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Effect of outlet water level raising and effluent recirculation on removal efficiency of pilot-scale, horizontal subsurface flow constructed wetlands
Desalination 248 (2009) 961–976
Effect of outlet water level raising and effluent recirculation on removal efficiency of pilot-scale, horizontal subsurface flow constructed wetlands Alexandros I. Stefanakis, Vassilios A. Tsihrintzis* Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, School of Engineering, Democritus University of Thrace, 67100 Xanthi, Greece Tel/Fax: +30-25410-79393, 78113; email: [email protected] Received 25 June 2008; accepted 27 August 2008
Abstract Two pilot-scale, horizontal subsurface flow (HSF) constructed wetlands (CWs) were constructed and operated continuously for 3 years, fed with synthetic wastewater. Both wetlands were of rectangular cross-section with dimensions 3.0 m in length and 0.75 m in width. After the first 2 years of operation (period A), two modifications were applied to the wetland units: (1) the outlet water level was raised in the first unit and (2) 50% effluent recirculation was applied to the second unit. Under these two modifications, the two CWs operated for one more year (period B). Water samples were collected every week from the influent, effluent and sampling pipes placed at onethird and two-thirds distance along each unit. The water samples were analysed immediately in the laboratory for BOD5, COD, Total Kjedlahl Nitrogen (TKN), ammonia nitrogen, nitrite, nitrate, total phosphorus (TP) and orthophosphates. The results show that outlet water level raising did not change significantly the performance of the wetland unit, but under this configuration the wetland managed to treat about 15% increased hydraulic and pollutant loads using the same surface area. On the other hand, effluent recirculation negatively affected wetland performance, resulting in a reduction of all pollutant removal rates. Keywords: Constructed wetlands; Horizontal subsurface flow; Effluent recirculation; Water level raising; Temperature; HRT; Organic matter; Nitrogen; Phosphorus
1. Introduction and background Constructed Wetlands (CWs) can be an economic, cost-effective and technically feasible *Corresponding author. Presented at the 2nd Conference on Small and Decentralized Water and Wastewater Treatment Plants (SWAT), Skiathos Island, Greece, May 2–4, 2008
solution to the problem of wastewater treatment [1–4]. The use of these systems has become very popular in many countries, especially in Europe and North America [5–7]. Horizontal subsurface flow (HSF) CWs are commonly used systems in Europe and in the United States [8–10]. The main advantages of these systems include great tolerance in cold climates and in high pollutant
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.08.008
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loads, minimization of odor and mosquito problems, and elimination of public health risk of contact with the treated wastewater [8, 11]. The HSF CWs are a reliable technology for elimination of organic pollutants (BOD5 and COD) and suspended solids; the removal of nutrients is generally more limited, because of a general lack of the necessary oxygen level to oxidize ammonium and of low sorption capacity of common substrate materials used for phosphorus retention [8, 10]. Today, research on these systems is targeting on optimizing and enhancing their performance. Two ways to do this are examined here: (1) partial effluent recirculation and (2) outlet water level raising above the porous media. Effluent recirculation has been proposed by various authors [12–15] as an operational modification to increase the treatment efficiency of CWs. The concept of this alternative method consists of taking part of the effluent and transferring it back to the inflow of the system. The theoretical hypothesis of effluent recirculation is that dilution of influent BOD5 and TSS through effluent recirculation may improve the treatment efficiency of the wetland. The main goal is to enhance aerobic microbial activity through intense interactions between pollutants and micro-organisms, which are close to the plant roots and onto the substrate surface, without significant alterations in system operation [16]. On the other hand, effluent recirculation may cause problems in HSF systems due to the increased hydraulic load, while in vertical flow systems with high hydraulic conductivity values it is suggested as an easy, applicable and effective method [12, 13, 17]. However, in fullscale operating facilities, this modification may increase operation costs, because of additional energy consumption for pumping. Water-level management plays a significant role in ecological functions occurring in wetland systems [11, 18] and is an important factor for the maintenance of wetland vegetation. Cattails grow well in submerged soils and can even dom-
inate where water depth is over 150 mm [19]. Control of outlet water level is also important for winter ice conditions, to prevent ice damage. Furthermore, water provides the environment for biochemical reactions to take place, and acts as a transport medium for oxygen. Waterlevel fluctuations in CWs have been suggested to physically increase aeration, thus enhancing microbial consumption of chemical oxygen demand and ammonia nitrogen [20]. In wetlands with control over inflow and outflow water, the flood level within the wetland will have to be actively and carefully managed. It is suggested that during the first growth season of the plants, water level is maintained where it just saturates the substrate between watering. As the plants grow in height, they have an increased ability to transport oxygen to the root zone. This allows the water level to be raised accordingly [18]. Water-level raising at the outlet on a subsurface flow CW results in the creation of a hybrid system, thus having an HSF CW with a free water surface (FWS) CW on top. The current study aims at examining two modifications applied on HSF CWs: first, to evaluate the effect of partial effluent recirculation on HSF wetland efficiency; and second, to observe if outlet water-level raising would still provide proper treatment in cases of an inflow increase, that is treatment of increased flow and loadings.
2. Materials and methods 2.1. Pilot-scale unit description Two similar pilot-scale HSF CWs were constructed and operated for 3 years, as part of a larger experiment containing five similar units [4]. Each wetland unit consists of a rectangular tank of dimensions 3.0 m in length, 0.75 m in width and 1 m in depth. A view of each wetland and a schematic section of the experimental layout are shown in Fig. 1. The first unit contained medium gravel (MG, d50 = 24.4 mm, range
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1.00 m
1.00 m
1.00 m
Effluent pipe
Diffuser
0.45 cm
963
Porous media
Plastic tank
Sample pipe 3.00 m (c)
Fig. 1. Photos of the two constructed wetland units taken in early winter of 2007: (a) MG-C; (b) FG-R; and (c) schematic section along each wetland unit.
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4–25 mm) obtained from a quarry, and was planted with cattails (Typha latifolia) (MG-C unit). Medium gravel is a carbonate rock (main elements: Si 3.39%; Al 0.90%; Fe 0.82%; Ca 27.20%; mg 4.53%; and P 0.03%). The other unit contained fine gravel (FG, d50 = 6.0 mm, range 0.25–16.0 mm) obtained from a river bed in the area, and was planted with common reeds (Phragmites australis) (FG-R unit). Fine gravel is an igneous rock (Si 28.50%; Al 7.95%; Fe 4.22%; Ca 3.62%; mg 1.76%; and P 0.11%). The river bed material is rich in Al, Fe and Ca, which are the three main elements responsible for phosphorus adsorption [21, 22]. On the contrary, the quarry material is only rich in Ca and is relatively poor in Al and Fe. The thickness of the porous media in both units was 0.45 m. The plants were collected from watercourses in the vicinity of the laboratory. The wetland units were equipped with inlet and outlet hydraulic structures, similar to those used in real systems. The effluent of each wetland unit was collected in a plastic tank placed at the unit outlet. The two wetland units operated initially for 2 years (period A), as described by Akratos and Tsihrintzis [4]. At the beginning of the third working year, wetland operation was changed. The outlet water level in the MG-C unit was raised from 45 to 60 cm, allowing for water to pond above the porous media at a mean depth of approximately 10 cm, thus creating a hybrid HSF-FWS system. Moreover, effluent recirculation started in the FG-R unit at 50% of inflowing wastewater. With these alterations, both units operated for one more year (period B). Both CWs were fed with synthetic wastewater, which was designed and used to simulate to the best the characteristics of domestic wastewater. The synthetic wastewater contained organic substances like nitrogen and phosphorus. The organic substances used were peptone (200 mg/L, 40% of the organic loading) as protein source, cane sugar (200 mg/L, 40% of the
organic loading) as saccharase source and acetic acid as source of organic acids (50 mg/L, 20% of the organic loading) [23]. The source of nitrogen was urea (60 mg/L) and of phosphorus hydrogen potassium phosphate (K2HPO4, 10 mg/L PO3 4 -P). For trace elements, a fertilizer was used. The synthetic wastewater was introduced three times a day, every 8 hours, at the upstream end of the wetlands. Hydraulic residence times (HRT) applied for both periods A and B were 6, 8 and 14 days. The daily inflow was increased by about 15% in the MG-C unit to account for the volume increase and maintain the same HRTs. 2.2. Sampling and chemical analyses The experiments during period A lasted from January 2004 until January 2006; and during period B, with the modified configurations, from February 2006 until February 2007. Water samples were collected once a week in the morning, from the influent, effluent and sampling pipes located at one-third and two-thirds of distances along each unit. The samples were analysed in the laboratory for determination of BOD5, COD, TKN, ammonia, nitrite and nitrate, total phosphorus (TP) and ortho-phosphates (PO3 4 --P). For BOD5 determination, respirometric bottles were used; for COD, the open reflux method was employed; for TKN and ammonia, the titrimetric method; for phosphorus, the stannous chloride method; for nitrite, the colorimetric method; and for nitrate, the cadmium reduction method [24]. 3. Results and discussion 3.1. Physicochemical parameters Table 1 contains measured physicochemical parameters for both wetland units during periods A and B. In case of pH, no significant variations occurred along the wetland length. Along the MG-C unit, pH was more or less stable, with a mean value around 6.45. An increase was
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Table 1 Mean values of physicochemical parameters at various points in both wetland units for both working periods Parameter (mean)
MG-C
FG-R
Period A Conductivity (mS/cm) pH DO (mg/L)
Influent 500 6.9 4.30
1/3 790 6.4 0.40
2/3 741 6.6 0.19
Effluent 757 6.7 1.31
1/3 864 6.8 0.31
2/3 842 7.1 0.39
Effluent 935 6.7 1.47
Period B Conductivity (mS/cm) pH DO (mg/L)
500 6.9 4.30
738 6.4 0.04
695 6.4 0.06
736 6.6 0.47
847 6.7 0.12
821 7.2 0.17
921 6.8 0.46
observed at two-thirds of distance along the FG-R unit to drop again in the effluent, with a mean value 6.9. On the whole, pH values in both pilot-scale units showed the trend to be kept on neutral or slightly acidic range, probably because of interactions between the substrate and the biofilm [1]. These interactions may have resulted in release of salts from the solid media to the water, explaining the slight increase of conductivity along the units. Increased effluent conductivity values have also been observed in other studies [25], and can be attributed to enhanced evapotranspiration and to substrate movement by plant roots due to wind effect. Dissolved oxygen concentrations were practically null, indicating oxygen consumption by pollutants. Figure 2 presents the temporal variation of all physicochemical parameters during period B for both wetland units. For conductivity and pH, it is obvious that there are no significant variations during period B. For DO, seasonal variations occurred, with higher values during the winter, when oxygen solubility in water was higher, and lower values during the summer. The level of oxygen appears to be higher in the FG-R unit, which implies that effluent recirculation performs better in DO increase than water level raising. Furthermore, the fine gravel in the FG-R unit enhances the interactions between the substrate and the biofilm, compared to the medium gravel in the MG-C unit, resulting in higher release of
salts to the water, thus explaining the higher conductivity and pH values in the FG-R unit. Overall, statistics of influent and effluent concentrations and removal efficiencies for both pilot-scale units and for both working periods are presented in Table 2. 3.2. Outlet water-level raising (MG-C unit) – overall performance The HRTs applied in period B were the same as in period A. During period B, to account for the additional volume after the water-level raising at the outlet, the wastewater inflow rate was increased, thus maintaining the same HRTs. In period B, the flow range was 31.5–59 L/d and the mean surface organic loading for BOD5 was 4.9–9.3 g/m2/d, with respective numbers 24– 55 L/d and 3.8–8.8 g/m2/d in period A. Figure 3 contains charts of the removal rates of various pollutants during period B, while Fig. 4 presents mean values of pollutant concentrations along the two wetland units for both working periods A and B. Organic matter removal in the MG-C unit did not show any significant differences. Mean BOD5 removal was slightly higher in period B (86.4% compared to 86.8% in period A), while COD removal slightly decreased (85.7% and 87.3% in periods B and A, respectively) (Table 2). Low standard deviation values for both periods indicate the low variability of organic matter
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MG-C
1.4
14 d
6d
FG-R 8d
DO (mg/L)
1.2 1 0.8 0.6 0.4 0.2
(a)
0 Feb-06 Apr-06
May-06
Jul-06
Aug-06 Oct-06 Nov-06 Jan-07
Feb-07
7.2
14 d
6d
8d
7
pH
6.8 6.6 6.4 6.2 6 Feb-06 Apr-06
May-06
Jul-06
Aug-06 Oct-06 Nov-06 Jan-07
Feb-07
(b) 1000
14 d
6d
8d
Conductivity (µS/cm)
950 900 850 800 750 700 650 600 Feb-06 Apr-06
(c)
Fig. 2. Continued.
May-06
Jul-06
Aug-06 Oct-06 Nov-06 Jan-07
Feb-07
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976 35
Temperature (°C)
30
Air temperature 14 d
FG-R (effluent)
967
MG-C (effluent)
6d
8d
25 20 15 10 5 0 Feb-06 Apr-06
May-06
Jul-06
Aug-06
Oct-06 Nov-06
Jan-07 Feb-07
(d)
Fig. 2. Variations of effluent physicochemical parameters during period B (HRT indicated) for (a) DO; (b) pH; (c) conductivity; and (d) air and wastewater temperature.
removal rates, also seen in Fig. 3. From Fig. 4, it is obvious that for both working periods, the organic matter is mainly removed in the first one-third of the wetland length, with small differentiations (Fig. 4a). In the case of nitrogen removal, the performance of the MG-C unit was lower in period B. Mean TKN removal dropped to 43.9% in period B from 58.4% in period A, and ammonia nitrogen retention reached 26.6%, compared to 43.2% in period A (Table 2). This has also been reported elsewhere [25], and possibly has to do with the ageing of the plants, but also with the increased nitrogen loading (0.91–1.72 g/d/m2 in period B compared to 0.64–1.60 g/d/m2 in period A). No significant variations occurred in the case of along-the-wetland-length removal between the two working periods (Fig. 4c). Phosphorus removal showed a different trend. TP retention increased significantly in period B (52.9%) compared to period A (39.9%), and showed less variability, as the low standard deviation values and Fig. 3 indicate, while the minimum removal observed in period B (1.9%) was greater compared to the respective value in period A (48.4%) (Table 2). On the other hand, ortho-phosphate retention was reduced to 49.9% from 55.9% in period A, something also reported elsewhere [26]. This probably has to do
with the fact that uptake of free ortho-phosphates by macrophytes was lower after 3 years of operation of the unit, while other forms of phosphorus (organically bound, e.g. phosphorylated sugar, polyphosphates) are still being removed. However, removal rates for TP and ortho-phosphates are comparable. PO3 4 --P loading was increased during period B (0.12–0.22 g/d/m2) compared to period A (0.10–0.18 g/d/m2). Moreover, as the main phosphorus removal mechanism is adsorption to the substrate and precipitation, decreased adsorption capacity of the substrate under slightly acidic pH values (Table 1) may be responsible for the reduced PO3 4 --P removal. The along-thewetland-length removal did not significantly change during period B, as Fig. 4e indicates. 3.3. Effluent recirculation (FG-R unit) – overall performance As for the MG-C unit, the HRTs applied in the FG-R unit during period B were the same as in period A. Figure 3 contains the removal rates of various pollutants during period B, while Fig. 4 presents mean values of pollutant concentrations along the wetland units for both working periods A and B. Organic matter removal showed a slight decrease in period B (50% effluent recirculation).
TP (mg/L)
PO3 4 P ðmg=LÞ
ðNO 3 þNO2 Þ N ðmgN=LÞ
NHþ 4 N ðmg=LÞ
TKN (mg/L)
COD (mg/L)
BOD5 (mg/L)
361.2 52.4 282.0 507.0 581.7 49.3 500.0 700.0 65.3 5.9 52.1 77.0 39.2 3.53 31.3 46.2 356.2 439.0 2.4 1511.0 8.4 1.1 6.0 10.7 9.2 1.1 7.7 13.0
Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max
508.0 0.0 2068.0 3.7 2.7 0.0 9.8 5.5 3.2 0.0 13.8
47.6 37.9 8.5 170.0 73.8 47.5 0.0 186.4 26.6 13.7 7.8 53.2 21.8 13.4 0.0 51.8 265.0
55.2 33.2 16.7 100.0 39.9 34.4 48.4 100.0
86.8 12.0 49.1 97.6 87.3 8.0 66.1 100.0 58.4 22.4 16.7 88.3 43.2 35.6 20.3 100.0
33.1 0.0 138.9 1.3 1.6 0.0 7.3 2.1 2.5 0.0 8.5
44.7 30.9 8.5 169.0 68.7 38.2 0.0 178.8 12.2 9.8 0.8 37.8 8.0 8.9 0.0 32.2 14.6
FG-R Effluent
84.9 19.9 14.1 100.0 76.7 28.2 6.1 100.0
87.2 9.9 47.4 97.8 88.2 6.6 67.5 100.0 81.3 14.7 44.9 98.8 79.4 22.3 20.5 100.0
Removal (%)
187.3 0.0 755.8 8.1 4.2 2.3 29.4 10.6 4.9 5.9 31.9
349.4 74.6 195.0 631.0 458.4 83.9 227.5 652.8 51.3 22.6 12.3 94.1 25.1 11.7 8.4 45.1 161.8 371.7 0.0 2187.0 3.5 2.8 0.0 10.3 5.0 4.0 0.0 21.4
47.5 30.3 5.7 117.0 65.0 37.6 8.2 136.0 24.2 13.9 4.2 51.2 18.0 12.6 3.1 50.4 79.5
Influent MG-C Effluent
Removal (%)
Influent
MG-C Effluent
Period B
Period A
49.9 39.1 22.9 100.0 52.9 32.9 1.9 100.0
86.4 8.4 69.1 98.3 85.7 8.4 68.1 97.2 43.9 34.6 47.2 94.4 26.6 36.4 42.9 87.0
Removal (%)
267.0 0.0 1219.0 2.1 1.7 0.0 8.2 3.5 2.7 0.0 16.0
50.4 32.4 13.0 186.0 63.6 38.0 4.8 201.4 17.8 12.4 2.5 45.4 14.9 11.8 1.4 38.4 75.7
69.1 27.5 25.0 100.0 64.9 21.7 1.9 100.0
85.4 9.0 51.8 95.0 85.4 10.0 48.8 98.9 61.0 26.0 11.3 96.2 37.5 46.5 106.0 95.2
FG-R Removal Effluent (%)
Table 2 Overall influent and effluent concentration and removal efficiency statistics in both constructed wetlands for both working periods
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BOD5 and COD removal rates were both 85.4% in period B (87.2% and 88.2% in period A, respectively). Reese [14] observed a lower BOD5 removal (77.5%) in a HSF CW with 40% effluent recirculation. In
along-the-wetland-length removal (Fig. 4b), no significant variations occurred in period B, compared to period A. Major portion of organic matter was still removed in the first one-third of unit length.
MG-C
BOD5 removal (%)
100 90 80 70 60 50 40 30 20 10 0
14 d
Feb-06 Apr-06
(a)
FG-R
6d
May-06
Jul-06
Aug-06
8d
Oct-06 Nov-06 Jan-07
Feb-07
100 90
COD removal (%)
80 70 60 50 40
14 d
6d
8d
30 20 10
(b)
0 Feb-06 Apr-06
May-06 Jul-06 Aug-06
Oct-06 Nov-06 Jan-07
Feb-07
TKN removal (%)
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 (c) Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
Fig. 3. Continued.
970
NH4+–N removal (%)
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TP removal (%)
(d)
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
−3
PO4 –P removal (%)
(e)
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
(f)
Fig. 3. Variation of removal rates during period B (HRT indicated) for: (a) BOD5; (b) COD; (c) TKN; (d) NHþ 4 --N; --P. (e) TP; and (f) PO3 4
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Period A − BOD5 Period B − BOD5 Period A − COD Period B − COD
In
(a)
70
1/3
2/3
Period A − TKN Period B − TKN Period A − NH3 Period B − NH3
MG-C
60
Nitrogen (mg/L)
Out
50 40 30 20
Organic matter (mg/L)
MG-C
600 550 500 450 400 350 300 250 200 150 100 50 0
1/3
2/3
Out
FG-R Period A – TKN Period B − TKN Period A − NH3 Period B − NH3
60 50 40 30 20
0
In
1/3
2/3
Period A − TP Period B − TP Period A − PO4 Period B − PO4
MG-C
11 10 9 8 7 6 5 4 3 2 1 0 In
1/3
Out
2/3
Out
In
(d)
Phophorous (mg/L)
Phophorous (mg/L)
In
10
0
(e)
Period A − BOD5 Period B − BOD5 Period A − COD Period B − COD
70
10
(c)
FG-R
600 550 500 450 400 350 300 250 200 150 100 0
(b)
Nitrogen (mg/L)
Organic matter (mg/L)
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976
(f)
11 10 9 8 7 6 5 4 3 2 1 0
1/3
2/3
Out
FG-R Period A − TP Period B − TP Period A − PO4 Period B − PO4
In
1/3
2/3
Out
Fig. 4. Mean constituent concentrations during both working periods (A and B) along MG-C and FG-R units for organic matter (a) and (b); nitrogen (c) and (d); and phosphorus (e) and (f).
Effluent recirculation did not enhance nutrient removal during period B. TKN and NHþ 4 --N removals were 61.0% and 37.5%, respectively, which were lower compared to those of period A (81.3% and 79.4%, respectively) without effluent recirculation. Although influent BOD5 was diluted with effluent recirculation, oxygen demands for organic matter decomposition
remained higher than those for nitrification, thus limiting nitrogen removal, as also reported by Reese [14]. However, these rates are higher than total nitrogen removal (20%) observed by Reese [14]. Small changes occurred in nitrogen removal in the last one-third of the unit length (Fig. 4d). Lower removals were also observed for phosphorus. TP and PO3 4 --P removal rates were
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64.9% and 69.1% in period B, respectively, with respective numbers 76.7% and 84.9% in period A. These rates are also higher, when compared with results obtained from Reese [14], who found only 40% removal of TP. As mentioned, phosphorus retention is mainly a result of adsorption on substrate media, plant consumption and chemical reactions between inorganic phosphorus and mineral components. These inorganic reactions are usually rapid processes, thus longer contact time between wastewater and substrate achieved with effluent recirculation seems not to affect phosphorus removal mechanisms [13]. As Fig. 4f depicts, phosphorus removal along the unit length did not show any important variations in period B, compared to period A. Generally, the results do not support the idea that effluent recirculation can improve constituent removal rates. The concept of effluent recirculation is that influent BOD concentration is diluted. Despite the dilution of influent BOD, oxygen demand for BOD decomposition stands still against nitrification demand. In this research, high influent BOD5 concentrations (349.4 mg/L; Table 2) did not allow for the process of nitrification to be enhanced, since most of available oxygen concentration was used for BOD decay. Additionally, the resulting increased hydraulic load due to effluent recirculation probably caused problems in the operation of the wetland system and limited its treatment performance, as also reported in the literature [13, 16]. It should also be taken into account that the wetland unit was operating for 2 years (period A) and had already reached high levels of efficiency, before effluent recirculation was adopted. All these factors contributed to the fact that effluent recirculation for nitrification enhancement by diluting influent BOD practically does not take place. 3.4. Effect of temperature on removal efficiency In general, lower efficiencies for all pollutants corresponded to lower temperatures and
the opposite. Figure 5 presents correlation charts of pollutant removals with wastewater temperatures. For organic pollutants (BOD5 and COD), the temperature dependence is not so significant [27], as Fig. 5a,b shows, which implies that organic matter removal is mostly a result of the activity of aerobic and anaerobic bacteria [5, 28, 29] which function even in temperatures as low as 58C. Porous media and plant roots keep the wastewater temperature in the winter higher than the air temperature by 2–38C (Fig. 2d), allowing thus the microbial activity to continue functioning [30]. In case of nitrogen removal, the dependence on temperature is much greater, and removal rates are higher in summer (Fig. 5c,d). This was expected, since the minimum temperature needed for nitrifying bacteria (Nitrosomonas and Nitrobacter) to grow is 4–58C [31], while the optimum temperature for nitrification ranges from 308C to 408C [32, 33]. Moreover, plant uptake for growth is greater during spring and summer months [34]. Phosphorus removal shows dependence on temperature too. Main phosphorus removal mechanisms – such as adsorption, plant uptake and microbial consumption – are not directly affected by temperature [1, 5]. Plant uptake through roots is higher in spring for growth and continues to summer months, when it reaches an optimum value [33]. For seasonal variations and negative values, the decomposition of microbial and plant biomass may be responsible during winter, resulting in pore fractionation, deeper water flow and decreased contact with decomposed organic matter (litter) on substrate surface [35]. Thus, phosphorus release takes place from the sediments back to the wetland system, due to substrate saturation [1, 36]. Table 3 contains pollutant removal statistics for temperatures below and above 158C during periods A and B for both wetland units. This temperature value was selected because below it neither the bacteria responsible for nitrogen removal nor the vegetation functions properly
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(a)
0
5
Linear FG-R
10
15
20
25
30
0
5
25
30
25
30
10 5 15 20 25 Wastewater temperature (°C)
30
10
15
20
Wastewater temperature (°C)
(b)
100 90 80 70 60 50 40 30 20 10 0
100 90 80 70 60 50 40 30 20 10 0
0
5
10 15 20 25 Wastewater temperature (°C)
30
0 (d)
–3
PO4 –P removal (%)
100 90 80 70 60 50 40 30 20 10 0
0 (e)
100 90 80 70 60 50 40 30 20 10 0
Wastewater temperature (°C)
(c)
TP removal (%)
Linear MG-C
NH4+ –N removal (%)
TKN removal (%)
FG-R
COD removal (%)
BOD5 removal (%)
MG-C 100 90 80 70 60 50 40 30 20 10 0
973
5
10 15 20 25 Wastewater temperature (°C)
(f)
10
15
20
Wastewater temperature (°C) 100 90 80 70 60 50 40 30 20 10 0
0
30
5
Fig. 5. Temperature and removal correlation charts for both wetland units during period B for (a) BOD5; (b) COD; 3 (c) TKN; (d) NHþ 4 --N; (e) TP; and (f) PO4 --P.
[32, 37]. Results of Table 3 reinforce previous remarks derived in Fig. 5. For BOD5 and COD, the mean removal rates for temperatures above 158C decreased during period B by 5–6% compared to period A, although they remained still satisfactory around
86% for both wetland units. For temperatures below 158C, the COD removal in the MG-C unit was slightly enhanced in period B and reached 86.1%, while in the FG-R unit remained at the same level (85.2%). BOD5 shows a small decrease in both units (Table 3). However, it
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Table 3 Removal efficiencies and statistics for temperatures below and above 158C for both wetland units, during both working periods Period A
BOD5 (%)
COD (%)
TKN (%)
NHþ 4 N ð%Þ
PO3 4 P ð%Þ
TP (%)
Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max
< 158C MG-C 88.0 29.7 49.1 97.6 84.4 31.8 66.1 100.0 52.4 29.5 16.7 88.3 28.7 34.0 20.3 89.5 37.6 30.7 16.7 89.3 23.2 29.3 48.4 86.4
Period B FG-R 86.8 29.3 47.4 95.5 85.2 31.6 67.5 97.7 71.7 23.2 44.6 94.7 63.9 25.0 11.3 97.1 73.4 22.1 14.1 100.0 59.6 30.7 6.1 100.0
> 158C MG-C 91.0 29.6 70.0 97.1 92.4 30.7 79.0 97.6 77.5 23.0 37.7 88.0 66.6 28.3 4.9 100.0 74.4 29.1 1.2 100.0 54.5 38.4 12.0 100.0
should be mentioned that the standard deviation values were significantly decreased during period B for both wetland units, which implies a more stable behavior in organic matter removal. With regard to nitrogen retention, temperature plays an important role, affecting microbial activity and vegetation. Higher removals were observed for higher temperatures (Table 3). However, comparison for temperatures below and above 158C and for both working periods shows that removal rates for both nitrogen pollutants (TKN and ammonia nitrogen) are significantly lower, especially in the FG-R unit; this implies that the nitrification rate was limited
FG-R 91.1 30.9 80.3 97.8 91.6 31.8 84.1 100.0 85.1 32.3 44.9 98.8 91.3 15.2 32.7 100.0 94.6 8.9 76.6 100.0 95.4 7.7 75.0 100.0
< 158C MG-C 86.2 8.7 73.7 98.3 86.1 8.3 70.2 97.2 39.1 36.2 47.2 84.4 17.4 30.9 35.8 78.4 30.1 32.2 22.9 86.1 37.0 26.2 1.9 82.7
FG-R 85.5 8.5 69.7 95.0 85.4 10.1 64.5 98.9 49.9 26.5 11.3 93.4 21.1 41.2 62.3 84.8 54.0 29.4 25.0 94.1 52.1 21.2 1.9 89.9
> 158C MG-C 86.7 8.3 69.1 96.4 85.4 8.8 68.1 95.3 49.2 32.9 28.0 94.4 36.8 40.2 42.9 87.0 72.1 34.7 10.1 100.0 70.6 31.0 7.1 100.0
FG-R 85.1 9.6 51.8 93.0 85.4 10.2 48.8 93.4 73.4 19.5 28.9 96.2 55.8 46.3 106.0 95.2 85.9 10.9 69.1 100.0 79.2 10.7 65.0 100.0
during period B. It seems that the available oxygen was mainly consumed for organic matter decomposition and partially for nitrification. TP and PO3 4 --P had a similar to nitrogen dependence on temperature. For the MG-C unit, the removal rates for TP improved during period B, while ortho-phosphate retention did not significantly change. On the other hand, phosphorus retention in the FG-R unit dropped significantly during period B. Plant decomposition and return of phosphorus in organic form in the wetland system, in addition to release from the precipitates [1], are possibly responsible for these alterations. However, it should be mentioned that the wetland
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units are in their third operational year, which might have affected plant function and substrate saturation. 4. Statistical analysis In order to investigate statistically how different the mean removal efficiency values are, the t-student confidence interval for 95% probability was used. Mean removals were investigated in the two wetland units for both working periods. The confidence interval for the difference of a pair of mean values was calculated and the results are presented in Table 4, where the upper and lower limits of these intervals are presented. When the interval includes the zero value, the two mean values in comparison are not statistically different; otherwise the two mean values are statistically different. For the MG-C unit, there are no statistical differences for all pollutants. This indicates that there was no significant variation in CW performance between periods A and B, when water level was raised at the outlet, despite the significantly increased loadings applied during period B compared to period A. This shows a positive effect of the modification on removal efficiency. On the other hand, in the FG-R unit, statistical differences occurred between the two working periods for nitrogen pollutants (TKN and Table 4 The t-student confidence interval for the comparison of the mean values in the MG-C and FG-R units for periods A and B
BOD5 COD TKN NHþ 4 --N PO3 4 --P TP
Period A
Period B
MG-C (5.7, 5.5) (2.9, 6.1) (1.6, 30.6) (3.3, 36.5) (15.5, 26.1) (30.5, 4.5)
FG-R (3.3, 6.9) (1.8, 7.4) (8.6, 32)* (21.8, 62.0)* (4.0, 27.6) (2.8, 28.8)*
* indicates statistically different mean values.
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ammonia nitrogen) and ortho-phosphates. This confirms that effluent recirculation during period B affected negatively the wetland performance. Therefore, outlet water-level raising seems to have a positive effect, because the wetland performance did not change while it was treating (with the same surface area) increased hydraulic loading. On the other hand, effluent recirculation is not recommended in HSF wetlands. 5. Conclusions Water-level raising at the outlet and effluent recirculation were applied in two HSF CWs during their third year of operation. Results were compared with previous operation without these modifications. Outlet water-level raising did not change the performance of the wetland, since removal rates of nitrogen pollutants, orthophosphates and COD were actually similar or slightly decreased. BOD5 removal remained steady, while TP retention showed even an improvement. However, under this configuration the wetland treated (without taking additional area) increased influent by 15%, increased organic load by 14%, increased nitrogen load by 18% and increased phosphorus load by 17%. Therefore, this modification is positive for wetland performance. Effluent recirculation at a rate of 50% seems to have negatively affected the performance of the wetland. Despite the influent BOD dilution, major portion of available oxygen is being used for BOD decomposition than for nitrification. Furthermore, the increased hydraulic load due to this modification is possibly responsible for limitation of wetland removal efficiency, since all pollutant removal rates were reduced after recirculation application, especially those of nutrients (nitrogen and phosphorus). References [1] R. Kadlec and R. Knight (Eds), Treatment Wetlands, CRC Press, 1996. [2] S. Cole, Environ. Sci. Technol. 32 (1998) 218–223.
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[3] Q. He and K. Mankin, J. Amer. Wat. Res. Assoc. 38 (2002) 1679–1689. [4] C. Akratos and V.A. Tsihrintzis, Ecol. Eng. 29 (2007) 173–191. [5] J. Vymazal, Ecol. Eng. 18 (2002) 633–646. [6] D.P.L. Rousseau, P.A. Vanrolleghem and N. De Pauw, Ecol. Eng. 23 (2004) 151–163. [7] H. Brix, H.-H. Schierup and C.A. Arias, Water Sci. Technol. 56 (3) (2007) 63–68. [8] EPA, Wastewater Technology Fact Sheet, Wetlands: Subsurface Flow, Office of Water, Washington DC, September 2000. [9] J. Vymazal, Ecol. Eng. 25 (2005) 478–490. [10] J. Vymazal, M. Greenway, K. Tonderski, H. Brix and U. Mander, Constructed Wetlands for Wastewater Treatment, Wetlands and Natural Resource Management, Springer-Verlag, Berlin, Heidelberg, 2006. [11] EPA, A Handbook of Constructed Wetlands – Volume 1: General Considerations, USEPA-Region III with USDA, NRCS, 1995. [12] C. Moreno, N. Farahbakhshazad and M.G. Morrison, Water Air Soil Poll. 135 (2002) 237–247. [13] G. Sun, K.R. Gray, A.J. Biddlestone, J.S. Allen and J.D. Cooper, Process Biochem. 39 (2003) 351–357. [14] C.S. Reese, Recirculation Effects on Total Nitrogen and Total Phosphorus Removal in a FullScale Subsurface Flow Constructed Wetland, PhD Thesis, Department of Civil and Environmental Engineering of the College of Engineering, University of Cincinnati, USA, 2005. [15] H. Lian-sheng, L. Hong-Liang, X. Bei-dou and Z. Ying-bo, Water Sci. Technol. 54(11–12) (2006) 137–149. [16] Y.Q. Zhao, G. Sun and J.S. Allen, Sci. Total Environ. 330 (2004) 1–8. [17] J. Laber, R. Perfler and R. Haberl, Water Sci. Technol. 35(5) (1997) 71–77. [18] M. Sundaravadivel and S. Vigneswara, Environ. Sci. Technol. 31(4) (2001) 351–409. [19] EPA, Design Manual – Constructed Wetlands and Aquatic Plant Systems for Municipal Wastewater Treatment, Office of Research and Development, Center for Environmental Research Information, Cincinnati, Ohio, September 1988. [20] C.C. Tanner, J. D’ Eugenio, B.G. McBride, P.S.J. Sukias and K. Thompson, Ecol. Eng. 12 (1999) 67–92. [21] T. Zhu, P.D. Jenssen, T. Maehlum and T. Krogstad, Water Sci. Technol. 35(5) (1997) 103–108.
[22] A. Drizo, A.C. Frost, J. Grace and A.K. Smith, Water Res. 33(17) (1999) 3595–3602. [23] Metcalf & Eddy Inc., Wastewater Engineering: Treatment and Reuse, fourth edition, Tchobanoglous G., Burton F.L. and Stensel D.H. (rev.), McGraw-Hill, New York, 2003. [24] APHA, AWWA (American Public Health Association, American Water Works Association), Standard Methods for the Examination of Water and Wastewater, (twientieth edition), Washington, DC, 1998. [25] R.K. Hench, K.G. Bissonnette, J.A. Sexstone, G.J. Coleman, K. Garbutt and G.J. Skousen, Water Res. 37 (2003) 921–927. [26] J. Garcı´a, P. Aguirre, J. Barraga´n, R. Mujeriego, V. Matamoros and M.J. Mayona, Ecol. Eng. 25 (2005) 405–418. [27] C.R. Steinmann, S. Weinhart and A. Melzer, Water Res. 37 (2003) 2035–2042. [28] M. Greenway and A. Woolley, Ecol. Eng. 12 (1999) 39–55. [29] D. Steer, L. Fraser, J. Boddy and B. Seibert, Ecol. Eng. 18 (2002) 429–440. [30] P. Hiley, Performance of wastewater treatment and € Mander and P. nutrient removal wetlands, In U. Jenssen (eds), Constructed Wetlands for Wastewater Treatment in Cold Climates, (Reedbeds) in Cold Temperature Climates, WIT Press, Southampton, 2003, pp. 1–18. [31] P.F. Cooper, G.D. Job, M.B. Green and R.B.E. Shutes, Reed Beds and Constructed Wetlands for Wastewater Treatment, Medmenham, Marlow, UK, WRc Publications, 1996, p. 184. [32] P. Kuschk, A. Wiebner, U. Kappelmeyer, E. Weibbrodt, M. Kastner and U. Stottmeister, Water Res. 37 (2003) 4236–4242. [33] J. Vymazal, Sci. Total Environ. (2006) doi: 10.1016/j.scitotenv.2006.09.014. [34] T.A.J. Verhoeven and F.M.A. Meuleman, Ecol. Eng. 12 (1999) 5–12. [35] J. Kyambadde, F. Kansiim, L. Gumaelius and G. Dalhammar, Water Res. 38 (2004) 475–485. [36] C. Vohla, E. P€ oldvere, A. Noorvee, V. Kuusemets € Mander, J. Environ. Sci. Heal A 40 (2006) and U. 1251–1264. [37] J. Vymazal (ed.), Nitrogen removal in constructed wetlands with horizontal sub-surface flow – can we determine the key process? In: Nutrient Cycling and Retention in Natural and Constructed Wetlands, Backhuys Publishers, Leiden, 1999, pp. 1–17.
Effect of outlet water level raising and effluent recirculation on removal efficiency of pilot-scale, horizontal subsurface flow constructed wetlands Alexandros I. Stefanakis, Vassilios A. Tsihrintzis* Laboratory of Ecological Engineering and Technology, Department of Environmental Engineering, School of Engineering, Democritus University of Thrace, 67100 Xanthi, Greece Tel/Fax: +30-25410-79393, 78113; email: [email protected] Received 25 June 2008; accepted 27 August 2008
Abstract Two pilot-scale, horizontal subsurface flow (HSF) constructed wetlands (CWs) were constructed and operated continuously for 3 years, fed with synthetic wastewater. Both wetlands were of rectangular cross-section with dimensions 3.0 m in length and 0.75 m in width. After the first 2 years of operation (period A), two modifications were applied to the wetland units: (1) the outlet water level was raised in the first unit and (2) 50% effluent recirculation was applied to the second unit. Under these two modifications, the two CWs operated for one more year (period B). Water samples were collected every week from the influent, effluent and sampling pipes placed at onethird and two-thirds distance along each unit. The water samples were analysed immediately in the laboratory for BOD5, COD, Total Kjedlahl Nitrogen (TKN), ammonia nitrogen, nitrite, nitrate, total phosphorus (TP) and orthophosphates. The results show that outlet water level raising did not change significantly the performance of the wetland unit, but under this configuration the wetland managed to treat about 15% increased hydraulic and pollutant loads using the same surface area. On the other hand, effluent recirculation negatively affected wetland performance, resulting in a reduction of all pollutant removal rates. Keywords: Constructed wetlands; Horizontal subsurface flow; Effluent recirculation; Water level raising; Temperature; HRT; Organic matter; Nitrogen; Phosphorus
1. Introduction and background Constructed Wetlands (CWs) can be an economic, cost-effective and technically feasible *Corresponding author. Presented at the 2nd Conference on Small and Decentralized Water and Wastewater Treatment Plants (SWAT), Skiathos Island, Greece, May 2–4, 2008
solution to the problem of wastewater treatment [1–4]. The use of these systems has become very popular in many countries, especially in Europe and North America [5–7]. Horizontal subsurface flow (HSF) CWs are commonly used systems in Europe and in the United States [8–10]. The main advantages of these systems include great tolerance in cold climates and in high pollutant
0011-9164/09/$– See front matter © 2009 Published by Elsevier B.V. doi:10.1016/j.desal.2008.08.008
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loads, minimization of odor and mosquito problems, and elimination of public health risk of contact with the treated wastewater [8, 11]. The HSF CWs are a reliable technology for elimination of organic pollutants (BOD5 and COD) and suspended solids; the removal of nutrients is generally more limited, because of a general lack of the necessary oxygen level to oxidize ammonium and of low sorption capacity of common substrate materials used for phosphorus retention [8, 10]. Today, research on these systems is targeting on optimizing and enhancing their performance. Two ways to do this are examined here: (1) partial effluent recirculation and (2) outlet water level raising above the porous media. Effluent recirculation has been proposed by various authors [12–15] as an operational modification to increase the treatment efficiency of CWs. The concept of this alternative method consists of taking part of the effluent and transferring it back to the inflow of the system. The theoretical hypothesis of effluent recirculation is that dilution of influent BOD5 and TSS through effluent recirculation may improve the treatment efficiency of the wetland. The main goal is to enhance aerobic microbial activity through intense interactions between pollutants and micro-organisms, which are close to the plant roots and onto the substrate surface, without significant alterations in system operation [16]. On the other hand, effluent recirculation may cause problems in HSF systems due to the increased hydraulic load, while in vertical flow systems with high hydraulic conductivity values it is suggested as an easy, applicable and effective method [12, 13, 17]. However, in fullscale operating facilities, this modification may increase operation costs, because of additional energy consumption for pumping. Water-level management plays a significant role in ecological functions occurring in wetland systems [11, 18] and is an important factor for the maintenance of wetland vegetation. Cattails grow well in submerged soils and can even dom-
inate where water depth is over 150 mm [19]. Control of outlet water level is also important for winter ice conditions, to prevent ice damage. Furthermore, water provides the environment for biochemical reactions to take place, and acts as a transport medium for oxygen. Waterlevel fluctuations in CWs have been suggested to physically increase aeration, thus enhancing microbial consumption of chemical oxygen demand and ammonia nitrogen [20]. In wetlands with control over inflow and outflow water, the flood level within the wetland will have to be actively and carefully managed. It is suggested that during the first growth season of the plants, water level is maintained where it just saturates the substrate between watering. As the plants grow in height, they have an increased ability to transport oxygen to the root zone. This allows the water level to be raised accordingly [18]. Water-level raising at the outlet on a subsurface flow CW results in the creation of a hybrid system, thus having an HSF CW with a free water surface (FWS) CW on top. The current study aims at examining two modifications applied on HSF CWs: first, to evaluate the effect of partial effluent recirculation on HSF wetland efficiency; and second, to observe if outlet water-level raising would still provide proper treatment in cases of an inflow increase, that is treatment of increased flow and loadings.
2. Materials and methods 2.1. Pilot-scale unit description Two similar pilot-scale HSF CWs were constructed and operated for 3 years, as part of a larger experiment containing five similar units [4]. Each wetland unit consists of a rectangular tank of dimensions 3.0 m in length, 0.75 m in width and 1 m in depth. A view of each wetland and a schematic section of the experimental layout are shown in Fig. 1. The first unit contained medium gravel (MG, d50 = 24.4 mm, range
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1.00 m
1.00 m
1.00 m
Effluent pipe
Diffuser
0.45 cm
963
Porous media
Plastic tank
Sample pipe 3.00 m (c)
Fig. 1. Photos of the two constructed wetland units taken in early winter of 2007: (a) MG-C; (b) FG-R; and (c) schematic section along each wetland unit.
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4–25 mm) obtained from a quarry, and was planted with cattails (Typha latifolia) (MG-C unit). Medium gravel is a carbonate rock (main elements: Si 3.39%; Al 0.90%; Fe 0.82%; Ca 27.20%; mg 4.53%; and P 0.03%). The other unit contained fine gravel (FG, d50 = 6.0 mm, range 0.25–16.0 mm) obtained from a river bed in the area, and was planted with common reeds (Phragmites australis) (FG-R unit). Fine gravel is an igneous rock (Si 28.50%; Al 7.95%; Fe 4.22%; Ca 3.62%; mg 1.76%; and P 0.11%). The river bed material is rich in Al, Fe and Ca, which are the three main elements responsible for phosphorus adsorption [21, 22]. On the contrary, the quarry material is only rich in Ca and is relatively poor in Al and Fe. The thickness of the porous media in both units was 0.45 m. The plants were collected from watercourses in the vicinity of the laboratory. The wetland units were equipped with inlet and outlet hydraulic structures, similar to those used in real systems. The effluent of each wetland unit was collected in a plastic tank placed at the unit outlet. The two wetland units operated initially for 2 years (period A), as described by Akratos and Tsihrintzis [4]. At the beginning of the third working year, wetland operation was changed. The outlet water level in the MG-C unit was raised from 45 to 60 cm, allowing for water to pond above the porous media at a mean depth of approximately 10 cm, thus creating a hybrid HSF-FWS system. Moreover, effluent recirculation started in the FG-R unit at 50% of inflowing wastewater. With these alterations, both units operated for one more year (period B). Both CWs were fed with synthetic wastewater, which was designed and used to simulate to the best the characteristics of domestic wastewater. The synthetic wastewater contained organic substances like nitrogen and phosphorus. The organic substances used were peptone (200 mg/L, 40% of the organic loading) as protein source, cane sugar (200 mg/L, 40% of the
organic loading) as saccharase source and acetic acid as source of organic acids (50 mg/L, 20% of the organic loading) [23]. The source of nitrogen was urea (60 mg/L) and of phosphorus hydrogen potassium phosphate (K2HPO4, 10 mg/L PO3 4 -P). For trace elements, a fertilizer was used. The synthetic wastewater was introduced three times a day, every 8 hours, at the upstream end of the wetlands. Hydraulic residence times (HRT) applied for both periods A and B were 6, 8 and 14 days. The daily inflow was increased by about 15% in the MG-C unit to account for the volume increase and maintain the same HRTs. 2.2. Sampling and chemical analyses The experiments during period A lasted from January 2004 until January 2006; and during period B, with the modified configurations, from February 2006 until February 2007. Water samples were collected once a week in the morning, from the influent, effluent and sampling pipes located at one-third and two-thirds of distances along each unit. The samples were analysed in the laboratory for determination of BOD5, COD, TKN, ammonia, nitrite and nitrate, total phosphorus (TP) and ortho-phosphates (PO3 4 --P). For BOD5 determination, respirometric bottles were used; for COD, the open reflux method was employed; for TKN and ammonia, the titrimetric method; for phosphorus, the stannous chloride method; for nitrite, the colorimetric method; and for nitrate, the cadmium reduction method [24]. 3. Results and discussion 3.1. Physicochemical parameters Table 1 contains measured physicochemical parameters for both wetland units during periods A and B. In case of pH, no significant variations occurred along the wetland length. Along the MG-C unit, pH was more or less stable, with a mean value around 6.45. An increase was
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Table 1 Mean values of physicochemical parameters at various points in both wetland units for both working periods Parameter (mean)
MG-C
FG-R
Period A Conductivity (mS/cm) pH DO (mg/L)
Influent 500 6.9 4.30
1/3 790 6.4 0.40
2/3 741 6.6 0.19
Effluent 757 6.7 1.31
1/3 864 6.8 0.31
2/3 842 7.1 0.39
Effluent 935 6.7 1.47
Period B Conductivity (mS/cm) pH DO (mg/L)
500 6.9 4.30
738 6.4 0.04
695 6.4 0.06
736 6.6 0.47
847 6.7 0.12
821 7.2 0.17
921 6.8 0.46
observed at two-thirds of distance along the FG-R unit to drop again in the effluent, with a mean value 6.9. On the whole, pH values in both pilot-scale units showed the trend to be kept on neutral or slightly acidic range, probably because of interactions between the substrate and the biofilm [1]. These interactions may have resulted in release of salts from the solid media to the water, explaining the slight increase of conductivity along the units. Increased effluent conductivity values have also been observed in other studies [25], and can be attributed to enhanced evapotranspiration and to substrate movement by plant roots due to wind effect. Dissolved oxygen concentrations were practically null, indicating oxygen consumption by pollutants. Figure 2 presents the temporal variation of all physicochemical parameters during period B for both wetland units. For conductivity and pH, it is obvious that there are no significant variations during period B. For DO, seasonal variations occurred, with higher values during the winter, when oxygen solubility in water was higher, and lower values during the summer. The level of oxygen appears to be higher in the FG-R unit, which implies that effluent recirculation performs better in DO increase than water level raising. Furthermore, the fine gravel in the FG-R unit enhances the interactions between the substrate and the biofilm, compared to the medium gravel in the MG-C unit, resulting in higher release of
salts to the water, thus explaining the higher conductivity and pH values in the FG-R unit. Overall, statistics of influent and effluent concentrations and removal efficiencies for both pilot-scale units and for both working periods are presented in Table 2. 3.2. Outlet water-level raising (MG-C unit) – overall performance The HRTs applied in period B were the same as in period A. During period B, to account for the additional volume after the water-level raising at the outlet, the wastewater inflow rate was increased, thus maintaining the same HRTs. In period B, the flow range was 31.5–59 L/d and the mean surface organic loading for BOD5 was 4.9–9.3 g/m2/d, with respective numbers 24– 55 L/d and 3.8–8.8 g/m2/d in period A. Figure 3 contains charts of the removal rates of various pollutants during period B, while Fig. 4 presents mean values of pollutant concentrations along the two wetland units for both working periods A and B. Organic matter removal in the MG-C unit did not show any significant differences. Mean BOD5 removal was slightly higher in period B (86.4% compared to 86.8% in period A), while COD removal slightly decreased (85.7% and 87.3% in periods B and A, respectively) (Table 2). Low standard deviation values for both periods indicate the low variability of organic matter
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MG-C
1.4
14 d
6d
FG-R 8d
DO (mg/L)
1.2 1 0.8 0.6 0.4 0.2
(a)
0 Feb-06 Apr-06
May-06
Jul-06
Aug-06 Oct-06 Nov-06 Jan-07
Feb-07
7.2
14 d
6d
8d
7
pH
6.8 6.6 6.4 6.2 6 Feb-06 Apr-06
May-06
Jul-06
Aug-06 Oct-06 Nov-06 Jan-07
Feb-07
(b) 1000
14 d
6d
8d
Conductivity (µS/cm)
950 900 850 800 750 700 650 600 Feb-06 Apr-06
(c)
Fig. 2. Continued.
May-06
Jul-06
Aug-06 Oct-06 Nov-06 Jan-07
Feb-07
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976 35
Temperature (°C)
30
Air temperature 14 d
FG-R (effluent)
967
MG-C (effluent)
6d
8d
25 20 15 10 5 0 Feb-06 Apr-06
May-06
Jul-06
Aug-06
Oct-06 Nov-06
Jan-07 Feb-07
(d)
Fig. 2. Variations of effluent physicochemical parameters during period B (HRT indicated) for (a) DO; (b) pH; (c) conductivity; and (d) air and wastewater temperature.
removal rates, also seen in Fig. 3. From Fig. 4, it is obvious that for both working periods, the organic matter is mainly removed in the first one-third of the wetland length, with small differentiations (Fig. 4a). In the case of nitrogen removal, the performance of the MG-C unit was lower in period B. Mean TKN removal dropped to 43.9% in period B from 58.4% in period A, and ammonia nitrogen retention reached 26.6%, compared to 43.2% in period A (Table 2). This has also been reported elsewhere [25], and possibly has to do with the ageing of the plants, but also with the increased nitrogen loading (0.91–1.72 g/d/m2 in period B compared to 0.64–1.60 g/d/m2 in period A). No significant variations occurred in the case of along-the-wetland-length removal between the two working periods (Fig. 4c). Phosphorus removal showed a different trend. TP retention increased significantly in period B (52.9%) compared to period A (39.9%), and showed less variability, as the low standard deviation values and Fig. 3 indicate, while the minimum removal observed in period B (1.9%) was greater compared to the respective value in period A (48.4%) (Table 2). On the other hand, ortho-phosphate retention was reduced to 49.9% from 55.9% in period A, something also reported elsewhere [26]. This probably has to do
with the fact that uptake of free ortho-phosphates by macrophytes was lower after 3 years of operation of the unit, while other forms of phosphorus (organically bound, e.g. phosphorylated sugar, polyphosphates) are still being removed. However, removal rates for TP and ortho-phosphates are comparable. PO3 4 --P loading was increased during period B (0.12–0.22 g/d/m2) compared to period A (0.10–0.18 g/d/m2). Moreover, as the main phosphorus removal mechanism is adsorption to the substrate and precipitation, decreased adsorption capacity of the substrate under slightly acidic pH values (Table 1) may be responsible for the reduced PO3 4 --P removal. The along-thewetland-length removal did not significantly change during period B, as Fig. 4e indicates. 3.3. Effluent recirculation (FG-R unit) – overall performance As for the MG-C unit, the HRTs applied in the FG-R unit during period B were the same as in period A. Figure 3 contains the removal rates of various pollutants during period B, while Fig. 4 presents mean values of pollutant concentrations along the wetland units for both working periods A and B. Organic matter removal showed a slight decrease in period B (50% effluent recirculation).
TP (mg/L)
PO3 4 P ðmg=LÞ
ðNO 3 þNO2 Þ N ðmgN=LÞ
NHþ 4 N ðmg=LÞ
TKN (mg/L)
COD (mg/L)
BOD5 (mg/L)
361.2 52.4 282.0 507.0 581.7 49.3 500.0 700.0 65.3 5.9 52.1 77.0 39.2 3.53 31.3 46.2 356.2 439.0 2.4 1511.0 8.4 1.1 6.0 10.7 9.2 1.1 7.7 13.0
Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max
508.0 0.0 2068.0 3.7 2.7 0.0 9.8 5.5 3.2 0.0 13.8
47.6 37.9 8.5 170.0 73.8 47.5 0.0 186.4 26.6 13.7 7.8 53.2 21.8 13.4 0.0 51.8 265.0
55.2 33.2 16.7 100.0 39.9 34.4 48.4 100.0
86.8 12.0 49.1 97.6 87.3 8.0 66.1 100.0 58.4 22.4 16.7 88.3 43.2 35.6 20.3 100.0
33.1 0.0 138.9 1.3 1.6 0.0 7.3 2.1 2.5 0.0 8.5
44.7 30.9 8.5 169.0 68.7 38.2 0.0 178.8 12.2 9.8 0.8 37.8 8.0 8.9 0.0 32.2 14.6
FG-R Effluent
84.9 19.9 14.1 100.0 76.7 28.2 6.1 100.0
87.2 9.9 47.4 97.8 88.2 6.6 67.5 100.0 81.3 14.7 44.9 98.8 79.4 22.3 20.5 100.0
Removal (%)
187.3 0.0 755.8 8.1 4.2 2.3 29.4 10.6 4.9 5.9 31.9
349.4 74.6 195.0 631.0 458.4 83.9 227.5 652.8 51.3 22.6 12.3 94.1 25.1 11.7 8.4 45.1 161.8 371.7 0.0 2187.0 3.5 2.8 0.0 10.3 5.0 4.0 0.0 21.4
47.5 30.3 5.7 117.0 65.0 37.6 8.2 136.0 24.2 13.9 4.2 51.2 18.0 12.6 3.1 50.4 79.5
Influent MG-C Effluent
Removal (%)
Influent
MG-C Effluent
Period B
Period A
49.9 39.1 22.9 100.0 52.9 32.9 1.9 100.0
86.4 8.4 69.1 98.3 85.7 8.4 68.1 97.2 43.9 34.6 47.2 94.4 26.6 36.4 42.9 87.0
Removal (%)
267.0 0.0 1219.0 2.1 1.7 0.0 8.2 3.5 2.7 0.0 16.0
50.4 32.4 13.0 186.0 63.6 38.0 4.8 201.4 17.8 12.4 2.5 45.4 14.9 11.8 1.4 38.4 75.7
69.1 27.5 25.0 100.0 64.9 21.7 1.9 100.0
85.4 9.0 51.8 95.0 85.4 10.0 48.8 98.9 61.0 26.0 11.3 96.2 37.5 46.5 106.0 95.2
FG-R Removal Effluent (%)
Table 2 Overall influent and effluent concentration and removal efficiency statistics in both constructed wetlands for both working periods
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BOD5 and COD removal rates were both 85.4% in period B (87.2% and 88.2% in period A, respectively). Reese [14] observed a lower BOD5 removal (77.5%) in a HSF CW with 40% effluent recirculation. In
along-the-wetland-length removal (Fig. 4b), no significant variations occurred in period B, compared to period A. Major portion of organic matter was still removed in the first one-third of unit length.
MG-C
BOD5 removal (%)
100 90 80 70 60 50 40 30 20 10 0
14 d
Feb-06 Apr-06
(a)
FG-R
6d
May-06
Jul-06
Aug-06
8d
Oct-06 Nov-06 Jan-07
Feb-07
100 90
COD removal (%)
80 70 60 50 40
14 d
6d
8d
30 20 10
(b)
0 Feb-06 Apr-06
May-06 Jul-06 Aug-06
Oct-06 Nov-06 Jan-07
Feb-07
TKN removal (%)
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 (c) Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
Fig. 3. Continued.
970
NH4+–N removal (%)
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976
TP removal (%)
(d)
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
−3
PO4 –P removal (%)
(e)
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
14 d 6d 8d 100 90 80 70 60 50 40 30 20 10 0 –10 Feb-06 Apr-06 May-06 Jul-06 Aug-06 Oct-06 Nov-06 Jan-07 Feb-07
(f)
Fig. 3. Variation of removal rates during period B (HRT indicated) for: (a) BOD5; (b) COD; (c) TKN; (d) NHþ 4 --N; --P. (e) TP; and (f) PO3 4
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Period A − BOD5 Period B − BOD5 Period A − COD Period B − COD
In
(a)
70
1/3
2/3
Period A − TKN Period B − TKN Period A − NH3 Period B − NH3
MG-C
60
Nitrogen (mg/L)
Out
50 40 30 20
Organic matter (mg/L)
MG-C
600 550 500 450 400 350 300 250 200 150 100 50 0
1/3
2/3
Out
FG-R Period A – TKN Period B − TKN Period A − NH3 Period B − NH3
60 50 40 30 20
0
In
1/3
2/3
Period A − TP Period B − TP Period A − PO4 Period B − PO4
MG-C
11 10 9 8 7 6 5 4 3 2 1 0 In
1/3
Out
2/3
Out
In
(d)
Phophorous (mg/L)
Phophorous (mg/L)
In
10
0
(e)
Period A − BOD5 Period B − BOD5 Period A − COD Period B − COD
70
10
(c)
FG-R
600 550 500 450 400 350 300 250 200 150 100 0
(b)
Nitrogen (mg/L)
Organic matter (mg/L)
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976
(f)
11 10 9 8 7 6 5 4 3 2 1 0
1/3
2/3
Out
FG-R Period A − TP Period B − TP Period A − PO4 Period B − PO4
In
1/3
2/3
Out
Fig. 4. Mean constituent concentrations during both working periods (A and B) along MG-C and FG-R units for organic matter (a) and (b); nitrogen (c) and (d); and phosphorus (e) and (f).
Effluent recirculation did not enhance nutrient removal during period B. TKN and NHþ 4 --N removals were 61.0% and 37.5%, respectively, which were lower compared to those of period A (81.3% and 79.4%, respectively) without effluent recirculation. Although influent BOD5 was diluted with effluent recirculation, oxygen demands for organic matter decomposition
remained higher than those for nitrification, thus limiting nitrogen removal, as also reported by Reese [14]. However, these rates are higher than total nitrogen removal (20%) observed by Reese [14]. Small changes occurred in nitrogen removal in the last one-third of the unit length (Fig. 4d). Lower removals were also observed for phosphorus. TP and PO3 4 --P removal rates were
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64.9% and 69.1% in period B, respectively, with respective numbers 76.7% and 84.9% in period A. These rates are also higher, when compared with results obtained from Reese [14], who found only 40% removal of TP. As mentioned, phosphorus retention is mainly a result of adsorption on substrate media, plant consumption and chemical reactions between inorganic phosphorus and mineral components. These inorganic reactions are usually rapid processes, thus longer contact time between wastewater and substrate achieved with effluent recirculation seems not to affect phosphorus removal mechanisms [13]. As Fig. 4f depicts, phosphorus removal along the unit length did not show any important variations in period B, compared to period A. Generally, the results do not support the idea that effluent recirculation can improve constituent removal rates. The concept of effluent recirculation is that influent BOD concentration is diluted. Despite the dilution of influent BOD, oxygen demand for BOD decomposition stands still against nitrification demand. In this research, high influent BOD5 concentrations (349.4 mg/L; Table 2) did not allow for the process of nitrification to be enhanced, since most of available oxygen concentration was used for BOD decay. Additionally, the resulting increased hydraulic load due to effluent recirculation probably caused problems in the operation of the wetland system and limited its treatment performance, as also reported in the literature [13, 16]. It should also be taken into account that the wetland unit was operating for 2 years (period A) and had already reached high levels of efficiency, before effluent recirculation was adopted. All these factors contributed to the fact that effluent recirculation for nitrification enhancement by diluting influent BOD practically does not take place. 3.4. Effect of temperature on removal efficiency In general, lower efficiencies for all pollutants corresponded to lower temperatures and
the opposite. Figure 5 presents correlation charts of pollutant removals with wastewater temperatures. For organic pollutants (BOD5 and COD), the temperature dependence is not so significant [27], as Fig. 5a,b shows, which implies that organic matter removal is mostly a result of the activity of aerobic and anaerobic bacteria [5, 28, 29] which function even in temperatures as low as 58C. Porous media and plant roots keep the wastewater temperature in the winter higher than the air temperature by 2–38C (Fig. 2d), allowing thus the microbial activity to continue functioning [30]. In case of nitrogen removal, the dependence on temperature is much greater, and removal rates are higher in summer (Fig. 5c,d). This was expected, since the minimum temperature needed for nitrifying bacteria (Nitrosomonas and Nitrobacter) to grow is 4–58C [31], while the optimum temperature for nitrification ranges from 308C to 408C [32, 33]. Moreover, plant uptake for growth is greater during spring and summer months [34]. Phosphorus removal shows dependence on temperature too. Main phosphorus removal mechanisms – such as adsorption, plant uptake and microbial consumption – are not directly affected by temperature [1, 5]. Plant uptake through roots is higher in spring for growth and continues to summer months, when it reaches an optimum value [33]. For seasonal variations and negative values, the decomposition of microbial and plant biomass may be responsible during winter, resulting in pore fractionation, deeper water flow and decreased contact with decomposed organic matter (litter) on substrate surface [35]. Thus, phosphorus release takes place from the sediments back to the wetland system, due to substrate saturation [1, 36]. Table 3 contains pollutant removal statistics for temperatures below and above 158C during periods A and B for both wetland units. This temperature value was selected because below it neither the bacteria responsible for nitrogen removal nor the vegetation functions properly
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976
(a)
0
5
Linear FG-R
10
15
20
25
30
0
5
25
30
25
30
10 5 15 20 25 Wastewater temperature (°C)
30
10
15
20
Wastewater temperature (°C)
(b)
100 90 80 70 60 50 40 30 20 10 0
100 90 80 70 60 50 40 30 20 10 0
0
5
10 15 20 25 Wastewater temperature (°C)
30
0 (d)
–3
PO4 –P removal (%)
100 90 80 70 60 50 40 30 20 10 0
0 (e)
100 90 80 70 60 50 40 30 20 10 0
Wastewater temperature (°C)
(c)
TP removal (%)
Linear MG-C
NH4+ –N removal (%)
TKN removal (%)
FG-R
COD removal (%)
BOD5 removal (%)
MG-C 100 90 80 70 60 50 40 30 20 10 0
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5
10 15 20 25 Wastewater temperature (°C)
(f)
10
15
20
Wastewater temperature (°C) 100 90 80 70 60 50 40 30 20 10 0
0
30
5
Fig. 5. Temperature and removal correlation charts for both wetland units during period B for (a) BOD5; (b) COD; 3 (c) TKN; (d) NHþ 4 --N; (e) TP; and (f) PO4 --P.
[32, 37]. Results of Table 3 reinforce previous remarks derived in Fig. 5. For BOD5 and COD, the mean removal rates for temperatures above 158C decreased during period B by 5–6% compared to period A, although they remained still satisfactory around
86% for both wetland units. For temperatures below 158C, the COD removal in the MG-C unit was slightly enhanced in period B and reached 86.1%, while in the FG-R unit remained at the same level (85.2%). BOD5 shows a small decrease in both units (Table 3). However, it
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Table 3 Removal efficiencies and statistics for temperatures below and above 158C for both wetland units, during both working periods Period A
BOD5 (%)
COD (%)
TKN (%)
NHþ 4 N ð%Þ
PO3 4 P ð%Þ
TP (%)
Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max Mean SD min max
< 158C MG-C 88.0 29.7 49.1 97.6 84.4 31.8 66.1 100.0 52.4 29.5 16.7 88.3 28.7 34.0 20.3 89.5 37.6 30.7 16.7 89.3 23.2 29.3 48.4 86.4
Period B FG-R 86.8 29.3 47.4 95.5 85.2 31.6 67.5 97.7 71.7 23.2 44.6 94.7 63.9 25.0 11.3 97.1 73.4 22.1 14.1 100.0 59.6 30.7 6.1 100.0
> 158C MG-C 91.0 29.6 70.0 97.1 92.4 30.7 79.0 97.6 77.5 23.0 37.7 88.0 66.6 28.3 4.9 100.0 74.4 29.1 1.2 100.0 54.5 38.4 12.0 100.0
should be mentioned that the standard deviation values were significantly decreased during period B for both wetland units, which implies a more stable behavior in organic matter removal. With regard to nitrogen retention, temperature plays an important role, affecting microbial activity and vegetation. Higher removals were observed for higher temperatures (Table 3). However, comparison for temperatures below and above 158C and for both working periods shows that removal rates for both nitrogen pollutants (TKN and ammonia nitrogen) are significantly lower, especially in the FG-R unit; this implies that the nitrification rate was limited
FG-R 91.1 30.9 80.3 97.8 91.6 31.8 84.1 100.0 85.1 32.3 44.9 98.8 91.3 15.2 32.7 100.0 94.6 8.9 76.6 100.0 95.4 7.7 75.0 100.0
< 158C MG-C 86.2 8.7 73.7 98.3 86.1 8.3 70.2 97.2 39.1 36.2 47.2 84.4 17.4 30.9 35.8 78.4 30.1 32.2 22.9 86.1 37.0 26.2 1.9 82.7
FG-R 85.5 8.5 69.7 95.0 85.4 10.1 64.5 98.9 49.9 26.5 11.3 93.4 21.1 41.2 62.3 84.8 54.0 29.4 25.0 94.1 52.1 21.2 1.9 89.9
> 158C MG-C 86.7 8.3 69.1 96.4 85.4 8.8 68.1 95.3 49.2 32.9 28.0 94.4 36.8 40.2 42.9 87.0 72.1 34.7 10.1 100.0 70.6 31.0 7.1 100.0
FG-R 85.1 9.6 51.8 93.0 85.4 10.2 48.8 93.4 73.4 19.5 28.9 96.2 55.8 46.3 106.0 95.2 85.9 10.9 69.1 100.0 79.2 10.7 65.0 100.0
during period B. It seems that the available oxygen was mainly consumed for organic matter decomposition and partially for nitrification. TP and PO3 4 --P had a similar to nitrogen dependence on temperature. For the MG-C unit, the removal rates for TP improved during period B, while ortho-phosphate retention did not significantly change. On the other hand, phosphorus retention in the FG-R unit dropped significantly during period B. Plant decomposition and return of phosphorus in organic form in the wetland system, in addition to release from the precipitates [1], are possibly responsible for these alterations. However, it should be mentioned that the wetland
A.I. Stefanakis et al. / Desalination 248 (2009) 961–976
units are in their third operational year, which might have affected plant function and substrate saturation. 4. Statistical analysis In order to investigate statistically how different the mean removal efficiency values are, the t-student confidence interval for 95% probability was used. Mean removals were investigated in the two wetland units for both working periods. The confidence interval for the difference of a pair of mean values was calculated and the results are presented in Table 4, where the upper and lower limits of these intervals are presented. When the interval includes the zero value, the two mean values in comparison are not statistically different; otherwise the two mean values are statistically different. For the MG-C unit, there are no statistical differences for all pollutants. This indicates that there was no significant variation in CW performance between periods A and B, when water level was raised at the outlet, despite the significantly increased loadings applied during period B compared to period A. This shows a positive effect of the modification on removal efficiency. On the other hand, in the FG-R unit, statistical differences occurred between the two working periods for nitrogen pollutants (TKN and Table 4 The t-student confidence interval for the comparison of the mean values in the MG-C and FG-R units for periods A and B
BOD5 COD TKN NHþ 4 --N PO3 4 --P TP
Period A
Period B
MG-C (5.7, 5.5) (2.9, 6.1) (1.6, 30.6) (3.3, 36.5) (15.5, 26.1) (30.5, 4.5)
FG-R (3.3, 6.9) (1.8, 7.4) (8.6, 32)* (21.8, 62.0)* (4.0, 27.6) (2.8, 28.8)*
* indicates statistically different mean values.
975
ammonia nitrogen) and ortho-phosphates. This confirms that effluent recirculation during period B affected negatively the wetland performance. Therefore, outlet water-level raising seems to have a positive effect, because the wetland performance did not change while it was treating (with the same surface area) increased hydraulic loading. On the other hand, effluent recirculation is not recommended in HSF wetlands. 5. Conclusions Water-level raising at the outlet and effluent recirculation were applied in two HSF CWs during their third year of operation. Results were compared with previous operation without these modifications. Outlet water-level raising did not change the performance of the wetland, since removal rates of nitrogen pollutants, orthophosphates and COD were actually similar or slightly decreased. BOD5 removal remained steady, while TP retention showed even an improvement. However, under this configuration the wetland treated (without taking additional area) increased influent by 15%, increased organic load by 14%, increased nitrogen load by 18% and increased phosphorus load by 17%. Therefore, this modification is positive for wetland performance. Effluent recirculation at a rate of 50% seems to have negatively affected the performance of the wetland. Despite the influent BOD dilution, major portion of available oxygen is being used for BOD decomposition than for nitrification. Furthermore, the increased hydraulic load due to this modification is possibly responsible for limitation of wetland removal efficiency, since all pollutant removal rates were reduced after recirculation application, especially those of nutrients (nitrogen and phosphorus). References [1] R. Kadlec and R. Knight (Eds), Treatment Wetlands, CRC Press, 1996. [2] S. Cole, Environ. Sci. Technol. 32 (1998) 218–223.
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