- Email: [email protected]
Treatment of high-strength wastewater in tropical vertical flow constructed wetlands planted with Typha angustifolia and Cyperus involucratus
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
Treatment of high-strength wastewater in tropical vertical flow constructed wetlands planted with Typha angustifolia and Cyperus involucratus Suwasa Kantawanichkul a,∗ , Supreeya Kladprasert a , Hans Brix b a b
Department of Environmental Engineering, Chiang Mai University, Thailand Department of Biological Sciences, Aarhus University, Ole Worms Allé, Building 1135, DK-8000 Århus C., Denmark
a r t i c l e
i n f o
Article history:
a b s t r a c t The ability of vertical flow (VF) constructed wetland systems to treat high-strength (ca.
Received 17 December 2007
300 mg L−1 of COD and ca. 300 mg L−1 total-nitrogen) wastewater under tropical climatic con-
Received in revised form
ditions was studied during a 5-month period. Nine 0.8-m diameter experimental VF units
30 May 2008
(depth 0.6 m) were used: three units were planted with Typha angustifolia L., another three
Accepted 2 June 2008
units were planted with Cyperus involucratus Rottb and three units were unplanted. Each set of units were operated at hydraulic loading rates (HLRs) of 20, 50 and 80 mm d−1 . Cyperus produced more shoots and biomass than the Typha, which was probably stressed because
Keywords:
of lack of water. The high evapotranspirative water loss from the Cyperus systems resulted
Cattail
in higher effluent concentrations of COD and total-P, but the mass removal of COD did not
Constructed wetland
differ significantly between planted and unplanted systems. Average mass removal rates
Cyperus involucratus
of COD, TKN and total-P at a HLR of 80 mm d−1 were 17.8, 15.4 and 0.69 g m−2 d−1 . The first-
Evapotranspiration
order removal rate constants at a HLR of 80 mm d−1 for COD, TKN and total-P were 49.8,
Oxygen transfer rate
30.1 and 13.5 m year−1 , respectively, which is in the higher range of k-values reported in the
Removal rate constant
literature. The oxygen transfer rates were ca. 80 g m−2 d−1 in the planted systems as opposed
Tropical wetland
to ca. 60 g m−2 d−1 in the unplanted systems. The number of Nitrosomonas was two to three
Typha angustifolia
orders of magnitude higher in the planted systems compared to the unplanted systems.
Umbrella Sedge
Planted systems thus had significantly higher removal rates of nitrogen and phosphorus,
Vertical flow
higher oxygen transfer rates, and higher quantities of ammonia-oxidizing bacteria. None of the systems did, however, fully nitrify the wastewater, even at low loading rates. The vertical filters did not provide sufficient contact time between the wastewater and the biofilm on the gravel medium of the filters probably because of the shallow bed depth (0.6 m) and the coarse texture of the gravel. It is concluded that vertical flow constructed wetland systems have a high capacity to treat high-strength wastewater in tropical climates. The gravel and sand matrix of the vertical filter must, however, be designed in a way so that the pulseloaded wastewater can pass through the filter medium at a speed that will allow the water to drain before the next dose arrives whilst at the same time holding the water back long enough to allow sufficient contact with the biofilm on the filter medium. © 2008 Elsevier B.V. All rights reserved.
∗
Corresponding author. Tel.: +66 53 944192; fax: +66 53 210328. E-mail address: [email protected] (S. Kantawanichkul). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.06.002
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
1.
Introduction
Many agro and food production industries in Thailand discharge poorly treated wastewater with high COD (chemical oxygen demand) and nitrogen (N) contents directly into canals and rivers, causing severe environmental problems in these. There is therefore an urgent need to improve the conditions by introducing proper treatment of the wastewater prior to discharge. Conventional high-tech wastewater treatment systems require large capital investments and operating costs, and for that reason these systems are not realistic solutions for producers that cannot afford such expensive treatment systems. Also, high-tech systems often fail in less-developed countries like Thailand (Brix et al., 2007). Constructed wetlands are, on the other hand, a cost-effective, reliable and robust alternative to conventional wastewater treatment systems that are known to be particularly efficient in removing COD, and have also successfully been used in temperate countries to treat wastewaters with high concentrations of N (e.g. Tanner et al., 1995; Gottschall et al., 2007; Poach et al., 2007). However, although the potential for application of constructed wetlands in tropical countries is enormous, the rate of adoption of the technology for wastewater treatment has been slow (Denny, 1997; Wilderer, 2004) partly because of the lack of generally adopted design criteria for tropical climates. The vertical flow constructed wetland system with unsaturated flow is known to be particularly efficient in treating many types of wastewater (Cooper, 1999; Molle et al., 2005; Brix and Arias, 2005b). The vertical flow systems process greater oxygen transport ability than the horizontal subsurface flow beds (Brix and Schierup, 1990; Cooper, 2005), and they are more effective for the mineralization of biodegradable organic matter and for nitrification through the activity of ammoniaoxidizing bacteria. However, this type of system is rather new and undocumented in Thailand, and there are no design criteria available. Experiences and design criteria described in the literature cannot be directly transferred to the tropical conditions of Thailand, because the studies are nearly exclusively conducted in temperate climates. Climate and other local conditions influence wastewater characteristics, plant growth and evaporation as well as the removal processes in the constructed wetland, particularly the microbial processes which are expected to be stimulated by the high temperatures. Therefore, there is a need to gain more experience on the per-
239
formance of vertical flow constructed wetlands from tropical climates in order to assess the capacity of the systems under tropical climatic conditions. Different species of wetland plants, including species of Reed (Phragmites australis (Cav.) Trin ex Steudel and Phragmites mauritianus Kunth), have been used in constructed wetland systems in tropical areas (Laber et al., 1999; Kaseva, 2004). Cattail (Typha angustifolia L.) is also widely used and is known to be highly tolerant to various types of wastewater (e.g. Koottatep et al., 2001, 2005), and Umbrella Sedge (Cyperus involucratus Rottb) is a species with a high value for ornamentation that can grow very well in a subsurface flow constructed wetland system (Kantawanichkul et al., 1999, 2003; Kantawanichkul and Somprasert, 2005). The objectives of this study were to assess the ability of vertical flow constructed wetland systems to treat high COD and high-N wastewater, and to evaluate the performance of systems planted with C. involucratus and T. angustifolia with unplanted systems. Furthermore, we wanted to estimate mass removal rates and removal rate constants to get more insight into the capacity of vertical flow systems under tropical climatic conditions.
2.
Materials and methods
2.1.
Experimental setup
The experiment was conducted in nine pilot-scale vertical flow constructed wetland units located at the university campus in Chiang Mai, Thailand (18◦ 47 24N, 98◦ 58 11E; elevation 318 m). The experiment was run for 5 months during November 2005–April 2006. Three units were planted with Narrow Leaf Cattail (T. angustifolia L.), another three tanks were planted with Umbrella Sedge (C. involucratus Rottb, syn Cyperus alternifolius L. ssp. flabelliformis (Rottb.) Kükenth.), and the last three tanks were unplanted. Planting density was 16 plants per square meter. Each unit consisted of a 0.8-m diameter circular precast concrete tank (height 0.6 m), each established with a three-layer gravel filter: 0.15 m of 30–60 mm gravel in the bottom followed by 0.15 m of 6–12 mm gravel in the middle and a 0.2 m layer of coarse 1–2 mm sand at the top (Fig. 1). The porosity of the media was 0.37. Each set of three tanks was loaded with synthetic wastewater at three different hydraulic loading rates (HLRs) of 20, 50 and 80 mm d−1 , respectively (Fig. 2). Each tank was operated with 30 min feeding every 2 h using
Fig. 1 – Schematic drawing of the experimental vertical flow constructed wetland units.
240
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Fig. 2 – Experimental set-up of the nine vertical flow constructed wetland units planted with Umbrella sedges (Cyperus involucratus), cattails (Typha angustifolia) or control (unplanted) and different loading rates (20, 50 and 80 mm d−1 ).
timer-controlled peristaltic pumps. Every week the loadings provided by the pumps were checked and when necessary adjusted to the desired level. Effluents were collected in drains in the bottom of the tanks and allowed to drain freely into effluent storage tanks. The amount of effluent from each tank was measured weekly to estimate water loss from the system due to evapotranspiration.
2.2.
Chemical analysis
The synthetic wastewater used was prepared from NH4 Cl, starch, NaHCO3 and KHPO4 to gain COD and TKN (Total Kjeldahl Nitrogen) concentration around 300 mg L−1 (average measured composition during the experiment is shown in Table 1). Inlet and effluent water samples from each tank were analyzed weekly for COD, TKN, NH4 -N, NO2 -N, NO3 -N, total-P and TSS (total suspended solids) following standard analytical procedures (APHA, 1998). The temperature of the effluent wastewater ranged between 21 and 29.5 ◦ C, dissolved oxygen concentration (as analysed by an oxygen electrode) between 0.2 and 2.0 mg L−1 , and pH between 6.60 and 7.85 during the experiment. Only data from day 43 and onwards are presented here as the initial period was considered a period of acclimation. Sampling was carried out weekly for 4 months and data were averaged over monthly periods before statistical analyses (n = 4).
2.3.
Plant growth and uptake
At the end of the experiment the number of plants in each unit was counted and nine representative Cyperus and three Typha plants from each unit harvested to estimate biomass. The plants were fractionated into belowground (roots and rhizomes) and aboveground (leaves and stems) tissues, and their dry weight measured. Subsamples of the dried plant tissue were homogenized and the contents of N analyzed by a standard Kjeldahl technique (National Institute of Agricultural Sciences, 1977). The uptake of N was calculated from the total biomass and the N concentration in the tissues.
Table 1 – Average (±1S.D.) composition (mg L−1 ) of the synthetic wastewater used in the experiment (n = 16)
−1
TSS (mg L ) COD (mg L−1 ) TKN (mg L−1 ) NH4 -N (mg L−1 ) NO2 + NO3 -N (mg L−1 ) Total-P (mg L−1 )
Average
Range
± ± ± ± ± ±
7–74 211–333 273–332 260–319 0–1.0 21.6–26.7
42 275 305 290 0.2 24.2
20 33 16 16 0.3 1.2
241
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
2.4.
Enumeration of bacteria
At the end of the experiment the gravel medium was sampled at four points in each unit at two depths; 10–15 cm and 35–40 cm. Samples from the same unit and same depth was pooled and mixed and analyzed for the quantity of ammonia-oxidizing bacteria (Nitrosomonas and Nitrobacter) and denitrifying bacteria by the MPN (most probable number) technique (Alexander, 1965).
2.5.
Removal rate kinetics
The rate constants, k (m year−1 ), for COD, TKN and total-P were calculated based on observed mass removal rates in the tanks using the following expression (Kadlec and Knight, 1996): k = HLR(ln Cin − ln Cout ) where HLR is the hydraulic loading rate (m year−1 ) and Cin and Cout are the mass of pollutant (COD, TKN or Total-P) loaded into the system and leaving the system per day, respectively. The mass removal rates were used here instead of the inlet and effluent concentrations as water loss from the systems due to evapotranspiration was high (often > 20 mm d−1 at high plant densities). Thus, the data obtained from this model should be used with great caution. Many studies indicate that the model can be used for BOD (Biochemical Oxygen Demand) removal in surface flow and horizontal subsurface flow wetlands (Reed et al., 1988; Kadlec and Knight, 1996), and data also suggest that it can be used for total-P with some confidence. However, it is questionable whether the model can be used for vertical flow constructed wetlands and for the removal of N.
2.6.
Oxygen transfer rate
The hypothetical O2 transfer rates (OTR, g m−2 d−1 ) were calculated as suggested by Cooper (2005) and Platzer (1999): OTR = 0.7HLR[CODin − CODout ] + 4.3[NH4 -Nin − NH4 − Nout ] where HLR is the hydraulic loading rate (m d−1 ), [CODin − CODout ] is the mass of COD removed in the system (g m−2 d−1 ) and [NH4 -Nin − NH4 -Nout ] is the mass of nitrogen (g m−2 d−1 ) nitrified in the beds. Usually BOD5 rather than COD has been used as a direct estimate of the biodegradable organic matter that is oxidized in the beds. The factor 0.7 is here used to compensate for that as suggested by Platzer (1999). The factor 4.3 comes from the fact that it takes 4.3 g of O2 to oxidize 1 g of NH4 -N. We did not make any allowance for O2 recovered from denitrification of the nitrate in the beds.
2.7.
Data analysis
Data analysis was performed using two-way ANOVA and Type III sum of squares with the software Statgraphics Plus Ver. 4.1 (Manugistics, Inc., Scottsdale, AZ, USA). Data were tested for normal distribution using the Kolmogorov–Smirnov test, and heterogeneities of variances within treatments were tested using Cochran’s C-test. If necessary, logarithmic or square root transformations were performed to ensure homogeneity of variance, but for clarity all data are presented as untrans-
Table 2 – F-Values and significance of a two-way ANOVA of effluent concentrations from vertical flow constructed wetland systems planted with different species (Cyperus involucratus, Typha angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 )
TSS COD TKN NH4 -N NO2 + NO3 -N Total-P
Species
Loading
6.60* 5.06* 32.05*** 14.25*** 0.93 26.93***
1.63 0.83 51.40*** 43.71*** 7.72** 24.81***
Species × loading 7.03*** 0.15 9.32*** 5.05** 1.04 13.92***
∗
P < 0.05. P < 0.01. ∗∗∗ P < 0.001. ∗∗
formed. Differences between individual means were identified using Tukey HSD-procedure at the 5% significance level.
3.
Results
3.1.
Plant growth and water loss
The growth of Cyperus was much better (average 547 shoots m−2 , 4.3 kg dw m−2 ) than that of Typha (average 78 shoots m−2 ; 0.9 kg dw m−2 ). The Typha plants seemed stressed with wilting leaves and did not grow well. The much higher growth and biomass of Cyperus also resulted in a significantly (P < 0.001) higher water loss due to evapotranspiration in these compared to the systems planted with Typha and the unplanted control. The average water loss was 20.1 mm d−1 in systems with Cyperus, 11.9 mm d−1 in systems with Typha and 7.9 mm d−1 in the unplanted systems.
3.2.
Effluent concentrations
The effluent concentration of TSS was generally <10 mg L−1 (data not shown) and a significant interaction between species (including the unplanted treatment) and loading was observed (Table 2). In systems planted with Cyperus the effluent concentrations were highest at high loading rates, whereas in systems planted with Typha the opposite was observed. In unplanted systems TSS effluent concentration did not differ between loading rates. For COD the effluent concentration was significantly higher in Cyperus than in Typha planted systems (Fig. 3a), probably because of the concentrating effect of the high evapotranspiration rates in the Cyperus beds. Effluent COD concentrations were independent on loading rate (Table 2). Both loading rate and species affected TKN and NH4 N effluent concentrations, and the significant interaction term in the ANOVA shows that the effects of loading rate depend on species (Table 2). TKN and NH4 -N effluent concentrations were generally higher in the systems planted with Cyperus, and concentrations increased with loading rate (Fig. 3b). Effluent concentrations of oxidized nitrogen (NO2+3 -N) did not differ between planted and unplanted systems, but concentrations decreased significantly with loading rate (Fig. 3c). Effluent concentrations of total-P were generally high, but lowest in unplanted systems at low loading rate rates (Fig. 3d). Loading
242
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Fig. 3 – Effluent concentrations (average ± 1S.E.) of (a) chemical oxygen demand, COD; (b) total Kjeldahl nitrogen, TKN; (c) oxidized nitrogen, NO2 + NO3 -N; and (d) total phosphorus, total-P, at hydraulic loading rates of 20, 50 and 80 mm d−1 for vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted).
rate affected total-P effluent concentration in the unplanted systems whereas no effect was observed in the planted systems.
3.3.
Mass removal rates
Mass removal rates were very dependent on loading rate with significantly higher removal rates at high loadings (Tables 3 and 4). However, the effects of loading depended on species as shown by the significant interaction terms in the ANOVA (Table 3), except the mass removal of COD which was independent on species. At low loading rates (20 and 50 mm d−1 ) the mass removal rates did not depend much on species, but at a HLR of 80 mm d−1 the mass removal rates were generally lower for most parameters in the unplanted systems compared to the planted systems (Table 4).
Table 3 – F-Values and significance of a two-way ANOVA of mass removal rates in vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 )
TSS COD TKN NH4 -N Total-P
Species
Loading
Species × loading
5.33* 0.39 9.47*** 10.19*** 6.41**
3288.99*** 362.70*** 285.68*** 258.86*** 47.60***
4.20** 0.47 7.43*** 7.07*** 5.81**
3.4.
Plant uptake of nitrogen
The concentration of N in the plant tissues varied between 2.8 and 3.4% of the dry weight for Cyperus and 1.8 and 2.4% of the dry weight for Typha. There was no significant difference between the N concentrations at the different loading rates. The average plant uptake of N throughout the experiment was 0.86 ± 0.25 g m−2 d−1 for Cyperus and 0.12 ± 0.06 g m−2 d−1 for Typha (mean ± 1S.D.). Comparing the plant uptake with the average mass removal rates of N in the systems, the uptake of N by Cyperus constituted between 6.7% of the mass N removal at a HLR of 80 mm d−1 to 16.9% at 20 mm d−1 . Because of the lower growth and N content in the tissues of Typha, the plant uptake only constituted 0.5–3.3% of the mass N removal rates in the Typha planted systems.
3.5.
Removal rate constants
Removal rate constants depended strongly on loading rate and the effects of loading rate different between the species (except for kCOD ) as shown by the significant interactions in the ANOVA (Table 5). Removal rate constants were generally much higher at high loading rates (Fig. 4), particularly in planted systems. The removal rate constant for COD did not differ significantly between species (Fig. 4a), but for N the removal rate constant was significantly higher in planted systems compared with unplanted systems (Fig. 4b). The removal rate constant for P was especially high in the Cyperus system at high loading rate (Fig. 4c).
3.6.
Oxygen transfer rates
∗
P < 0.05. ∗∗ P < 0.01. ∗∗∗ P < 0.001.
The calculated O2 transfer rate depended strongly on loading rate, and the significant interaction in the ANOVA (F = 7.16;
243
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Table 4 – Mass removal rates (average ± S.E.) of TSS, COD, TKN, NH4 -N and total-P in vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 ) Treatment
Parameter
Hydraulic loading rate 20 mm d−1
50 mm d−1
80 mm d−1
C. involucratus T. angustifolia Unplanted
TSS (g m−2 d−1 )
0.78 ± 0.01 0.72 ± 0.02 0.74 ± 0.01
1.82 ± 0.06 1.98 ± 0.03 1.87 ± 0.04
3.08 ± 0.05 3.19 ± 0.03 3.00 ± 0.07
C. involucratus T. angustifolia Unplanted
COD (g m−2 d−1 )
4.5 ± 0.2 4.6 ± 0.2 4.4 ± 0.3
10.2 ± 0.9 10.8 ± 1.3 10.8 ± 0.3
18.1 ± 0.9 18.2 ± 0.5 17.1 ± 0.8
C. involucratus T. angustifolia Unplanted
TKN (g m−2 d−1 )
4.5 ± 0.2 4.6 ± 0.2 5.1 ± 0.1
9.0 ± 1.0 10.8 ± 0.6 8.7 ± 0.3
16.9 ± 0.9 16.6 ± 0.7 12.6 ± 0.9
C. involucratus T. angustifolia Unplanted
NH4 -N (g m−2 d−1 )
4.5 ± 0.2 4.4 ± 0.2 4.9 ± 0.1
8.7 ± 1.0 10.3 ± 0.6 8.2 ± 0.3
16.2 ± 0.8 15.8 ± 0.6 11.8 ± 0.9
C. involucratus T. angustifolia Unplanted
Total-P (g m−2 d−1 )
0.22 ± 0.03 0.17 ± 0.02 0.27 ± 0.02
0.45 ± 0.11 0.47 ± 0.08 0.36 ± 0.06
0.96 ± 0.09 0.64 ± 0.08 0.48 ± 0.07
P = 0.0005) shows that the dependency differs between planted and unplanted treatments (Fig. 5). The O2 transfer rates were ca. 80 g m−2 d−1 in the planted systems at a HLR of 80 mm d−1 and only ca. 60 g m−2 d−1 at the same HLR in the unplanted system.
3.7.
Ammonia-oxidizing and denitrifying bacteria
There were no significant differences between the number of bacteria and depth of sampling or loading rate. However, the number of Nitrosomonas was two and three orders of magnitude higher in the systems planted with Typha and Cyperus, respectively, compared to the unplanted systems (Table 6). The quantity of Nitrobacter was also highest, but not statistically significant, in the Cyperus planted systems. The number of denitrifiers was of the same magnitude in all systems. The numbers reported here should, however, be regarded more as relative numbers rather than absolute numbers as the enumeration by the MPN method is known to have the possibility of underestimation (Konuma et al., 2001).
Table 5 – F-Values and significance of a two-way ANOVA of removal rate constants (k) in vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 ) Species TSS COD TKN NH4 -N Total-P *
P < 0.05.
∗∗
P < 0.01. P < 0.001.
∗∗∗
**
8.60 0.82 7.15** 7.85** 6.13**
Loading ***
223.59 81.32*** 77.45*** 68.96*** 25.32***
Species × loading 4.64** 0.57 7.16*** 6.95*** 5.42**
4.
Discussion
This study clearly shows that unsaturated vertical flow constructed wetland systems have a high capacity to treat high-strength wastewater. Many studies from temperate areas have documented that vertical flow constructed wetland systems are very efficient in removing biodegradable organic matter as well as able to nitrify ammonium (Laber et al., 1997; Cooper, 1999; Brix et al., 2002; Weedon, 2003; Brix and Arias, 2005b), and the present study confirms that this is also true in tropical climates. The effluent concentrations reached are lower than the maximum permitted values for medium sized pig farms in Thailand (BOD 100 mg L−1 ; COD 400 mg L−1 ; TSS 200 mg L−1 ; TKN 200 mg L−1 ) as published by the Ministry of Natural Resources and Environment. T. angustifolia did not grow well in this study, as opposed to C. involucratus, which had a very vigorous growth and biomass production. This is surprising, as Typha sp. often grow well at very eutrophic sites and are also a common plant in constructed wetlands (e.g. Koottatep et al., 2005). The poor growth in the present study might be because of lack of adequate water in the unsaturated vertical filter. Typha is a wetland plant with a very high water demand and a limited capacity to close stomata and hence reduce water loss by transpiration. Cyperus in nature is found not only in wetlands, but also on drier sites which indicates that this species has a higher capacity to tolerate water stress compared to Typha. In this study the water loss from the planted systems was very high, probably caused by factors like the ‘oasis’ effect. The oasis effect is the phenomenon where warmer and dry air in equilibrium with dry areas flows across a vegetation of plants with a high water availability (Verma et al., 1978; Brix and Arias, 2005a). Evapotranspiration from isolated expanses of vegetation like the experimental systems used in this study, on a per unit area basis, can therefore be significantly greater than cal-
244
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Fig. 4 – Removal rate constants (average ± 1S.E.) for (a) chemical oxygen demand, kCOD ; (b) total Kjeldahl nitrogen, kTKN ; and (c) total phosphorus, kTP , at hydraulic loading rates of 20, 50 and 80 mm d−1 for vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted).
Fig. 5 – Oxygen transfer rates (OTR, average ± 1S.E.) for vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) at hydraulic loading rates of 20, 50 and 80 mm d−1 .
culated potential evapotranspiration (Rosenberg, 1969). In large full-scale systems, water loss by evapotranspiration can be expected to be less than measured in the present study.
Both mass removal rates and the estimated removal rate constants increased with loading rate. This clearly shows that the capacity of the systems to remove biodegradable organic matter (BOD and COD) as well as nitrogen is higher than found at the highest loading rate in this study. Average mass removal rates of COD and TKN at a HLR of 80 mm d−1 were 17.8 and 15.4 g m−2 d−1 , respectively. Also the removal rate constants for COD and TKN were high: 49.8 and 30.1 m year−1 , respectively, which is in the higher range of k-values reported in the literature probably partially because of the higher temperature (Kadlec and Knight, 1996; Knight et al., 2000; Oovel et al., 2007; Konnerup et al., 2008). We did not include the irreducible background wetland concentration (C*) in the model used to estimate k-values in the present study, as we did not have a reliable estimate of C*. Inclusion of C* in the model would result in higher estimated k-values. Hence, additional studies aiming at estimating the irreducible background concentration (C*) and studies at higher loading rates are needed to better estimate k-values of vertical flow systems under tropical conditions. However, great caution should be used in using the estimated rate constants for design purposes, as we used mass loading and removal rates in the present study. Removal of P was, as expected, low in the vertical flow sys-
Table 6 – Average numbers of ammonia-oxidizing bacteria (Nitrosomonas and Nitrobacter) and denitrifiers (no. per g dry weight) in the substrate of vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) as estimated by the MPN technique, and result of ANOVA (F-ratios) C. involucratus Nitrosomonas Nitrobacter Denitrifiers NS = not significant. ∗∗∗
P < 0.001.
9.63E+05 1.12E+05 2.17E+03
T. angustifolia
Unplanted
F-Ratio
1.13E+05 1.73E+04 3.31E+03
1.39E+03 1.13E+04 1.46E+03
37.30*** 1.75NS 0.28NS
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
tems as the gravel medium used had a low binding capacity for P. Effluent concentrations of P were generally high. The concentrating effect of evapotranspirative water loss, particularly at low hydraulic loading rates, probably contributed to the high concentrations. The average mass removal rate of P was 0.69 g m−2 d−1 and the removal rate constant was 13.5 m year−1 at a HLR of 80 mm d−1 . This is at the same level as reported in other studies (Kadlec and Knight, 1996; Oovel et al., 2007). Nitrogen processing in the wetland was high, but not complete. Even though the concentrations of TKN and NH4 were reduced significantly, the concentrations in the effluent were still high. Nitrate was produced, but concentrations were relatively low compared with TKN-levels in the influent. Ammonia-oxidizing bacteria were present at relative high densities in the filters, particularly the planted filters, confirming the capacity of the filters to nitrify. And even though there were differences between the N processing at the different loading rates, the pattern was the same: nitrification was far from complete. This indicates that the physical structure of the vertical filter did not hold the water back long enough to allow adequate contact with the bacteria growing on the surface of the gravel to secure both ammonification and nitrification. The filters were relative shallow (0.6 m) and consisted of coarser gravel compared with vertical flow systems used in temperate regions (e.g. Brix and Arias, 2005b). The retention time of the water in the filters was therefore very short, even at low loading rates. The vertical filters should be deeper and should consist of finer sand or gravel to provide enough contact time for sequential ammonification and nitrification, particularly when treating high-strength wastewater. Plant uptake did contribute to the N removal, particularly at low loading rates, but plant uptake was quantitatively of low importance compared to the microbial N processes as has also been shown in many other studies (e.g. Tanner et al., 2002). Nitrogen removal in constructed wetlands can also occur by ammonia volatilization, and at high ammonium concentrations in wastewater volatilization may be a significant factor even at neutral pH (Hunt and Poach, 2001). There are a number of approaches used by different researchers for estimating the needed surface area of vertical flow constructed wetland system (e.g. Cooper, 2005). The oxygen transfer rate (OTR) from the atmosphere to the filter, which is a physical process driven by convective air flow and diffusion, is considered of prime importance by many, because oxygen is needed for degradation of organic matter as well as for nitrification. Design OTRs of around 30 g m−2 d−2 have been recommended (Vymazal et al., 1998; Cooper, 1999; Platzer, 1999), but many researchers have found much higher rates (Noorvee et al., 2005; Arias et al., 2005; Cooper, 2005; Brix and Arias, 2005b; Vymazal et al., 2006). In the present study the OTRs were ca. 80 g m−2 d−1 in planted and ca. 60 g m−2 d−1 in un-planted systems at a HLR of 80 mm d−1 . As expected, the OTR increased with loading rate, and the low effects of loading rate on the effluent concentrations indicate that it is not the OTR that has limited degradation in the filters. Rather, it is the lack of sufficient contact time of the wastewater in the filter that limited performance.
245
The effects of plants in vertical constructed wetland systems has been a matter of controversy in the literature (Brix, 1994, 1997). Many studies have found no or very little difference between the treatment performance of planted and unplanted controls in small-scale mesocosm studies (e.g. Tanner, 2001). In the present study we did find statistically significant effects of plants, particularly on N removal, but plants also affected P removal, and the number of ammonia-oxidizing bacteria was much higher in planted systems. The high evapotranspirative water loss from the planted systems, particularly the systems planted with Cyperus, affected the effluent pollutant levels by increasing the effluent concentrations. This effect will be much less in full-scale systems. Direct uptake of nutrients into the plant tissues were significant, but of little quantitative importance at high loading rates. Other plant effects could be physical effects of the plant growth and the presence of plant roots in the gravel filter. The presence and growth of roots can both decrease and increase the hydraulic conductivity of the gravel filter, depending on grain size distribution, and other factors. We did not notice any of these potential effects in the present short-term study, and it is likely that such effects will become of greater importance in the longer term.
5.
Conclusion
Vertical flow constructed wetland systems with unsaturated flow have a high capacity to treat high-strength wastewater in tropical climates. The gravel and sand matrix of the vertical systems must, however, be designed in a way so that the pulseloaded wastewater can pass through the bed at a speed that will allow the water to pass through the bed before the next dose arrives whilst at the same time holding the water back long enough to allow sufficient contact with the biofilm growing on the bed medium to achieve the desired treatment. Also, the surface area must be large enough to secure a sufficient oxygen transfer to cover the need for microbial degradation of organic matter and nitrification of ammonium. The vertical filters used in the present study did not provide sufficient contact time between the wastewater and the bed medium to secure complete nitrification of the high-strength wastewater. Oxygen transfer into the bed medium was high and probably did not limit treatment performance. Rather, the bed depth, which was only 0.6 m, and the composition of the bed matrix, which was relatively coarse sand and gravel, limited performance because of insufficient contact time. Therefore, for high-strength wastewater the beds should be deeper than 0.6 m, and the bed matrix should be of a finer texture than used in the present study to secure sufficient contact time between water and the biofilm. However, care should be taken, as the selection of finer sand or gravel may lead to surface clogging and hence flooding. The plants have an important role in this respect as they may help stabilize the hydraulic conductivity of the bed medium and counteract clogging. Additional studies are needed to identify the optimal bed design for the treatment of high-strength wastewater in tropical climates.
246
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Acknowledgement The authors acknowledge the financial support by the Faculty of Engineering, CMU.
references
Alexander, M., 1965. Most-probable-number method for microbial populations. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. American Society of Agronomy, Madison, WI, pp. 1467–1472. APHA, 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Arias, C.A., Brix, H., Marti, E., 2005. Recycling of treated effluents enhances removal of total nitrogen in vertical flow constructed wetlands. J. Environ. Sci. Health Part A: Toxic/Hazard. Subst. Environ. Eng. 40, 1431–1443. Brix, H., 1994. Functions of macrophytes in constructed wetlands. Water Sci. Technol. 29, 71–78. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Sci. Technol. 35, 11–17. Brix, H., Arias, C.A., 2005a. Danish guidelines for small-scale constructed wetland systems for onsite treatment of domestic sewage. Water Sci. Technol. 51, 1–9. Brix, H., Arias, C.A., 2005b. The use of vertical flow constructed wetlands for on-site treatment of domestic wastewater: New Danish guidelines. Ecol. Eng. 25, 491–500. Brix, H., Arias, C.A., Johansen, N.H., 2002. BOD and nitrogen removal from municipal wastewater in an experimental two-stage vertical flow constructed wetland system with recycling. In: Proceedings of the 8th International Conference on Wetland Systems for Water Pollution Control, vol. 1, Arusha, Dar es Salam, Tanzania, September 16–19, 2002, pp. 400–410. Brix, H., Koottatep, T., Laugesen, C.H., 2007. Wastewater treatment in tsunami affected areas of Thailand by constructed wetlands. Water Sci. Technol. 56, 69–74. Brix, H., Schierup, H.-H., 1990. Soil oxygenation in constructed reed beds: the role of macrophyte and soil atmosphere interface oxygen transport. In: Cooper, P.F., Findlater, B.C. (Eds.), Constructed Wetlands in Water Pollution Control. Pergamon Press, London, pp. 53–66. Cooper, P., 1999. A review of the design and performance of vertical-flow and hybrid reed bed treatment systems. Water Sci. Technol. 40, 1–9. Cooper, P., 2005. The performance of vertical flow constructed wetland systems with special reference to the significance of oxygen transfer and hydraulic loading rates. Water Sci. Technol. 51, 81–90. Denny, P., 1997. Implementation of constructed wetlands in developing countries. Water Sci. Technol. 35, 27–34. Gottschall, N., Boutin, C., Crolla, A., Kinsley, C., Champagne, P., 2007. The role of plants in the removal of nutrients at a constructed wetland treating agricultural (dairy) wastewater, Ontario, Canada. Ecol. Eng. 29, 154–163. Hunt, P.G., Poach, M.E., 2001. State of the art for animal wastewater treatment in constructed wetlands. Water Sci. Technol. 44, 19–25. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. Lewis Publishers, Boca Raton/New York/London/Tokyo. Kantawanichkul, S., Pilaila, S., Tanapiyawanich, W., Tikampornpittaya, W., Kamkrua, S., 1999. Wastewater treatment by tropical plants in vertical-flow constructed wetlands. Water Sci. Technol. 40, 173–178.
Kantawanichkul, S., Somprasert, S., 2005. Using a compact combined constructed wetland system to treat agricultural wastewater with high nitrogen. Water Sci. Technol. 51, 47–53. Kantawanichkul, S., Somprasert, S., Aekasin, U., Shutes, R.B.E., 2003. Treatment of agricultural wastewater in two experimental combined constructed wetland systems in a tropical climate. Water Sci. Technol. 48, 199–205. Kaseva, M.E., 2004. Performance of a sub-surface flow constructed wetland in polishing pre-treated wastewater—a tropical case study. Water Res. 38, 681–687. Knight, R.L., Payne, V.W.E., Borer, R.E., Clarke, R.A., Pries, J.H., 2000. Constructed wetlands for livestock wastewater management. Ecol. Eng. 15, 41–55. Konnerup, D., Koottatep, T., Brix, H., 2009. Treatment of domestic wastewater in tropical, subsurface flow constructed wetlands planted with Canna and Heliconia. Ecol. Eng. 35, 248–257. Konuma, S., Satoh, H., Mino, T., Matsuo, T., 2001. Comparison of enumeration methods for ammonia-oxidizing bacteria. Water Sci. Technol. 43, 107–114. Koottatep, T., Polprasert, C., Oanh, N.T.K., Heinss, U., Montangero, A., Strauss, M., 2001. Septage dewatering in vertical-flow constructed wetlands located in the tropics. Water Sci. Technol. 44, 181–188. Koottatep, T., Surinkul, N., Polprasert, C., Kamal, A.S.M., Kone, D., Montangero, A., Heinss, U., Strauss, M., 2005. Treatment of septage in constructed wetlands in tropical climate: lessons learnt from seven years of operation. Water Sci. Technol. 51, 119–126. Laber, J., Haberl, R., Shrestha, R., 1999. Two-stage constructed wetland for treating hospital wastewater in Nepal. Water. Sci. Technol. 40, 317–324. Laber, J., Perfler, R., Haberl, R., 1997. Two strategies for advanced nitrogen elimination in vertical flow constructed wetlands. Water Sci. Technol. 35, 71–77. Molle, P., Lienard, A., Boutin, C., Merlin, G., Iwema, A., 2005. How to treat raw sewage with constructed wetlands: an overview of the French systems. Water Sci. Technol. 51, 11–21. National Institute of Agricultural Sciences, 1977. Official Methods of Analysis of Fertilizers. Ministry of Agriculture and Forestry, Nishigahara, Japan. Noorvee, A., Repp, K., Poldvere, E., Mander, U., 2005. The effects of aeration and the application of the k-C* model in a subsurface flow constructed wetland. J. Environ. Sci. Health Part A: Toxic/Hazard. Subst. Environ. Eng. 40, 1445–1456. Oovel, M., Tooming, A., Mauring, T., Mander, U., 2007. Schoolhouse wastewater purification in a LWA-filled hybrid constructed wetland in Estonia. Ecol. Eng. 29, 17–26. Platzer, C., 1999. Design recommendations for subsurface flow constructed wetlands for nitrification and denitrification. Water Sci. Technol. 40, 257–263. Poach, M.E., Hunt, P.G., Reddy, G.B., Stone, K.C., Johnson, M.H., Grubbs, A., 2007. Effect of intermittent drainage on swine wastewater treatment by marsh-pond-marsh constructed wetlands. Ecol. Eng. 30, 43–50. Reed, S.C., Middlebrooks, E.J., Crites, R.W., 1988. Natural Systems for Waste Management and Treatment. McGraw-Hill Inc., New York, NY. Rosenberg, N.J., 1969. Advective contribution of energy utilised in evapotranspiration by alfalfa in the East Central Great Plains. Agric. Meteorol. 6, 179–184. Tanner, C.C., 2001. Plants as ecosystem engineers in subsurface-flow treatment wetlands. Water Sci. Technol. 44, 9–17. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. 1. Removal of oxygen demand, suspended solids and faecal coliforms. Water Res. 29, 17–26.
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Tanner, C.C., Kadlec, R.H., Gibbs, M.M., Sukias, J.P.S., Nguyen, M.L., 2002. Nitrogen processing gradients in subsurface-flow treatment wetlands—influence of wastewater characteristics. Ecol. Eng. 18, 499–520. Verma, S.B., Rosenberg, N.J., Blad, B.L., 1978. Turbulent exchange coefficients for sensible heat and water vapour under advective conditions. J. Appl. Meteorol. 17, 330–338. Vymazal, J., Brix, H., Cooper, P.F., Haberl, R., Perfler, R., Laber, J., 1998. Removal mechanisms and types of constructed wetlands. In: Vymazal, J., Brix, H., Cooper, P.F., Green, M.B., Haberl, R. (Eds.), Constructed Wetlands for Wastewater Treatment in Europe. Backhuys Publisher, Leiden, The Netherlands, pp. 17–66.
247
Vymazal, J., Greenway, M., Tonderski, K., Brix, H., Mander, U., 2006. Constructed wetlands for wastewater treatment. In: Verhoeven, J.T.A., Beltman, B., Bobbink, R., Whigham, D.F. (Eds.), Wetlands and Natural Resource Management. Springer, pp. 69–96. Weedon, C.M., 2003. Compact vertical flow constructed wetland systems—first two years’ performance. Water Sci. Technol. 48, 15–23. Wilderer, P.A., 2004. Applying sustainable water management concepts in rural and urban areas: some thoughts about reasons, means and needs. Water Sci. Technol. 49, 8–16.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
Treatment of high-strength wastewater in tropical vertical flow constructed wetlands planted with Typha angustifolia and Cyperus involucratus Suwasa Kantawanichkul a,∗ , Supreeya Kladprasert a , Hans Brix b a b
Department of Environmental Engineering, Chiang Mai University, Thailand Department of Biological Sciences, Aarhus University, Ole Worms Allé, Building 1135, DK-8000 Århus C., Denmark
a r t i c l e
i n f o
Article history:
a b s t r a c t The ability of vertical flow (VF) constructed wetland systems to treat high-strength (ca.
Received 17 December 2007
300 mg L−1 of COD and ca. 300 mg L−1 total-nitrogen) wastewater under tropical climatic con-
Received in revised form
ditions was studied during a 5-month period. Nine 0.8-m diameter experimental VF units
30 May 2008
(depth 0.6 m) were used: three units were planted with Typha angustifolia L., another three
Accepted 2 June 2008
units were planted with Cyperus involucratus Rottb and three units were unplanted. Each set of units were operated at hydraulic loading rates (HLRs) of 20, 50 and 80 mm d−1 . Cyperus produced more shoots and biomass than the Typha, which was probably stressed because
Keywords:
of lack of water. The high evapotranspirative water loss from the Cyperus systems resulted
Cattail
in higher effluent concentrations of COD and total-P, but the mass removal of COD did not
Constructed wetland
differ significantly between planted and unplanted systems. Average mass removal rates
Cyperus involucratus
of COD, TKN and total-P at a HLR of 80 mm d−1 were 17.8, 15.4 and 0.69 g m−2 d−1 . The first-
Evapotranspiration
order removal rate constants at a HLR of 80 mm d−1 for COD, TKN and total-P were 49.8,
Oxygen transfer rate
30.1 and 13.5 m year−1 , respectively, which is in the higher range of k-values reported in the
Removal rate constant
literature. The oxygen transfer rates were ca. 80 g m−2 d−1 in the planted systems as opposed
Tropical wetland
to ca. 60 g m−2 d−1 in the unplanted systems. The number of Nitrosomonas was two to three
Typha angustifolia
orders of magnitude higher in the planted systems compared to the unplanted systems.
Umbrella Sedge
Planted systems thus had significantly higher removal rates of nitrogen and phosphorus,
Vertical flow
higher oxygen transfer rates, and higher quantities of ammonia-oxidizing bacteria. None of the systems did, however, fully nitrify the wastewater, even at low loading rates. The vertical filters did not provide sufficient contact time between the wastewater and the biofilm on the gravel medium of the filters probably because of the shallow bed depth (0.6 m) and the coarse texture of the gravel. It is concluded that vertical flow constructed wetland systems have a high capacity to treat high-strength wastewater in tropical climates. The gravel and sand matrix of the vertical filter must, however, be designed in a way so that the pulseloaded wastewater can pass through the filter medium at a speed that will allow the water to drain before the next dose arrives whilst at the same time holding the water back long enough to allow sufficient contact with the biofilm on the filter medium. © 2008 Elsevier B.V. All rights reserved.
∗
Corresponding author. Tel.: +66 53 944192; fax: +66 53 210328. E-mail address: [email protected] (S. Kantawanichkul). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.06.002
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
1.
Introduction
Many agro and food production industries in Thailand discharge poorly treated wastewater with high COD (chemical oxygen demand) and nitrogen (N) contents directly into canals and rivers, causing severe environmental problems in these. There is therefore an urgent need to improve the conditions by introducing proper treatment of the wastewater prior to discharge. Conventional high-tech wastewater treatment systems require large capital investments and operating costs, and for that reason these systems are not realistic solutions for producers that cannot afford such expensive treatment systems. Also, high-tech systems often fail in less-developed countries like Thailand (Brix et al., 2007). Constructed wetlands are, on the other hand, a cost-effective, reliable and robust alternative to conventional wastewater treatment systems that are known to be particularly efficient in removing COD, and have also successfully been used in temperate countries to treat wastewaters with high concentrations of N (e.g. Tanner et al., 1995; Gottschall et al., 2007; Poach et al., 2007). However, although the potential for application of constructed wetlands in tropical countries is enormous, the rate of adoption of the technology for wastewater treatment has been slow (Denny, 1997; Wilderer, 2004) partly because of the lack of generally adopted design criteria for tropical climates. The vertical flow constructed wetland system with unsaturated flow is known to be particularly efficient in treating many types of wastewater (Cooper, 1999; Molle et al., 2005; Brix and Arias, 2005b). The vertical flow systems process greater oxygen transport ability than the horizontal subsurface flow beds (Brix and Schierup, 1990; Cooper, 2005), and they are more effective for the mineralization of biodegradable organic matter and for nitrification through the activity of ammoniaoxidizing bacteria. However, this type of system is rather new and undocumented in Thailand, and there are no design criteria available. Experiences and design criteria described in the literature cannot be directly transferred to the tropical conditions of Thailand, because the studies are nearly exclusively conducted in temperate climates. Climate and other local conditions influence wastewater characteristics, plant growth and evaporation as well as the removal processes in the constructed wetland, particularly the microbial processes which are expected to be stimulated by the high temperatures. Therefore, there is a need to gain more experience on the per-
239
formance of vertical flow constructed wetlands from tropical climates in order to assess the capacity of the systems under tropical climatic conditions. Different species of wetland plants, including species of Reed (Phragmites australis (Cav.) Trin ex Steudel and Phragmites mauritianus Kunth), have been used in constructed wetland systems in tropical areas (Laber et al., 1999; Kaseva, 2004). Cattail (Typha angustifolia L.) is also widely used and is known to be highly tolerant to various types of wastewater (e.g. Koottatep et al., 2001, 2005), and Umbrella Sedge (Cyperus involucratus Rottb) is a species with a high value for ornamentation that can grow very well in a subsurface flow constructed wetland system (Kantawanichkul et al., 1999, 2003; Kantawanichkul and Somprasert, 2005). The objectives of this study were to assess the ability of vertical flow constructed wetland systems to treat high COD and high-N wastewater, and to evaluate the performance of systems planted with C. involucratus and T. angustifolia with unplanted systems. Furthermore, we wanted to estimate mass removal rates and removal rate constants to get more insight into the capacity of vertical flow systems under tropical climatic conditions.
2.
Materials and methods
2.1.
Experimental setup
The experiment was conducted in nine pilot-scale vertical flow constructed wetland units located at the university campus in Chiang Mai, Thailand (18◦ 47 24N, 98◦ 58 11E; elevation 318 m). The experiment was run for 5 months during November 2005–April 2006. Three units were planted with Narrow Leaf Cattail (T. angustifolia L.), another three tanks were planted with Umbrella Sedge (C. involucratus Rottb, syn Cyperus alternifolius L. ssp. flabelliformis (Rottb.) Kükenth.), and the last three tanks were unplanted. Planting density was 16 plants per square meter. Each unit consisted of a 0.8-m diameter circular precast concrete tank (height 0.6 m), each established with a three-layer gravel filter: 0.15 m of 30–60 mm gravel in the bottom followed by 0.15 m of 6–12 mm gravel in the middle and a 0.2 m layer of coarse 1–2 mm sand at the top (Fig. 1). The porosity of the media was 0.37. Each set of three tanks was loaded with synthetic wastewater at three different hydraulic loading rates (HLRs) of 20, 50 and 80 mm d−1 , respectively (Fig. 2). Each tank was operated with 30 min feeding every 2 h using
Fig. 1 – Schematic drawing of the experimental vertical flow constructed wetland units.
240
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Fig. 2 – Experimental set-up of the nine vertical flow constructed wetland units planted with Umbrella sedges (Cyperus involucratus), cattails (Typha angustifolia) or control (unplanted) and different loading rates (20, 50 and 80 mm d−1 ).
timer-controlled peristaltic pumps. Every week the loadings provided by the pumps were checked and when necessary adjusted to the desired level. Effluents were collected in drains in the bottom of the tanks and allowed to drain freely into effluent storage tanks. The amount of effluent from each tank was measured weekly to estimate water loss from the system due to evapotranspiration.
2.2.
Chemical analysis
The synthetic wastewater used was prepared from NH4 Cl, starch, NaHCO3 and KHPO4 to gain COD and TKN (Total Kjeldahl Nitrogen) concentration around 300 mg L−1 (average measured composition during the experiment is shown in Table 1). Inlet and effluent water samples from each tank were analyzed weekly for COD, TKN, NH4 -N, NO2 -N, NO3 -N, total-P and TSS (total suspended solids) following standard analytical procedures (APHA, 1998). The temperature of the effluent wastewater ranged between 21 and 29.5 ◦ C, dissolved oxygen concentration (as analysed by an oxygen electrode) between 0.2 and 2.0 mg L−1 , and pH between 6.60 and 7.85 during the experiment. Only data from day 43 and onwards are presented here as the initial period was considered a period of acclimation. Sampling was carried out weekly for 4 months and data were averaged over monthly periods before statistical analyses (n = 4).
2.3.
Plant growth and uptake
At the end of the experiment the number of plants in each unit was counted and nine representative Cyperus and three Typha plants from each unit harvested to estimate biomass. The plants were fractionated into belowground (roots and rhizomes) and aboveground (leaves and stems) tissues, and their dry weight measured. Subsamples of the dried plant tissue were homogenized and the contents of N analyzed by a standard Kjeldahl technique (National Institute of Agricultural Sciences, 1977). The uptake of N was calculated from the total biomass and the N concentration in the tissues.
Table 1 – Average (±1S.D.) composition (mg L−1 ) of the synthetic wastewater used in the experiment (n = 16)
−1
TSS (mg L ) COD (mg L−1 ) TKN (mg L−1 ) NH4 -N (mg L−1 ) NO2 + NO3 -N (mg L−1 ) Total-P (mg L−1 )
Average
Range
± ± ± ± ± ±
7–74 211–333 273–332 260–319 0–1.0 21.6–26.7
42 275 305 290 0.2 24.2
20 33 16 16 0.3 1.2
241
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
2.4.
Enumeration of bacteria
At the end of the experiment the gravel medium was sampled at four points in each unit at two depths; 10–15 cm and 35–40 cm. Samples from the same unit and same depth was pooled and mixed and analyzed for the quantity of ammonia-oxidizing bacteria (Nitrosomonas and Nitrobacter) and denitrifying bacteria by the MPN (most probable number) technique (Alexander, 1965).
2.5.
Removal rate kinetics
The rate constants, k (m year−1 ), for COD, TKN and total-P were calculated based on observed mass removal rates in the tanks using the following expression (Kadlec and Knight, 1996): k = HLR(ln Cin − ln Cout ) where HLR is the hydraulic loading rate (m year−1 ) and Cin and Cout are the mass of pollutant (COD, TKN or Total-P) loaded into the system and leaving the system per day, respectively. The mass removal rates were used here instead of the inlet and effluent concentrations as water loss from the systems due to evapotranspiration was high (often > 20 mm d−1 at high plant densities). Thus, the data obtained from this model should be used with great caution. Many studies indicate that the model can be used for BOD (Biochemical Oxygen Demand) removal in surface flow and horizontal subsurface flow wetlands (Reed et al., 1988; Kadlec and Knight, 1996), and data also suggest that it can be used for total-P with some confidence. However, it is questionable whether the model can be used for vertical flow constructed wetlands and for the removal of N.
2.6.
Oxygen transfer rate
The hypothetical O2 transfer rates (OTR, g m−2 d−1 ) were calculated as suggested by Cooper (2005) and Platzer (1999): OTR = 0.7HLR[CODin − CODout ] + 4.3[NH4 -Nin − NH4 − Nout ] where HLR is the hydraulic loading rate (m d−1 ), [CODin − CODout ] is the mass of COD removed in the system (g m−2 d−1 ) and [NH4 -Nin − NH4 -Nout ] is the mass of nitrogen (g m−2 d−1 ) nitrified in the beds. Usually BOD5 rather than COD has been used as a direct estimate of the biodegradable organic matter that is oxidized in the beds. The factor 0.7 is here used to compensate for that as suggested by Platzer (1999). The factor 4.3 comes from the fact that it takes 4.3 g of O2 to oxidize 1 g of NH4 -N. We did not make any allowance for O2 recovered from denitrification of the nitrate in the beds.
2.7.
Data analysis
Data analysis was performed using two-way ANOVA and Type III sum of squares with the software Statgraphics Plus Ver. 4.1 (Manugistics, Inc., Scottsdale, AZ, USA). Data were tested for normal distribution using the Kolmogorov–Smirnov test, and heterogeneities of variances within treatments were tested using Cochran’s C-test. If necessary, logarithmic or square root transformations were performed to ensure homogeneity of variance, but for clarity all data are presented as untrans-
Table 2 – F-Values and significance of a two-way ANOVA of effluent concentrations from vertical flow constructed wetland systems planted with different species (Cyperus involucratus, Typha angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 )
TSS COD TKN NH4 -N NO2 + NO3 -N Total-P
Species
Loading
6.60* 5.06* 32.05*** 14.25*** 0.93 26.93***
1.63 0.83 51.40*** 43.71*** 7.72** 24.81***
Species × loading 7.03*** 0.15 9.32*** 5.05** 1.04 13.92***
∗
P < 0.05. P < 0.01. ∗∗∗ P < 0.001. ∗∗
formed. Differences between individual means were identified using Tukey HSD-procedure at the 5% significance level.
3.
Results
3.1.
Plant growth and water loss
The growth of Cyperus was much better (average 547 shoots m−2 , 4.3 kg dw m−2 ) than that of Typha (average 78 shoots m−2 ; 0.9 kg dw m−2 ). The Typha plants seemed stressed with wilting leaves and did not grow well. The much higher growth and biomass of Cyperus also resulted in a significantly (P < 0.001) higher water loss due to evapotranspiration in these compared to the systems planted with Typha and the unplanted control. The average water loss was 20.1 mm d−1 in systems with Cyperus, 11.9 mm d−1 in systems with Typha and 7.9 mm d−1 in the unplanted systems.
3.2.
Effluent concentrations
The effluent concentration of TSS was generally <10 mg L−1 (data not shown) and a significant interaction between species (including the unplanted treatment) and loading was observed (Table 2). In systems planted with Cyperus the effluent concentrations were highest at high loading rates, whereas in systems planted with Typha the opposite was observed. In unplanted systems TSS effluent concentration did not differ between loading rates. For COD the effluent concentration was significantly higher in Cyperus than in Typha planted systems (Fig. 3a), probably because of the concentrating effect of the high evapotranspiration rates in the Cyperus beds. Effluent COD concentrations were independent on loading rate (Table 2). Both loading rate and species affected TKN and NH4 N effluent concentrations, and the significant interaction term in the ANOVA shows that the effects of loading rate depend on species (Table 2). TKN and NH4 -N effluent concentrations were generally higher in the systems planted with Cyperus, and concentrations increased with loading rate (Fig. 3b). Effluent concentrations of oxidized nitrogen (NO2+3 -N) did not differ between planted and unplanted systems, but concentrations decreased significantly with loading rate (Fig. 3c). Effluent concentrations of total-P were generally high, but lowest in unplanted systems at low loading rate rates (Fig. 3d). Loading
242
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Fig. 3 – Effluent concentrations (average ± 1S.E.) of (a) chemical oxygen demand, COD; (b) total Kjeldahl nitrogen, TKN; (c) oxidized nitrogen, NO2 + NO3 -N; and (d) total phosphorus, total-P, at hydraulic loading rates of 20, 50 and 80 mm d−1 for vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted).
rate affected total-P effluent concentration in the unplanted systems whereas no effect was observed in the planted systems.
3.3.
Mass removal rates
Mass removal rates were very dependent on loading rate with significantly higher removal rates at high loadings (Tables 3 and 4). However, the effects of loading depended on species as shown by the significant interaction terms in the ANOVA (Table 3), except the mass removal of COD which was independent on species. At low loading rates (20 and 50 mm d−1 ) the mass removal rates did not depend much on species, but at a HLR of 80 mm d−1 the mass removal rates were generally lower for most parameters in the unplanted systems compared to the planted systems (Table 4).
Table 3 – F-Values and significance of a two-way ANOVA of mass removal rates in vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 )
TSS COD TKN NH4 -N Total-P
Species
Loading
Species × loading
5.33* 0.39 9.47*** 10.19*** 6.41**
3288.99*** 362.70*** 285.68*** 258.86*** 47.60***
4.20** 0.47 7.43*** 7.07*** 5.81**
3.4.
Plant uptake of nitrogen
The concentration of N in the plant tissues varied between 2.8 and 3.4% of the dry weight for Cyperus and 1.8 and 2.4% of the dry weight for Typha. There was no significant difference between the N concentrations at the different loading rates. The average plant uptake of N throughout the experiment was 0.86 ± 0.25 g m−2 d−1 for Cyperus and 0.12 ± 0.06 g m−2 d−1 for Typha (mean ± 1S.D.). Comparing the plant uptake with the average mass removal rates of N in the systems, the uptake of N by Cyperus constituted between 6.7% of the mass N removal at a HLR of 80 mm d−1 to 16.9% at 20 mm d−1 . Because of the lower growth and N content in the tissues of Typha, the plant uptake only constituted 0.5–3.3% of the mass N removal rates in the Typha planted systems.
3.5.
Removal rate constants
Removal rate constants depended strongly on loading rate and the effects of loading rate different between the species (except for kCOD ) as shown by the significant interactions in the ANOVA (Table 5). Removal rate constants were generally much higher at high loading rates (Fig. 4), particularly in planted systems. The removal rate constant for COD did not differ significantly between species (Fig. 4a), but for N the removal rate constant was significantly higher in planted systems compared with unplanted systems (Fig. 4b). The removal rate constant for P was especially high in the Cyperus system at high loading rate (Fig. 4c).
3.6.
Oxygen transfer rates
∗
P < 0.05. ∗∗ P < 0.01. ∗∗∗ P < 0.001.
The calculated O2 transfer rate depended strongly on loading rate, and the significant interaction in the ANOVA (F = 7.16;
243
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Table 4 – Mass removal rates (average ± S.E.) of TSS, COD, TKN, NH4 -N and total-P in vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 ) Treatment
Parameter
Hydraulic loading rate 20 mm d−1
50 mm d−1
80 mm d−1
C. involucratus T. angustifolia Unplanted
TSS (g m−2 d−1 )
0.78 ± 0.01 0.72 ± 0.02 0.74 ± 0.01
1.82 ± 0.06 1.98 ± 0.03 1.87 ± 0.04
3.08 ± 0.05 3.19 ± 0.03 3.00 ± 0.07
C. involucratus T. angustifolia Unplanted
COD (g m−2 d−1 )
4.5 ± 0.2 4.6 ± 0.2 4.4 ± 0.3
10.2 ± 0.9 10.8 ± 1.3 10.8 ± 0.3
18.1 ± 0.9 18.2 ± 0.5 17.1 ± 0.8
C. involucratus T. angustifolia Unplanted
TKN (g m−2 d−1 )
4.5 ± 0.2 4.6 ± 0.2 5.1 ± 0.1
9.0 ± 1.0 10.8 ± 0.6 8.7 ± 0.3
16.9 ± 0.9 16.6 ± 0.7 12.6 ± 0.9
C. involucratus T. angustifolia Unplanted
NH4 -N (g m−2 d−1 )
4.5 ± 0.2 4.4 ± 0.2 4.9 ± 0.1
8.7 ± 1.0 10.3 ± 0.6 8.2 ± 0.3
16.2 ± 0.8 15.8 ± 0.6 11.8 ± 0.9
C. involucratus T. angustifolia Unplanted
Total-P (g m−2 d−1 )
0.22 ± 0.03 0.17 ± 0.02 0.27 ± 0.02
0.45 ± 0.11 0.47 ± 0.08 0.36 ± 0.06
0.96 ± 0.09 0.64 ± 0.08 0.48 ± 0.07
P = 0.0005) shows that the dependency differs between planted and unplanted treatments (Fig. 5). The O2 transfer rates were ca. 80 g m−2 d−1 in the planted systems at a HLR of 80 mm d−1 and only ca. 60 g m−2 d−1 at the same HLR in the unplanted system.
3.7.
Ammonia-oxidizing and denitrifying bacteria
There were no significant differences between the number of bacteria and depth of sampling or loading rate. However, the number of Nitrosomonas was two and three orders of magnitude higher in the systems planted with Typha and Cyperus, respectively, compared to the unplanted systems (Table 6). The quantity of Nitrobacter was also highest, but not statistically significant, in the Cyperus planted systems. The number of denitrifiers was of the same magnitude in all systems. The numbers reported here should, however, be regarded more as relative numbers rather than absolute numbers as the enumeration by the MPN method is known to have the possibility of underestimation (Konuma et al., 2001).
Table 5 – F-Values and significance of a two-way ANOVA of removal rate constants (k) in vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) and different loading rates (20, 50 and 80 mm d−1 ) Species TSS COD TKN NH4 -N Total-P *
P < 0.05.
∗∗
P < 0.01. P < 0.001.
∗∗∗
**
8.60 0.82 7.15** 7.85** 6.13**
Loading ***
223.59 81.32*** 77.45*** 68.96*** 25.32***
Species × loading 4.64** 0.57 7.16*** 6.95*** 5.42**
4.
Discussion
This study clearly shows that unsaturated vertical flow constructed wetland systems have a high capacity to treat high-strength wastewater. Many studies from temperate areas have documented that vertical flow constructed wetland systems are very efficient in removing biodegradable organic matter as well as able to nitrify ammonium (Laber et al., 1997; Cooper, 1999; Brix et al., 2002; Weedon, 2003; Brix and Arias, 2005b), and the present study confirms that this is also true in tropical climates. The effluent concentrations reached are lower than the maximum permitted values for medium sized pig farms in Thailand (BOD 100 mg L−1 ; COD 400 mg L−1 ; TSS 200 mg L−1 ; TKN 200 mg L−1 ) as published by the Ministry of Natural Resources and Environment. T. angustifolia did not grow well in this study, as opposed to C. involucratus, which had a very vigorous growth and biomass production. This is surprising, as Typha sp. often grow well at very eutrophic sites and are also a common plant in constructed wetlands (e.g. Koottatep et al., 2005). The poor growth in the present study might be because of lack of adequate water in the unsaturated vertical filter. Typha is a wetland plant with a very high water demand and a limited capacity to close stomata and hence reduce water loss by transpiration. Cyperus in nature is found not only in wetlands, but also on drier sites which indicates that this species has a higher capacity to tolerate water stress compared to Typha. In this study the water loss from the planted systems was very high, probably caused by factors like the ‘oasis’ effect. The oasis effect is the phenomenon where warmer and dry air in equilibrium with dry areas flows across a vegetation of plants with a high water availability (Verma et al., 1978; Brix and Arias, 2005a). Evapotranspiration from isolated expanses of vegetation like the experimental systems used in this study, on a per unit area basis, can therefore be significantly greater than cal-
244
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Fig. 4 – Removal rate constants (average ± 1S.E.) for (a) chemical oxygen demand, kCOD ; (b) total Kjeldahl nitrogen, kTKN ; and (c) total phosphorus, kTP , at hydraulic loading rates of 20, 50 and 80 mm d−1 for vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted).
Fig. 5 – Oxygen transfer rates (OTR, average ± 1S.E.) for vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) at hydraulic loading rates of 20, 50 and 80 mm d−1 .
culated potential evapotranspiration (Rosenberg, 1969). In large full-scale systems, water loss by evapotranspiration can be expected to be less than measured in the present study.
Both mass removal rates and the estimated removal rate constants increased with loading rate. This clearly shows that the capacity of the systems to remove biodegradable organic matter (BOD and COD) as well as nitrogen is higher than found at the highest loading rate in this study. Average mass removal rates of COD and TKN at a HLR of 80 mm d−1 were 17.8 and 15.4 g m−2 d−1 , respectively. Also the removal rate constants for COD and TKN were high: 49.8 and 30.1 m year−1 , respectively, which is in the higher range of k-values reported in the literature probably partially because of the higher temperature (Kadlec and Knight, 1996; Knight et al., 2000; Oovel et al., 2007; Konnerup et al., 2008). We did not include the irreducible background wetland concentration (C*) in the model used to estimate k-values in the present study, as we did not have a reliable estimate of C*. Inclusion of C* in the model would result in higher estimated k-values. Hence, additional studies aiming at estimating the irreducible background concentration (C*) and studies at higher loading rates are needed to better estimate k-values of vertical flow systems under tropical conditions. However, great caution should be used in using the estimated rate constants for design purposes, as we used mass loading and removal rates in the present study. Removal of P was, as expected, low in the vertical flow sys-
Table 6 – Average numbers of ammonia-oxidizing bacteria (Nitrosomonas and Nitrobacter) and denitrifiers (no. per g dry weight) in the substrate of vertical flow constructed wetland systems planted with different species (C. involucratus, T. angustifolia or unplanted) as estimated by the MPN technique, and result of ANOVA (F-ratios) C. involucratus Nitrosomonas Nitrobacter Denitrifiers NS = not significant. ∗∗∗
P < 0.001.
9.63E+05 1.12E+05 2.17E+03
T. angustifolia
Unplanted
F-Ratio
1.13E+05 1.73E+04 3.31E+03
1.39E+03 1.13E+04 1.46E+03
37.30*** 1.75NS 0.28NS
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
tems as the gravel medium used had a low binding capacity for P. Effluent concentrations of P were generally high. The concentrating effect of evapotranspirative water loss, particularly at low hydraulic loading rates, probably contributed to the high concentrations. The average mass removal rate of P was 0.69 g m−2 d−1 and the removal rate constant was 13.5 m year−1 at a HLR of 80 mm d−1 . This is at the same level as reported in other studies (Kadlec and Knight, 1996; Oovel et al., 2007). Nitrogen processing in the wetland was high, but not complete. Even though the concentrations of TKN and NH4 were reduced significantly, the concentrations in the effluent were still high. Nitrate was produced, but concentrations were relatively low compared with TKN-levels in the influent. Ammonia-oxidizing bacteria were present at relative high densities in the filters, particularly the planted filters, confirming the capacity of the filters to nitrify. And even though there were differences between the N processing at the different loading rates, the pattern was the same: nitrification was far from complete. This indicates that the physical structure of the vertical filter did not hold the water back long enough to allow adequate contact with the bacteria growing on the surface of the gravel to secure both ammonification and nitrification. The filters were relative shallow (0.6 m) and consisted of coarser gravel compared with vertical flow systems used in temperate regions (e.g. Brix and Arias, 2005b). The retention time of the water in the filters was therefore very short, even at low loading rates. The vertical filters should be deeper and should consist of finer sand or gravel to provide enough contact time for sequential ammonification and nitrification, particularly when treating high-strength wastewater. Plant uptake did contribute to the N removal, particularly at low loading rates, but plant uptake was quantitatively of low importance compared to the microbial N processes as has also been shown in many other studies (e.g. Tanner et al., 2002). Nitrogen removal in constructed wetlands can also occur by ammonia volatilization, and at high ammonium concentrations in wastewater volatilization may be a significant factor even at neutral pH (Hunt and Poach, 2001). There are a number of approaches used by different researchers for estimating the needed surface area of vertical flow constructed wetland system (e.g. Cooper, 2005). The oxygen transfer rate (OTR) from the atmosphere to the filter, which is a physical process driven by convective air flow and diffusion, is considered of prime importance by many, because oxygen is needed for degradation of organic matter as well as for nitrification. Design OTRs of around 30 g m−2 d−2 have been recommended (Vymazal et al., 1998; Cooper, 1999; Platzer, 1999), but many researchers have found much higher rates (Noorvee et al., 2005; Arias et al., 2005; Cooper, 2005; Brix and Arias, 2005b; Vymazal et al., 2006). In the present study the OTRs were ca. 80 g m−2 d−1 in planted and ca. 60 g m−2 d−1 in un-planted systems at a HLR of 80 mm d−1 . As expected, the OTR increased with loading rate, and the low effects of loading rate on the effluent concentrations indicate that it is not the OTR that has limited degradation in the filters. Rather, it is the lack of sufficient contact time of the wastewater in the filter that limited performance.
245
The effects of plants in vertical constructed wetland systems has been a matter of controversy in the literature (Brix, 1994, 1997). Many studies have found no or very little difference between the treatment performance of planted and unplanted controls in small-scale mesocosm studies (e.g. Tanner, 2001). In the present study we did find statistically significant effects of plants, particularly on N removal, but plants also affected P removal, and the number of ammonia-oxidizing bacteria was much higher in planted systems. The high evapotranspirative water loss from the planted systems, particularly the systems planted with Cyperus, affected the effluent pollutant levels by increasing the effluent concentrations. This effect will be much less in full-scale systems. Direct uptake of nutrients into the plant tissues were significant, but of little quantitative importance at high loading rates. Other plant effects could be physical effects of the plant growth and the presence of plant roots in the gravel filter. The presence and growth of roots can both decrease and increase the hydraulic conductivity of the gravel filter, depending on grain size distribution, and other factors. We did not notice any of these potential effects in the present short-term study, and it is likely that such effects will become of greater importance in the longer term.
5.
Conclusion
Vertical flow constructed wetland systems with unsaturated flow have a high capacity to treat high-strength wastewater in tropical climates. The gravel and sand matrix of the vertical systems must, however, be designed in a way so that the pulseloaded wastewater can pass through the bed at a speed that will allow the water to pass through the bed before the next dose arrives whilst at the same time holding the water back long enough to allow sufficient contact with the biofilm growing on the bed medium to achieve the desired treatment. Also, the surface area must be large enough to secure a sufficient oxygen transfer to cover the need for microbial degradation of organic matter and nitrification of ammonium. The vertical filters used in the present study did not provide sufficient contact time between the wastewater and the bed medium to secure complete nitrification of the high-strength wastewater. Oxygen transfer into the bed medium was high and probably did not limit treatment performance. Rather, the bed depth, which was only 0.6 m, and the composition of the bed matrix, which was relatively coarse sand and gravel, limited performance because of insufficient contact time. Therefore, for high-strength wastewater the beds should be deeper than 0.6 m, and the bed matrix should be of a finer texture than used in the present study to secure sufficient contact time between water and the biofilm. However, care should be taken, as the selection of finer sand or gravel may lead to surface clogging and hence flooding. The plants have an important role in this respect as they may help stabilize the hydraulic conductivity of the bed medium and counteract clogging. Additional studies are needed to identify the optimal bed design for the treatment of high-strength wastewater in tropical climates.
246
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Acknowledgement The authors acknowledge the financial support by the Faculty of Engineering, CMU.
references
Alexander, M., 1965. Most-probable-number method for microbial populations. In: Black, C.A. (Ed.), Methods of Soil Analysis. Part 2. Chemical and Microbiological Properties. American Society of Agronomy, Madison, WI, pp. 1467–1472. APHA, 1998. Standard Methods for the Examination of Water and Wastewater. American Public Health Association, Washington, DC. Arias, C.A., Brix, H., Marti, E., 2005. Recycling of treated effluents enhances removal of total nitrogen in vertical flow constructed wetlands. J. Environ. Sci. Health Part A: Toxic/Hazard. Subst. Environ. Eng. 40, 1431–1443. Brix, H., 1994. Functions of macrophytes in constructed wetlands. Water Sci. Technol. 29, 71–78. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Sci. Technol. 35, 11–17. Brix, H., Arias, C.A., 2005a. Danish guidelines for small-scale constructed wetland systems for onsite treatment of domestic sewage. Water Sci. Technol. 51, 1–9. Brix, H., Arias, C.A., 2005b. The use of vertical flow constructed wetlands for on-site treatment of domestic wastewater: New Danish guidelines. Ecol. Eng. 25, 491–500. Brix, H., Arias, C.A., Johansen, N.H., 2002. BOD and nitrogen removal from municipal wastewater in an experimental two-stage vertical flow constructed wetland system with recycling. In: Proceedings of the 8th International Conference on Wetland Systems for Water Pollution Control, vol. 1, Arusha, Dar es Salam, Tanzania, September 16–19, 2002, pp. 400–410. Brix, H., Koottatep, T., Laugesen, C.H., 2007. Wastewater treatment in tsunami affected areas of Thailand by constructed wetlands. Water Sci. Technol. 56, 69–74. Brix, H., Schierup, H.-H., 1990. Soil oxygenation in constructed reed beds: the role of macrophyte and soil atmosphere interface oxygen transport. In: Cooper, P.F., Findlater, B.C. (Eds.), Constructed Wetlands in Water Pollution Control. Pergamon Press, London, pp. 53–66. Cooper, P., 1999. A review of the design and performance of vertical-flow and hybrid reed bed treatment systems. Water Sci. Technol. 40, 1–9. Cooper, P., 2005. The performance of vertical flow constructed wetland systems with special reference to the significance of oxygen transfer and hydraulic loading rates. Water Sci. Technol. 51, 81–90. Denny, P., 1997. Implementation of constructed wetlands in developing countries. Water Sci. Technol. 35, 27–34. Gottschall, N., Boutin, C., Crolla, A., Kinsley, C., Champagne, P., 2007. The role of plants in the removal of nutrients at a constructed wetland treating agricultural (dairy) wastewater, Ontario, Canada. Ecol. Eng. 29, 154–163. Hunt, P.G., Poach, M.E., 2001. State of the art for animal wastewater treatment in constructed wetlands. Water Sci. Technol. 44, 19–25. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. Lewis Publishers, Boca Raton/New York/London/Tokyo. Kantawanichkul, S., Pilaila, S., Tanapiyawanich, W., Tikampornpittaya, W., Kamkrua, S., 1999. Wastewater treatment by tropical plants in vertical-flow constructed wetlands. Water Sci. Technol. 40, 173–178.
Kantawanichkul, S., Somprasert, S., 2005. Using a compact combined constructed wetland system to treat agricultural wastewater with high nitrogen. Water Sci. Technol. 51, 47–53. Kantawanichkul, S., Somprasert, S., Aekasin, U., Shutes, R.B.E., 2003. Treatment of agricultural wastewater in two experimental combined constructed wetland systems in a tropical climate. Water Sci. Technol. 48, 199–205. Kaseva, M.E., 2004. Performance of a sub-surface flow constructed wetland in polishing pre-treated wastewater—a tropical case study. Water Res. 38, 681–687. Knight, R.L., Payne, V.W.E., Borer, R.E., Clarke, R.A., Pries, J.H., 2000. Constructed wetlands for livestock wastewater management. Ecol. Eng. 15, 41–55. Konnerup, D., Koottatep, T., Brix, H., 2009. Treatment of domestic wastewater in tropical, subsurface flow constructed wetlands planted with Canna and Heliconia. Ecol. Eng. 35, 248–257. Konuma, S., Satoh, H., Mino, T., Matsuo, T., 2001. Comparison of enumeration methods for ammonia-oxidizing bacteria. Water Sci. Technol. 43, 107–114. Koottatep, T., Polprasert, C., Oanh, N.T.K., Heinss, U., Montangero, A., Strauss, M., 2001. Septage dewatering in vertical-flow constructed wetlands located in the tropics. Water Sci. Technol. 44, 181–188. Koottatep, T., Surinkul, N., Polprasert, C., Kamal, A.S.M., Kone, D., Montangero, A., Heinss, U., Strauss, M., 2005. Treatment of septage in constructed wetlands in tropical climate: lessons learnt from seven years of operation. Water Sci. Technol. 51, 119–126. Laber, J., Haberl, R., Shrestha, R., 1999. Two-stage constructed wetland for treating hospital wastewater in Nepal. Water. Sci. Technol. 40, 317–324. Laber, J., Perfler, R., Haberl, R., 1997. Two strategies for advanced nitrogen elimination in vertical flow constructed wetlands. Water Sci. Technol. 35, 71–77. Molle, P., Lienard, A., Boutin, C., Merlin, G., Iwema, A., 2005. How to treat raw sewage with constructed wetlands: an overview of the French systems. Water Sci. Technol. 51, 11–21. National Institute of Agricultural Sciences, 1977. Official Methods of Analysis of Fertilizers. Ministry of Agriculture and Forestry, Nishigahara, Japan. Noorvee, A., Repp, K., Poldvere, E., Mander, U., 2005. The effects of aeration and the application of the k-C* model in a subsurface flow constructed wetland. J. Environ. Sci. Health Part A: Toxic/Hazard. Subst. Environ. Eng. 40, 1445–1456. Oovel, M., Tooming, A., Mauring, T., Mander, U., 2007. Schoolhouse wastewater purification in a LWA-filled hybrid constructed wetland in Estonia. Ecol. Eng. 29, 17–26. Platzer, C., 1999. Design recommendations for subsurface flow constructed wetlands for nitrification and denitrification. Water Sci. Technol. 40, 257–263. Poach, M.E., Hunt, P.G., Reddy, G.B., Stone, K.C., Johnson, M.H., Grubbs, A., 2007. Effect of intermittent drainage on swine wastewater treatment by marsh-pond-marsh constructed wetlands. Ecol. Eng. 30, 43–50. Reed, S.C., Middlebrooks, E.J., Crites, R.W., 1988. Natural Systems for Waste Management and Treatment. McGraw-Hill Inc., New York, NY. Rosenberg, N.J., 1969. Advective contribution of energy utilised in evapotranspiration by alfalfa in the East Central Great Plains. Agric. Meteorol. 6, 179–184. Tanner, C.C., 2001. Plants as ecosystem engineers in subsurface-flow treatment wetlands. Water Sci. Technol. 44, 9–17. Tanner, C.C., Clayton, J.S., Upsdell, M.P., 1995. Effect of loading rate and planting on treatment of dairy farm wastewaters in constructed wetlands. 1. Removal of oxygen demand, suspended solids and faecal coliforms. Water Res. 29, 17–26.
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 238–247
Tanner, C.C., Kadlec, R.H., Gibbs, M.M., Sukias, J.P.S., Nguyen, M.L., 2002. Nitrogen processing gradients in subsurface-flow treatment wetlands—influence of wastewater characteristics. Ecol. Eng. 18, 499–520. Verma, S.B., Rosenberg, N.J., Blad, B.L., 1978. Turbulent exchange coefficients for sensible heat and water vapour under advective conditions. J. Appl. Meteorol. 17, 330–338. Vymazal, J., Brix, H., Cooper, P.F., Haberl, R., Perfler, R., Laber, J., 1998. Removal mechanisms and types of constructed wetlands. In: Vymazal, J., Brix, H., Cooper, P.F., Green, M.B., Haberl, R. (Eds.), Constructed Wetlands for Wastewater Treatment in Europe. Backhuys Publisher, Leiden, The Netherlands, pp. 17–66.
247
Vymazal, J., Greenway, M., Tonderski, K., Brix, H., Mander, U., 2006. Constructed wetlands for wastewater treatment. In: Verhoeven, J.T.A., Beltman, B., Bobbink, R., Whigham, D.F. (Eds.), Wetlands and Natural Resource Management. Springer, pp. 69–96. Weedon, C.M., 2003. Compact vertical flow constructed wetland systems—first two years’ performance. Water Sci. Technol. 48, 15–23. Wilderer, P.A., 2004. Applying sustainable water management concepts in rural and urban areas: some thoughts about reasons, means and needs. Water Sci. Technol. 49, 8–16.