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sub-surface flow: A review of the field experience. Jan Vymazala,b,c,⁎, ... Available online 25 September 2008. Constructed ...... Wat Sci Tech 2001;44(11/12):441–8. Börner T, von ... Florida, USA: Lewis Publisher/CRC Press; 1999. p. 205–22.
S CI EN C E OF TH E T OTAL EN V I RO N M EN T 4 0 7 ( 2 0 09 ) 39 1 1–3 92 2

a v a i l a b l e a t w w w. s c i e n c e d i r e c t . c o m

w w w. e l s e v i e r. c o m / l o c a t e / s c i t o t e n v

Removal of organics in constructed wetlands with horizontal sub-surface flow: A review of the field experience Jan Vymazal a,b,c,⁎, Lenka Kröpfelová a a

ENKI, o.p.s., Dukelská 145, 379 01 Třeboň, Czech Republic Institute of Systems Biology and Ecology, Czech Academy of Sciences, Dukelská 145, 379 01 Třeboň, Czech Republic c Czech University of Life Sciences in Prague, Faculty of Environmental Sciences, Department of Landscape Ecology, Náměstí Smiřických 1, 281 63 Kostelec nad Černými lesy, Czech Republic b

AR TIC LE D ATA

ABSTR ACT

Article history:

Constructed wetlands with horizontal sub-surface flow (HF CWs) have successfully been

Received 7 January 2008

used for treatment various types of wastewater for more than four decades. Most systems

Received in revised form

have been designed to treat municipal sewage but the use for wastewaters from

20 August 2008

agriculture, industry and landfill leachate in HF CWs is getting more attention

Accepted 20 August 2008

nowadays. The paper summarizes the results from more than 400 HF CWs from 36

Available online 25 September 2008

countries around the world. The survey revealed that the highest removal efficiencies for BOD5 and COD were achieved in systems treating municipal wastewater while the lowest

Keywords:

efficiency was recorded for landfill leachate. The survey also revealed that HF CWs are

BOD5

successfully used for both secondary and tertiary treatment. The highest average inflow

COD

concentrations of BOD5 (652 mg l− 1) and COD (1865 mg l− 1) were recorded for industrial

Constructed wetlands

wastewaters followed by wastewaters from agriculture for BOD5 (464 mg l− 1) and landfill

Horizontal flow

leachate for COD (933 mg l− 1). Hydraulic loading data reveal that the highest loaded

Macrophytes

systems are those treating wastewaters from agriculture and tertiary municipal

Organic loading

wastewaters (average hydraulic loading rate 24.3 cm d− 1). On the other hand, landfill leachate systems in the survey were loaded with average only 2.7 cm d− 1. For both BOD5 and COD, the highest average loadings were recorded for agricultural wastewaters (541 and 1239 kg ha− 1 d− 1, respectively) followed by industrial wastewaters (365 and 1212 kg ha− 1 d− 1, respectively). The regression equations for BOD5 and COD inflow/outflow concentrations yielded very loose relationships. Much stronger relationships were found for inflow/outflow loadings and especially for COD. The influence of vegetation on removal of organics in HF CWs is not unanimously agreed but most studies indicated the positive effect of macrophytes. © 2008 Elsevier B.V. All rights reserved.

1.

Introduction

Organic matter is decomposed in constructed wetlands with horizontal sub-surface flow (HF CWs) by both aerobic and anaerobic microbial processes as well as by sedimentation and filtration of particulate organic matter. Because of heavy organic

loading and continuous saturation of the filtration bed anoxic/ anaerobic processes prevail while aerobic processes are restricted to small zones adjacent to roots and rhizomes (radial oxygen loss) and to a thin surface layer where oxygen diffusion from the atmosphere may occur. In lightly-loaded systems dissolved oxygen may be also carried out by inflowing wastewater.

⁎ Corresponding author. ENKI, o.p.s., Dukelská 145, 379 01 Třeboň, Czech Republic. Tel.: +420 233 350 180. E-mail address: [email protected] (J. Vymazal). 0048-9697/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.scitotenv.2008.08.032

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Aerobic degradation of soluble organic matter is governed by the aerobic heterotrophic bacteria according to the following reaction:

butyric, and lactic Eq. (3) acids, alcohols Eq. (4) and the gases CO2 and H2 (Mitsch and Gosselink, 2000; Reddy and Graetz, 1988; Vymazal, 1995; Megonikal et al., 2004):

C6 H12 O6 þ 6O2 →6CO2 þ 6H2 O þ 12e− þ energy

C6 H12 O6 →3CH3 COOH þ H2

ð2Þ

C6 H12 O6 →2CH3 CHOHCOOHðlactic acidÞ

ð3Þ

C6 H12 O6 →2CO2 þ 2CH3 CH2 OHðethanolÞ

ð4Þ

ð1Þ

Insufficient supply of oxygen to this group will greatly reduce the performance of aerobic biochemical oxidation. However, if the oxygen supply is not limited, aerobic degradation will be governed by the amount of active organic matter available to the organisms. In most types of wastewaters with the exception of some industrial wastewaters and runoff waters the supply of dissolved organic matter is sufficient and aerobic degradation is limited by dissolved oxygen concentration (Vymazal, 2001). Organic matter is composed of a complex mixture of biopolymers (Megonikal et al., 2004). Some of these compounds, such as proteins, carbohydrates, and lipids are easily degraded by microorganisms (i.e., labile), while other compounds, such as lignin and hemicellulose, are resistant to decomposition (i.e., recalcitrant). Biopolymers are degraded in a multi-step process (Fig. 1). First, microorganisms simplify polymers to monomers such as amino acids, fatty acids, and monosaccharides (Megonikal et al., 2004). The primary endproducts of fermentation are fatty acids such as acetic Eq. (1),

Next, primary fermentation products are mineralized to CO2 and CH4, or they undergo secondary fermentation to smaller volatile fatty acids (Fig. 1). Acetic acid is the primary acid formed in most flooded soils and sediments. Strictly anaerobic sulfate-reducing bacteria Eq. (5) and methaneforming bacteria Eqs. (6) and (7) then utilize the end-products of fermentation and, in fact, depend on the complex community of fermentative bacteria to supply substrate for their metabolic activities. Both groups play an important role in organic matter decomposition and carbon cycling in wetland soil environments (Valiela, 1984; Grant and Long, 1985; Vymazal, 1995): CH3 COOH þ H2 SO4 →2CO2 þ 2H2 O þ H2 S

ð5Þ

CH3 COOH þ 4H2 →2CH4 þ 2H2 O

ð6Þ

4H2 þ CO2 →2CH4 þ 2H2 O

ð7Þ

Megonikal et al. (2004) pointed out that whereas hydrogenotrophy Eq. (7) is widespread among the methanotrophs, acetotrophy Eq. (8), also known as acetate fermentation or acetoclastic methanogenesis) is restricted to just two genera, the Methanosarcina and Methanosaeta, which comprise about 10% of methanogenic species: CH3 COOH→CH4 þ CO2

Fig. 1 – Metabolic scheme for the degradation of complex organic matter, culminating in methanogenesis. Polymers are cleaved via extracellular or cell-surface associated enzymes to monomers that are fermented to organic products, H2 and CO2. Methane is formed primarily from the oxidation of H2 coupled to CO2 reduction or by the fermentation of acetate. Acetate is formed by primary fermentation, acetogenesis from H2/CO2 and from secondary fermentation of primary fermentation products. From Megonikal et al. (2004).

ð8Þ

First-order degradation has been used for design and to predict removal performance for basically all pollutants of interest in constructed wetlands (e.g., Reed et al., 1995; Reed, 1993; Cooper et al., 1996; Vymazal et al., 1998; Mitchell et al., 1998; Dahab et al., 2001). Its inadequacies has been recognized (Bavor et al., 1988, Kadlec and Knight, 1996) but it is still seen as the most appropriate design equation describing pollutant removal in light of present knowledge (Kadlec et al., 2000). Mitchell and McNevin (2001) suggested the model based on the assumption that the biological processes in wetlands, like other biological systems, exhibit Monod kinetics. Recently, more complex models have been presented (Wynn and Liehr, 2001; Mashauri and Kayombo, 2002; Langergraber, 2003; Rousseau et al., 2004; Marsili-Libelli and Checchi, 2005). However, most of these models require many parameters which are often difficult to measure and also include many presumptions which may not be valid for all systems. The primary objective of this paper is to evaluate the field experience on removal of organics in horizontal flow constructed wetlands for various types of wastewaters. In our survey only full-scale treatment systems and pilot-scale outdoor experimental systems were taken into consideration. Also, HF CWs which are a part of hybrid systems were considered when data for HF stage were available. We did

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Table 1 – Number of entries (in most cases annual means) and constructed wetlands included in the survey. Type of wastewater

COD

BOD5 No. of entries

−1

Municipal (b 40 mg l ) Municipal (N 40 mg l− 1) Agriculture a Industry b Landfill leachate c All results

281 746 43 48 25 1143

No. of CWs 122 261 19 23 13 438

(20) (32) (12) (11) (5) (36)

No. of entries 556⁎ 38 40 7 641

No. of CWs 244 17 25 6 292

(29) (13) (10) (3) (36)

Numbers in parentheses denote number of countries. Only systems where both inflow and outflow concentrations were available for the given period. ⁎Just one group. a Includes farm effluents (e.g., Finlayson et al., 1987; Wang et al., 1994; Gasiunas et al., 2005; Strusevičius and Strusevičiene, 2003; Lee et al., 2004; Junsan et al., 2000) chicken manure (Vymazal, 1990), shrimp aquaculture effluent (Lin et al., 2005), trout farm effluent (Schulz et al., 2003), dairy parlor effluent Mantovi et al. (2002, 2003). b Includes abattoir and meat processing effluents (e.g., Finlayson and Chick, 1983; Lavigne and Jankiewicz, 2000; Poggi-Varaldo et al., 2002; Gasiunas et al., 2005), food processing effluents (e.g., Vrhovšek et al., 1996; White, 1994; Pucci et al., 2000; Wallace, 2002), distillery effluents (Billore et al., 2001), winery effluents (Masi et al., 2002), petrochemical effluents (Ji et al., 2002), lignite pyrolysis effluents (Wiessner et al., 1999) or mixed industrial effluents (Wang et al., 1994). c Urbanc-Berčič (1997), Maehlum et al. (1999), Eckhardt et al. (1999), Kinsley et al. (2006), Wojciechowska and Obarska-Pempkowiak (2008).

not include any indoor systems or systems fed with artificial wastewater. We primarily looked at long-term results and most entries are annual averages. In Table 1, a summary of the data used in this survey is given. Indeed, the majority of results have come from the treatment of municipal wastewater. For municipal wastewaters we evaluated separately systems with inflow BOD5 concentrations b40 mg/l to denote tertiary treatment.

2.

Results and discussion

Removal of BOD5 from various types of wastewater is presented in Fig. 2. The “total” values are affected mostly by secondary municipal wastewater results and, therefore, these two values are quite similar. It is also seen that the highest inflow concentrations were recorded for industrial wastewaters. Industrial and agricultural wastewaters also exhibited the greatest variations in inflow concentrations. Besides

tertiary municipal wastewaters the lowest inflow BOD5 concentrations were recorded for landfill leachate systems. This is not surprising as organics in landfill leachate are very often recalcitrant and therefore, not analysed as BOD5 but rather COD. This suggestion is supported by the data in Fig. 3 which presents the COD results. From this graph it is also quite clear that by far the highest inflow concentrations of COD are recorded for industrial wastewaters with the highest inflow value in the dataset being 8420 mg l− 1 for distillery effluent (Billore et al., 2001). The data in Table 2 and Figs. 2 and 3 also reveal very low treatment efficiency for landfill leachate. For BOD5 and COD, the average treatment efficiencies were only 32.8% and 24.9%, respectively. The highest removal efficiencies were recorded for municipal wastewater (Table 2). The fact that HF constructed wetlands for municipal sewage treatment usually exhibit higher treatment efficiency was demonstrated by Puigagut et al. (2007). In their world-wide survey, BOD5 and COD removal efficiencies varied between 75 and 93% and between 64 and 82%, respectively. In Table 3,

Fig. 2 – Average inflow and outflow BOD5 concentrations for various types of wastewater. For number of entries see Table 2.

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Fig. 3 – Average inflow and outflow COD concentrations for various types of wastewater. For number of entries see Table 2.

examples of removal of organics from municipal wastewater are presented. Municipal wastewater usually does not contain elevated concentrations of recalcitrant organics and therefore,

most organics are labile and thus easily degradable. The results presented in Table 3 also indicate that HF CWs may be successfully used as tertiary treatment step. In Figs. 4 and 5 charts showing BOD5 and COD loadings are presented. For both BOD5 and COD, the highest average loadings were recorded for agriculture wastewaters followed by industrial wastewaters. The fact that organics in landfill leachate are very often recalcitrant is demonstrated by the influent COD:BOD5 ratio of 11.8 while for agriculture, industry and municipal wastewaters the ratios were 2.3, 3.4 and 2.2, respectively. In Fig. 6, specific area for various types of wastewater is presented. Population equivalent (PE) represents 60 g BOD5 d− 1 person− 1. However, it is questionable what this value means at present. The figure of 60 g BOD5 was set

Table 2 – Average removal efficiencies (in %) for various types of wastewater.

Agriculture Industry Landfill leachate Municipal tertiary Municipal secondary Total

BOD5

COD

68.2 60.1 32.8 60.7 80.7 73.4

63.0 63.1 24.9 63.2 62.7

Table 3 – Examples of removal of organics in HF constructed wetlands. Location

Onšov Leek Wootton Ashby Folville Himley Fare Sejerslev Wodonga Baggiolino Uggerhalne Ondřejov Holtby Koloděje Hasselt-Kiewit Brondum Middleton Glavotok Carrión de los Céspedes Agronomica

Country

Czech Rep. UK UK UK Denmark Denmark Australia Italy Denmark Czech Rep. UK Czech Rep. Belgium Denmark UK Croatia Spain Brazil

Area

Flow

BOD5

(m2)

(m3 d− 1)

In

Out

In

Out

2,100 825 825 463 1,500 7,931 97 96 2,640 806 612 4,495 896 437 168 360 229 450

92 306 164 90 19.5 492 4 6 103 50 30 176 23.3 8.1 10 40 5.8 6.6

5.9 8.5 14.8 16 30 54 76 81 115 143 189 204 232 330 390 427 513 979

2.7 2.3 3.0 3.1 3.0 7.0 13 7.2 6.0 14.8 18.5 15 6.0 16 25 56 67 19

26.5 82 103 89 85 137 226 226 330 334

12.3 35 43 41 27 34 49 33 63 36

398 536 780 831 611 1034 1005

47 46 78 195 76 134 19

COD

Ref.

1 2 2 2 3 3 4 5 6 1 2 1 7 3 2 8 9 10

The systems are ranked according to the inflow BOD5 concentration. 1 — unpubl. results, 2 — CWA (2006), 3 — Schierup et al. (1990a), 4 — Thomas et al. (1994), 5 — Pucci et al. (2004), 6 — Kadlec et al. (2000), 7 — VMM (2006), 8 — Shalabi (2004), 9 — Sardón et al. (2006), 10 — Philippi et al. (2006). Concentrations in mg l- 1.

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Fig. 4 – Average inflow and outflow BOD5 loadings for various types of wastewater. Total number of entries: 967; tertiary municipal: 230; secondary municipal: 624. For other entries see Table 2.

based on the observations made in numerous wastewater treatment plants during the 1940s and 1950s (Imhoff, 1960). However, the household “technologies” have changed immensely since then and, at present the production of organics per person is usually lower, especially in small communities. In Table 4, values of first-order areal rate constant kA and hydraulic loading rate (HLR) values for various types wastewater are presented. The kA values vary widely among various types of wastewater with the highest value of 0.298 m d− 1 for tertiary treatment municipal wastewater and the lowest value of 0.012 m d− 1 for landfill leachate. Hydraulic loading data reveal that the highest loaded systems are those treating wastewaters from agriculture and tertiary municipal wastewaters (24.3 cm d− 1). On the other hand, landfill leachate systems in the survey were loaded with only 2.7 cm d− 1. The regression equations based on inflow and outflow concentrations usually do not provide a good fit (Table 5). Important factors such as flow, climate, bed material, bed

design (length, width, depth), are neglected, leading to a variety of regression equations (Rousseau et al., 2004). The regression equations for BOD5 inflow/outflow concentrations for our dataset Eqs. (9) and (10) also reveal very loose relationships. However, the relationship for secondary wastewaters Eq. (5) is much better than that for tertiary wastewaters Eq. (9). Cout ¼ 0:25Cin þ 1:95ðn ¼ 300; Cin : 2–40mgl−1 ; R2 ¼ 0:25Þ Cout ¼ 0:34Cin ðn ¼ 845; Cin : 40:1–2957mgl−1 ; R2 ¼ 0:53Þ

ð9Þ ð10Þ

Better fit between COD inflow/outflow concentrations than for BOD5 has been reported in the literature (Table 5). Also results in our dataset yielded better regression Eq. (11): Cout ¼ 0:40Cin ðn ¼ 641; Cin : 4:3–8420mgl−1 R2 ¼ 0:66Þ

ð11Þ

However, detailed analysis of the data revealed that for inflow COD concentrations up to 200 mg l− 1 the regression is

Fig. 5 – Average inflow and outflow COD loadings for various types of wastewater. Total number of entries: 578; municipal: 493. For other entries see Table 2.

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Fig. 6 – Mean and median specific area (m2 PE− 1) used to treat various types of wastewaters. 1 PE = 60 g BOD5 per person per day. The specific area is calculated using the real organic loading.

much weaker (R2 = 0.31, n = 261). Also the results revealed that with increasing inflow concentrations the relationships with outflow concentrations are getting stronger. For example, for the inflow range up to 500 mg l− 1 the R2 = 0.41, (n = 518) and for inflow between 2000 and 8420 mg l− 1 the R2 is 0.62 (n = 18). The relationships between inflow and outflow BOD5 (Fig. 7) and COD loadings (Fig. 8) provide much stronger fit than that for concentrations. Vymazal (2001) reported for 18 Czech HF constructed wetlands regression coefficient R2 = 0.53 for BOD5 inflow/outflow relationship. Using the data from Danish (Schierup et al., 1990a) and North American (Reed 1993; Kadlec and Knight, 1996) databases, similar relationships also showed good correlation (R2 = 0.71, n = 54 and 0.67, n = 18, respectively). Reed and Brown (1995) reported regression coefficient for BOD5 as high as R2 = 0.95 for 14 HF constructed wetlands in the United States. There is a very high regression coefficient for COD inflow/ outflow loading relationship (Fig. 8) which is higher than those reported in the literature or those that could be calculated from the literature data. Vymazal (2001) reported R2 = 0.64 for 18 HF in the Czech Republic. Data from the Danish database (Schierup et al., 1990a) yielded R2 = 0.54 for 49 HF wetlands. Results from 31 HF constructed wetlands (96 annual means) in

Table 4 – Average first-order areal rate constant kA and hydraulic loading rate (HLR) values for various types wastewater. kA (m d− 1) BOD5 Agriculture Industry Landfill leachate Municipal tertiary Municipal secondary Total

0.182 0.061 0.012 0.298 0.122 0.160

(0.41) (0.054) (0.022) (1.47) (0.249) (0.756)

COD 0.655 (1.546) 0.059 (0.058) 0.100 (0.010) 0.104 (0.210) 0.136 (0.462)

HLR (cm d− 1) BOD5 24.3 8.1 2.7 24.3 6.9 11.8

(65) (11.6) (3.8) (71.6 (14.9) (40.5)

Standard deviations in parentheses. For number of entries see legends for Figs. 4 and 5.

the Czech Republic yielded R2 = 0.58 (Vymazal and Kröpfelová, unpubl.). It has been found that the highest removal of organics takes place within several meters of the inflow zone and further decrease in concentrations is much smaller (e.g., Bavor et al., 1987; Vymazal, 2003; Fonder and Xanthoulis, 2007, see also Fig. 9). Dense beds of emergent wetland plants are the most obvious visual feature of HF constructed wetlands. It is obvious that plants provide many benefits to the treatment process but their role in overall treatment performance is still not unanimous (Tanner, 2001). 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, a source of carbon compounds through root exudates and a micro-aerobic environment via root oxygen release (e.g., Gersberg et al., 1986; Tanner, 2001; Gagnon et al., 2006). Higher microbial densities in planted systems were reported, for example, by Hatano et al. (1993) or Münch et al. (2005). Kaseva (2004) reported positive effect of plants on treatment performance of a HF constructed wetland in Tanzania, especially for removal of COD. The inflow COD concentration of 106 mg l− 1 was reduced to 71 mg l− 1, 46.5 mg l− 1 and 41.8 mg l− 1 in unplanted cell and cells planted with Phragmites mauritianus and Typha latifolia, respectively. Also Mbuligwe (2004) observed better treatment performance of cells planted with T. latifolia and Colocasia esculenta as compared to unplanted cell in a system treating anaerobically pretreated wastewater in Dar es Salaam, Tanzania. Karathanasis et al. (2003) studied the effect of various plants (T. latifolia, Festuca arundinacea, Iris pseudacorus, Canna x. generalis, Hemerocallis fulva, Hibiscus moscheutos, Scirpus validus and Mentha spicata) on the performance of twelve HF constructed wetlands for domestic wastewater from a single house in Kentucky. The vegetated systems showed significantly greater (P b 0.05) removal efficiencies for BOD (N75%) than the unplanted systems (63%) throughout the year. Naylor et al. (2003) reported that during the use of experimental

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Table 5 – Examples of regression coefficients for BOD5 and COD inflow/outflow relationships reported in the literature. Country

R2

Number of systems

Reference

BOD5 USA a Poland a Denmark Portugal Czech Republic Czech Republic Denmark Slovenia a UK a

0.03 0.044 0.08 0.18 0.33 0.36 0.38 0.41 0.46

16 11 90 34 23 (55) 32 (131) 69 8 75 (268)

Kadlec and Knight (1996) Kalisz and Salbut (1995), Kowalik and Obarska-Pempkowiak (1998) Brix (1998) Dias and Pacheco (2001) Vymazal (2001c) Unpubl. results Schierup et al. (1990b) Urbanc-Berčič et al. (1998) CWA (2006)

COD Denmarka Italy Polanda Slovenia a Germany a Czech Republic UKa Czech Republic

0.29 0.38 0.44 0.47 0.52 0.69 0.70 0.81

60 10 8 8 7 24 (53) 40 (124) 30 (121)

Schierup et al. (1990b) Masi et al. (2000) (Kowalik and Obarska-Pempkowiak) 1998 Urbanc-Berčič et al. (1998) Börner et al. (1998) Vymazal (2001c) CWA (2006) Unpubl. results

Number of annual means in parentheses. a Calculated from the source.

HF constructed wetlands to treat diluted sludge from a freshwater fish farm anaerobic digester planted wetlands (Phragmites australis, T. latifolia) clearly outperforming unplanted units in term of BOD5 and COD. El Hafiane and El Hamouri (2004) found that HF wetlands planted with Arundo donax removed 21–25% COD more than unplanted units in Rabat, Morocco. Pandey et al. (2006) found substantially higher removal of BOD5 and COD in HF system in Nepal planted with Phragmites karka. BOD5 inflow concentration of 321 mg l− 1 was reduced to 48 mg l− 1 and 89 mg l− 1 in planted and unplanted wetlands. COD inflow concentration of 654 mg l− 1 was reduced to 87 mg l− 1 and 122 mg l− 1, respectively. Dallas and Ho (2005) reported higher removal of BOD5 in HF experimental wetlands planted with Coix lacryma-jobi as compared to unplanted units in Costa Rica.

Fig. 7 – Relationship between inflow and outflow BOD5 loadings (n = 988, inflow range 0.3–8580 kg ha− 1 d− 1). Highest 12 values are not shown.

On the other hand, Akratos and Tsihrintzis (2007) found in experimental HF constructed wetlands that the presence of plants and namely T. latifolia improved the removal of COD only slightly — 89.3% as compared to 87.2% in unplanted units. Also Burgoon et al. (1989) reported that removal of BOD5 in an unplanted mesocosms was 87.6% while removals in planted mesocosms Sagittaria latifolia, T. latifolia, Scirpus pungens and P. australis) were only slightly higher and varied between 85.2 and 93.2%. Tanner et al. (1995) reported that removal of BOD5 was not affected by the presence of vegetation in experimental wetlands planted with S. validus in Ruakura, New Zealand. The opinion of the effect of season or temperature on the HF constructed wetlands treatment performance is far from being unanimous. In the literature, there are reports suggesting little or now seasonal effects as well as reports indicating strong seasonal dependence. In reality, HF constructed wetlands are successfully operated in cold climates. For example, Giæver (2003) reported on the HF system in Fagernes, which is located in the northern part of Norway at 68° northern latitude. There are also other fine examples of the use of HF constructed wetlands in cold climates (e.g., Wallace and Knight, 2006; Navara, 1996). Hook et al. (2003) pointed out that the presence of plants can strongly affect the seasonal patterns of water treatment in HF constructed wetlands. The authors observed in temperature-controlled experimental units that compared to controls, plants enhanced COD removal overall, and they either attenuated (T. latifolia) or eliminated (Carex rostrata and Scirpus acutus) the seasonal decrease in performance expected at low temperatures. Kadlec et al. (2003) described two adjacent HF system in Minnesota with average annual air and wetland water temperatures of 5 °C and 9 °C with air temperatures reaching below − 40 °C. The results revealed the highest removal of BOD5 during autumn and summer with substantial decrease

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Fig. 8 – Relationship between inflow and outflow COD loadings (n = 579, inflow range 3.3–14,769 kg ha− 1 d− 1). Highest 12 values are not shown.

in treatment efficiency during winter and spring. On the other hand, Züst and Schönborn (2003) could not find any influence of wastewater temperature on the removal efficiency of COD in constructed wetlands located at the altitude of 730 m with average annual temperature of 8.4 °C in Switzerland. The treatment efficiency was still very good at temperatures as low as 0.5 °C. Steinmann et al. (2003) did not observed any seasonal changes in BOD5 and COD removal in a HF system in Mörlbach, Germany. Similar results were reported by Dahab et al. (2001) from Nebraska, U.S.A. Removals of BOD5 and COD were quite similar in summer (mean influent temperature 17.7 °C) and winter (10.1 °C) periods. Also Mæhlum and Jenssen (2003) reported that there was no significant difference in efficiency between cold (b4 °C) and warm (N11 °C) periods for all parameters tested (TP, TN, COD, BOD7, TSS) in 9 constructed wetlands in Norway. Vymazal (2001) reported very little difference between removal efficiency of organics in summer and winter in two HF constructed wetlands in the Czech Republic during the period 1994–1999 (Table 6).

Fig. 9 – Removal of organics along the longitudinal profile of the vegetated beds of HF constructed wetlands. Data from the Czech Republic (top, Vymazal, 2003) and Belgium (bottom, Fonder and Xanthoulis, 2007).

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Table 6 – Removal of BOD5 and COD in HF constructed wetlands at Ondřejov and Koloděje, Czech Republic. COD

BOD5 Summer

Koloděje Ondřejov

Winter

Summer

Winter

In

Out

In

Out

In

Out

In

Out

92 96

9.8 13.1

101 87

10.8 10.5

232 215

42 48

252 247

48 41

“Summer” and “winter” periods: May–October and November– April, respectively. Average “summer” water temperatures 12.9 °C at Ondřejov and 13.8 °C at Koloděje. “Winter” average water temperatures 6.2 °C at Ondřejov and 5.2 °C at Koloděje. From Vymazal and Kröpfelová (2008), data from Vymazal (2001). Values in mg l-1.

Ham et al. (2004) reported that BOD5 inflow concentrations of 161 mg l− 1 and 151 mg l− 1 were reduced to 21 mg l− 1 and 62 mg l− 1 during the growing and winter seasons, respectively in a HF CW at Konkuk University campus, Seoul, Korea during the period of July 1998–December 2003. However, the loading data revealed that during the growing and winter seasons quite similar loads − 60 and 55 kg ha− 1 d− 1, respectively — were removed. This example indicates that loading data usually provide better information than concentrations.

3.

Conclusions

Constructed wetlands with horizontal sub-surface flow (HF CWs) have successfully been used for treatment various types of wastewater for more than four decades. Most systems have been designed to treat municipal sewage but the use for wastewaters from agriculture, industry and landfill leachate in HF CWs is getting more attention nowadays. The survey of more than 400 HF CWs from 36 countries around the world revealed that the highest removal efficiencies for BOD5 and COD were achieved in systems treating municipal wastewater while the lowest efficiency was recorded for landfill leachate. This is caused by the fact that municipal wastewaters contain predominantly labile organics while landfill leachate contains often recalcitrant organics which are difficult to degrade. The highest average inflow concentrations of BOD5 (652 mg l− 1) and COD (1865 mg l− 1) were recorded for industrial wastewaters followed by wastewaters from agriculture for BOD5 (464 mg l− 1) and landfill leachate for COD (933 mg l− 1). Hydraulic loading data reveal that the highest loaded systems are those treating wastewaters from agriculture and tertiary municipal wastewaters (average hydraulic loading rate 24.3 cm d− 1). On the other end, landfill leachate systems in the survey were loaded with average only 2.7 cm d− 1. For both BOD5 and COD, the highest average loadings were recorded for agricultural wastewaters (541 and 1239 kg ha− 1 d− 1, respectively) followed by industrial wastewaters (365 and 1212 kg ha− 1 d− 1, respectively). The regression equations for BOD5 and COD inflow/outflow concentrations yielded very loose relationships but much stronger relationships were found for inflow/outflow loadings and especially for COD.

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The influence of vegetation on removal of organics in HF CWs is not unanimous but most studies indicated the positive effect of macrophytes. The reports on seasonal effect on BOD5 and COD removal in HF CWs differ from those suggesting little or now seasonal effects to those indicating strong seasonal dependence.

Acknowledgements The study was supported by grant No. 206/06/0058 “Monitoring of Heavy Metals and Selected Risk Elements during Wastewater Treatment in Constructed Wetlands” from the Czech Science Foundation and Grant. No. 2B06023 “Development of Mass and Energy Flows Evaluation in Selected Ecosystems” from the Ministry of Education, Youth and Sport of the Czech Republic.

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