for the control wetland. 5.3. Type of vegetation While the positive role of macrophytes on contaminant removal in CWs has been established, species selection is always one of the important considerations. A number of studies on the removal of conventional pollutants demonstrated that selection of macrophyte species matters (Konnerup et al., 2009; Kyambadde et al., 2004; Ranieri et al., 2013b). Different vegetation species have different capacities for nutrient uptake and accumulation of heavy metals (Tanner, 1995), as well as variable effects on the functioning and structure of bacterial communities involved in the removal of contaminants in the treatment cells of a CW (Ruiz-Rueda et al., 2009). It has been generally accepted that macrophyte species can influence treatment efficiency of nutrients in a CW (Yang et al., 2007; Kyambadde et al., 2004). In a comparative study of C. papyrus and

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M. violaceum-based CWs for wastewater treatment, Kyambadde et al. (2004) showed that plant uptake and storage was the major factor responsible for N and P removal in CWs planted with C. papyrus (where the plant contributed to 69.5% for N and 88.8% for P of the total N and P removal). On the other hand, for the CWplanted with M. violaceum, the plants only accounted for 15.8% N and 30.7% P of the total N and P removal. Their results also indicated greater removal of both TP and NH4eN in CWs dominated by C. papyrus (83% and 75%, respectively) than in those with M. violaceum (49% and 62%, respectively). The authors also observed significantly higher dissolved oxygen concentrations in CWs planted with C. papyrus than in those with M. violaceum. In an investigation of biomass production and nutrient uptake by different plant species in a gravel-based SSF CW system in Thailand, Konnerup et al. (2009) reported that the removal of TN was higher in the beds planted with Canna generalis than those planted with Heliconia psittacorum, mainly due to the higher growth rate of C. generalis (3100  470 g DW m2 yr1) compared to H. psittacorum (550  90 g DW m2 yr1). The authors concluded that Canna is the preferred species from the treatment perspective because of its more vigorous growth, and the results emphasized the important role of macrophytes for nutrient removal in tropical regions with high plant growth rates year-round. Senzia et al. (2003) conducted a field investigation on pilot-scale HSSF CW units located downstream of waste stabilization ponds in Tanzania. The CW system was designed to treat effluent from a facultative pond before discharging into a nearby river. Based on the total inflow nitrogen loading of 1.457 gN m2 d1, the CW system planted with Phragmites mauritianus showed nitrogen removal of 54%, while those planted with Typha domingensis had a nitrogen removal of 44%, mostly due to accumulation of organic nitrogen during the process of sedimentation and filtration. The authors indicated that T. domingensis has relatively shallow roots, which pose less impact on filtration and sedimentation as compared to P. mauritianus. In addition, plant communities consisting of multiple plant species with different seasonal growth patterns and root characteristics may be able to enhance the performance of a wetland. In a comparative study of the efficiency of contaminant removal between five emergent plant species, Yang et al. (2007) reported that Pennise tumpurpureum had the highest nutrient removal rates during the period from May to June, and Canna indica showed the highest removal rate during the month of August. The removal rate of Phragmites communis was the highest during the month of December. This finding implied that the removal efficiency of contaminants varied with season and patterns of plant growth, and the most vigorous growth period of the plants corresponded to high contaminant removal rates. 5.4. Seasonal variation Reduction of nutrient concentrations in a wetland is mainly by biotic, temperature-dependent activity. Consequently, the

influence of temperature is an important parameter when the pollutant treatment effectiveness of a CW is evaluated. In general, the efficiency of treatment in a CW decreases at low temperature primarily because of reduced biotic activity. Song et al. (2009) evaluated wastewater treatment effectiveness and seasonal performance of a full-scale CW system with a total area of 80 ha and treatment capability of 2.0  104 m3 d1 in Shanghai, China. COD, NH4eN, and TP removal efficiencies displayed seasonal variations. COD removal was more efficient in the spring (65.4%) and summer (66.3%) than in autumn (61.1%) and winter (59.4%), whereas NH4eN and TP removal was more efficient in summer (54.5% for NH4eN and 35.0% for TP) and autumn (43.3% for NH4eN and 34.0%) than in spring (33.7% for NH4eN and 28.2% for TP) and winter (32.4% for NH4eN and 28.9% for TP). In an investigation on the potential of CWs in treating the eutrophic lake water of Taihu, China, Li et al. (2008) reported that no statistically significant seasonal differences were found for COD removal. However, the nutrient removal rates fluctuated during the 12-month investigation. Higher NH4eN and TN removal effects appeared in autumn and summer, while in winter (DecembereFebruary), the elimination of NH4eN and TN remained relatively the lowest in all CWs. No statistically significant seasonal variation was detected for NO3eN removal efficiencies treatment (p > 0.05). By contrast, the removal rate of TP was higher in the first several months (AugusteOctober) in all CWs, then decreased and/or fluctuated over the rest experimental period. 6. Cost and space requirement 6.1. Cost requirement Table 6 shows a comparison of the capital and operational cost for a traditional wastewater treatment plant (WWTP) and CWs in developing countries. Although conventional WWTP and activated sludge processes are efficient in wastewater treatment, the costeffectiveness can only be achieved in densely populated urban areas. In contrast, the application of CWs is more affordable as the wastewater treatment for small communities. Table 6 shows that although the CW systems do not present an apparent advantage in construction cost, the cost of treatment and operation and maintenance (O/M) for CW systems are much lower than those for a conventional WWTP and activated sludge processes. In China, the cost of CW construction (US$ 164e460 m3) accounts for only one-third to one-half of building a WWTP (US$ 246e657 m3) (Liu et al., 2008). Furthermore, CW systems have extremely low O/M cost (US$ 0.0082e0.039 m3of wastewater) including power for pumps, harvesting of vegetation, and insect/ pest control, compared to that for a WWTP(US$ 0.1151e0.2465 m3 wastewater).Wang et al. (2006a) reported that the total capital cost of an ecosystem consisting of integrated ponds and a CW system (Shandong, China) was US$ 82 m3 d1, which is about half of the cost of conventional systems based on the activated sludge process.

Table 6 Comparison of cost requirements between CWs and WWTPs. Design capacity (m3 d1)

Total capital cost (US$)

Conventional WWTP

Unit capital cost (US$ m3)

Treatment cost (US$ m3)

246e657

CWs in the present study Dongying, Shandon, China Bogota Savannah, Colombia Longdao River, Beijing, China

100,000 65 200

8.2 million 14,672 32,616

References

164e460

0.0082e0.039

82 225.72 163.08

0.012 0.0134 0.014

Wang et al. (2005) Arias and Brown (2009) Chen et al. (2008)

0.0223

0.1151e0.2465 0.6362

Energy cost (US$ m3)

Liu et al. (2008) Hernández-Sancho and Sala-Garrido (2009) Liu et al. (2008)

0.7717 CWs in China

O/M cost (US$ m3)

0.1036

D.Q. Zhang et al. / Journal of Environmental Management 141 (2014) 116e131

The O/M cost is US$ 0.012 m3, only one-fifth that of conventional treatment systems. Chen et al. (2008) also reported that at a treatment capacity of 200 m3 d1, the construction cost of the Longdao River CW (Beijing, China) was estimated at US$ 0.02 m3, and the average total treatment cost was calculated at US$ 0.03 m3, which was equal to one-fifth of that for a conventional WWTP. 6.2. Land requirement CW systems for wastewater treatment are usually land intensive and require more space than that for conventional wastewater treatment systems (Kivaisi, 2001; Brissaud, 2007). The high land requirement for CWs is the main barrier for expanding the application of CWs, especially in densely populated areas where land prices are often too high. CW treatment may be economical relative to other options if land is available and affordable. According to Vymazal (2011), the area of an HSSF CW is usually about 5 m2 PE1 (PE ¼ population equivalent ¼ 60 BOD5 d1), while a VSSF CW requires less land, usually 1e3 m2 PE1 for municipal sewage. However, in the Asian Cities Programme (WAC) which was aimed at helping developing countries tackle difficult water and environmental sanitation problems through practical community based approaches in small and medium sized towns, UN-HABITAT (2008) revealed that an area of 1e2 m2 PE1 would be required for an HSSF CW while a specific area of 0.8e1.5 m2 PE1 would be needed for a VSSF CW. There are reported CWs with less land requirement. Zhai et al. (2011) studied a new type of hybrid CW system consisting of both a vertical-baffled flow CW and an HSSF CW to treat municipal wastewater in Southern China. The land area required for municipal sewage treatment with their hybrid CW system was 0.70e 0.93 m2 PE1 for vegetated wetland beds, which is much smaller than that for a conventional HF CW (5 m2 PE1) and VSSF CWs (1e 3 m2 PE1) (Vymazal, 2011). The new hybrid CW system can treat municipal wastewater with similar or even higher removal efficiencies than the conventional CW system but with less land required. Dallas et al. (2004) introduced a low-cost grey water treatment system in a typical Costa Rican community in Central America. The reed bed serving as an HSSF CW was treating approximately 755 L d1 of greywater from seven people. The reed bed had a land requirement of 2.4 m2 PE1 for grey water treatment. 7. Conclusion CWs are recognized as a reliable wastewater treatment technology and are emerging as a suitable, cost-effective solution for treatment of various types of wastewater in developing countries. The focus of this review has been on the performance of contaminant removal in different types of CWs in developing countries. Leveraging the advantages of individual systems, hybrid systems that include various types of CWs achieved higher treatment effects and were more efficient for the removal of TSS (93.82%), COD (85.65%), NH4eN (80.11%), and TN (66.88%), as compared to other types of CWs. Both HSSF (65.96%) and VSSF (59.61%) showed superior TP removal, compared to FWS CWs (49.16%), hybrid system (54.75%), and FTWs (49.58%). VSSF systems showed the best BOD (89.29%) removal compared to other types of CWs. A comparison between VSSF and HSSF CWs showed that VSSF CWs removed more NH4eN (61.20%) compared to HSSF CWs (53.52%), since VSSF CWs were intermittently loaded and had unsaturated flow resulting in a higher transfer of oxygen to the filter medium compared to HSSF CWs. FTW systems were a novel treatment concept and outperformed FWS CWs for all the parameters investigated.

129

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