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Contaminant removal efficiency depending on primary treatment and operational strategy in horizontal subsurface flow treatment wetlands
Ecological Engineering 37 (2011) 372–380
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Contaminant removal efficiency depending on primary treatment and operational strategy in horizontal subsurface flow treatment wetlands Anna Pedescoll, Angélica Corzo, Eduardo Álvarez, Jaume Puigagut, Joan García ∗ Environmental Engineering Division, Department of Hydraulic, Maritime and Environmental Engineering. Technical University of Catalonia, c/Jordi Girona 1-3, Building D1, 08034 Barcelona, Spain
a r t i c l e
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Article history: Received 3 August 2010 Received in revised form 1 December 2010 Accepted 10 December 2010 Available online 11 January 2011 Keywords: Constructed wetlands Reed beds Design Anaerobic digester Intermittent feeding
a b s t r a c t This study aimed to evaluate the contaminant removal efficiency of shallow horizontal subsurface flow treatment wetlands (SSF TWs) as a function of (1) primary treatment (hydrolytic upflow sludge blanket (HUSB) reactor vs. conventional settling) and (2) operation strategy (alternation of saturated/unsaturated phases vs. permanently saturated). An experimental plant was constructed, operated and surveyed for the main water quality parameters over a period of 2.5 years. The plant had 3 treatment lines: a control line (settler-wetland permanently saturated), a batch line (settler-wetland operated with saturated/unsaturated phases) and an anaerobic line (HUSB reactor-wetland permanently saturated). In each line wetlands had a surface area of 2.80 m2 , a water depth of 25 cm and a granular medium D60 = 7.3 mm, and were planted with common reed. During the study period the wetlands were operated at a hydraulic and organic load of 28.5 mm/d and about 4.7 g BOD/m2 d, respectively. Effluent average redox potential was lower for the anaerobic line (−45 ± 78 mV) than for the other two lines (3 ± 92.7 and −5 ± 71 mV for control and batch, respectively). Overall, chemical oxygen demand (COD), biochemical oxygen demand (BOD5 ) and ammonium mass removal efficiencies were slightly greater for the batch line (88%, 96% and 87%, respectively) than for the control line (83%, 94% and 80%) and the anaerobic line (80%, 87% and 73%). During cold seasons, COD and ammonium removal in the batch line was around 30% and 50% higher than in the control line, respectively. The results of this study indicate that the implementation of a HUSB reactor as primary treatment did not enhance the treatment capacity of the system (in comparison with a conventional settler). The efficiency of treatment wetland systems with horizontal subsurface flow can be improved using a batch operation strategy. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Subsurface flow treatment wetlands (SSF TWs) are extensive systems used for wastewater treatment that are particularly suitable for the sanitation of small communities (Rousseau et al., 2004; Puigagut et al., 2007; Vymazal and Kröpfelová, 2009). Contaminant removal efficiency attained by wetlands (such as ammonium removal) depends to a large extent on the redox status of the systems (Caselles-Osorio and García, 2007a). Accordingly, more oxidising conditions lead to greater contaminant removal efficiencies. Horizontal SSF TWs are complex bioreactors in which the removal of organic contaminants occurs mainly by means of anaerobic reactions and, to a lesser extent, by means of anoxic and
∗ Corresponding author. Tel.: +34 93 401 6464; fax: +34 93 401 7357. E-mail address: [email protected] (J. García). 0925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2010.12.011
aerobic pathways (Calheiros et al., 2009). Water depth is one of the main design factors of horizontal SSF TWs influencing the redox status of the system (Faulwetter et al., 2009). Accordingly, the lower the depth the higher the oxidising conditions within the bed (Singh et al., 2009). The higher efficiency of shallow horizontal SSF TWs (0.3 m) has been demonstrated for a wide variety of contaminants (Song et al., 2009; García et al., 2005; Matamoros et al., 2005; Huang et al., 2004). Overall, conventional horizontal SSF TWs with a water depth of about 0.5–0.7 m are considered to rarely have ammonium removal efficiencies above 40–55% because of their inability to provide enough oxidised conditions for nitrification (Austin and Nivala, 2009; Vymazal and Kröpfelová, 2009). Primary treatment is a previous key step to horizontal SSF TWs that is most often achieved using septic or Imhoff tanks (Knowles et al., 2011). However, other technologies, such as upflow anaerobic sludge blanket (UASB) reactors, have been recently investigated as a suitable primary treatment for SSF TWs, mainly because they can provide effluents with lower total suspended solids (TSS) and COD concentrations than standard primary treatments (Barros et al.,
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2008). Hydrolytic upflow sludge blanket (HUSB) reactors have been little investigated within the context of treatment wetland technology (Álvarez et al., 2008a,b). HUSB reactors are essentially UASB reactors operated at a lower hydraulic retention time (HRT) (from 2 to 5 h) in order to avoid methanogenesis reaction wherever possible. In general, solids retention time (SRT) in HUSB reactors is maintained for over 15 days in order to achieve high hydrolysis rates of wastewater solids. Field experience gained during the last few years has demonstrated that alternating unsaturated phases during the operation of SSF TWs is of capital importance to maintaining aerobic conditions within the wetlands (Molle et al., 2005). Accordingly, several studies have shown that wetlands operated under batch conditions perform better than those operated under continuous conditions (Caselles-Osorio and García, 2007a; Stein et al., 2003; Tanner et al., 1999). Higher contaminant removal efficiencies reported for batchoperated systems have also been linked to higher water level fluctuations (mediated by evapotranspiration) in comparison with continuous feeding systems. These fluctuations expose more granular medium to the atmosphere, thus promoting more oxidised conditions within the bed (Breen, 1997; Tanner et al., 1999) and with a positive effect on the removal of several contaminants, including COD and ammonium (Vymazal and Masa, 2003). This study aims to evaluate the contaminant removal efficiency of experimental shallow horizontal SSF TWs as a function of: (1) type of primary treatment (HUSB reactor or conventional settling) and (2) operating strategy (alternating batch-unsaturated phases and permanently saturated). For this purpose an experimental plant was constructed and operated over a period of two and half years after system start-up. To our knowledge, this is the first time that the effect of the type of primary treatment and operating strategy has been evaluated on rigorously controlled horizontal SSF TWs (in terms of hydraulic and organic loading rates) and over a sufficiently long period of time. Our aim was to obtain results useful for improving the design and operation of treatment wetlands.
2. Materials and methods 2.1. Experimental plant The plant used is set in the open at the experimental facility of the Department of Hydraulic, Maritime and Environmental Engineering of the Universitat Politècnica de Cataluya, Barcelona, Spain. Built in 2006, the plant became operational in February 2007 and treats urban wastewater pumped directly using 2 pumps from a nearby municipal sewer. Firstly, the wastewater is coarsely screened and subsequently stored in a 1.2 m3 plastic tank, which is continuously stirred in order to avoid sedimentation of solids. This tank is equipped with level buoys that control the operation of the feeding pumps. Accordingly, the pumps start automatically when the volume of wastewater in the tank is about 600 L and stop when the volume is about 1000 L. Wastewater retention time in this tank is approximately 12 h. From the storage tank, the wastewater is conveyed to 3 different treatment lines which, for ease of understanding, have been named batch, control and anaerobic lines (Fig. 1). Differences between treatment lines are related to the type of primary treatment and the operation strategy applied. The layout of the wetlands is the same in all three lines: three small wetlands in parallel (0.65 m2 each), two of them connected to a big wetland in series (1.65 m2 ) (Fig. 1). One of the three small wetlands was left unplanted and discharges directly to the sewer and, strictly speaking, does not belong to the treatment lines. These unplanted wetlands were constructed in order to study plant influence on clogging processes. However, clogging processes fall
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outside the scope of this paper and unplanted wetlands will therefore not be considered here. The two small parallel wetlands were necessary for the operation of the batch line. This system was also adopted in the other two lines for comparative purposes. These two small wetlands have a joint surface area (1.3 m2 ), which is approximately 45% of the total surface area of the treatment line (2.95 m2 ). Note that the appearance of clogging in this type of wetlands is more evident in the inlet zone of the wetlands (Caselles-Osorio and García, 2007b), and this is why the total surface of the wetland area was split in two (one big and two small wetlands), because this experimental set up was also designed to study clogging processes. Each of the three lines received a flow of 84 L/d (not including the unplanted wetlands). The wetlands were therefore operated at a hydraulic loading rate of 28.5 mm/d. The wetlands of each line were designed to have a maximum surface loading rate of approximately 6 g BOD/m2 d, as recommended by Kadlec and Knight (1996), García et al. (2005) or Akratos and Tsihrintzis (2007). 2.1.1. Primary treatment Batch and control lines have cylindrical PVC static settlers as primary treatment and are filled with screened wastewater pumped from the storage tank every 4 h. The control line has three settlers, one for each small wetland (planted and unplanted), with an internal diameter of 190 mm and an effective volume of 7 L (Fig. 2). The batch line has only two settlers because the small planted wetlands operate alternately. Settlers for the batch line have an internal diameter of 300 mm and an effective volume of 14 L (Fig. 2). In the control line, after 2 h of settling from each settler, 7 L of wastewater are discharged into the small wetlands (in total 14 L to the planted wetlands at each discharge). In the batch line, after 2 h of settling, 14 L of wastewater are discharged into one small wetland. Irrespective of the treatment line (control or batch), wastewater is discharged from settlers by means of electrovalves, while sludge wasting is realized by means of pumps that convey sludge back to the municipal sewer. The effect of differences in diameter and volume of the settlers between the control and the batch line was evaluated at the beginning of the study and there were no significant differences for TSS removal efficiency (results not shown). The anaerobic line has a cylindrical PVC HUSB reactor as primary treatment, which has an internal diameter of 300 mm, a total height of 1900 mm and an effective volume of 105 L (Fig. 2). The reactor is continuously fed with wastewater from the storage tank by means of a peristaltic pump that supplies a known flow. The HUSB reactor was operated at two different HRTs during the study period in order to assess optimum operational conditions. Specifically, the HUSB reactor was operated at 3 h of HRT from February 2007 to December 2008 and at 5 h of HRT from January 2009 to July 2009. The sludge blanket inside the HUSB reactor was kept as much as possible at a volatile solids (VS) concentration of <10 g/L by means of manual purges. HUSB reactor had 5 taps located in a vertical series starting from a height of 480 mm and each placed at a respective distance of 150 mm (Fig. 2). Overall, VS concentration was estimated every two weeks measuring concentrations at each tap. The minimum and maximum solids retention time (SRTmin and SRTmax ) were estimated according to the following equation (Álvarez et al., 2008b): SRTmax =
VXR QW XW
SRTmin =
VXR [(QW XW ) + (QXe )]
where V is the reactor volume (m3 ), XR is the weighted average biomass concentration in the reactor (kg VS/m3 ), Qw is the purged biomass flow rate (m3 /d), Xw is the concentration of purged
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Fig. 1. Schematic diagram of the experimental plant and side view of the batch line. The set-up of wetlands in the other lines was similar except for the presence of the electrovalve between big and small wetlands. The electrovalve in the batch line enabled draining of the small wetlands.
Fig. 2. Schematic side view of the primary treatments. Distances expressed in cm.
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Fig. 3. Schematic diagram of an operation cycle (4 days) of the batch line compared with the control and the anaerobic line (small wetlands). * The operation of one of the two small wetlands is shown for control and anaerobic lines only.
biomass (kg VS/m3 ), Q is the influent flow rate (m3 /d) and Xe is the effluent volatile solids concentration (kg VS/m3 ). Every 4 h the content of the upper part of the HUSB reactor was discharged into three distribution tanks (one per each small wetland) with an internal diameter of 300 mm and an effective volume of 7 L. Without time for particle settling (tanks were charged and discharged in 10 min), wastewater was discharged from these tanks into the small wetlands by means of electrovalves. This set-up ensured a flow of 7 L to each small wetland every 4 h. 2.1.2. Wetlands Big and small wetlands consist of plastic containers 1.5 m long, 1.1 m wide and 0.50 m high, and 0.95 m long, 0.70 m wide and 0.45 m high, respectively. Wastewater from the primary treatment was discharged by means of perforated pipes located along the width of the wetlands. Each container had a drainage pipe on the flat bottom for effluent discharge. The uniform gravel layer (D60 = 7.3 mm, Cu = 0.83, 40% initial porosity) was 0.3 m deep and the water level was kept 0.05 m below the gravel surface to give a water depth of 0.25 m. The theoretical hydraulic retention time (HRT) was 3.5 d per line. In April 2007 the wetlands were planted with developed rhizomes of common reed (Phragmites australis) and by July 2007 the plants were well-established and covered the entire surface of the wetlands. In anaerobic and control lines all the wetlands remained permanently saturated. The batch line, however, operated under a scheme of a four-day cycle and the small wetlands were not permanently saturated (Fig. 3). Accordingly, for the first two days of the cycle the small wetlands were fed in the same way as the control line, on the third day the wetland rested under saturated conditions but received no influent and on the fourth day the wetland was drained (by means of an electrovalve, see Fig. 1) and rested under unsaturated conditions. Note that the two small wetlands of the batch line are at different phases at any time (Fig. 3).
Effluent from each line was discharged into a tank (flow meter) with a water sensor level that was activated every 5 L of effluent. When this level was reached, the tank emptied. The total effluent volume was then calculated by multiplying the times that the sensor had been activated by 5 L. Flow measurements were recorded on a daily basis. 2.2. Samples and analyses In order to evaluate the removal efficiency of the experimental plant samples of the influent (storage tank), primary treatments (settlers and HUSB reactor) and effluent (big wetlands) were taken (always at 9:00 and 9:30 am) 3–4 times per month from April 2007 to July 2009. Samples were analysed for pH, redox potential, turbidity, COD, TSS, ammonium and sulphates. In addition, from October 2007 BOD5 , dissolved COD, nitrites, nitrates, TKN and total phosphorus were also analysed approximately once a month. Redox potential values were corrected for the potential of the hydrogen electrode. Analyses were carried out following the methods described in APHA-AWWA-WPCF (2001). The results will be presented in concentration and mass per area unit, though removal efficiencies were only calculated in terms of areal mass removal. Differences between lines were statistically evaluated through the two-way (time and treatment) ANOVA (without replication) test using the software package SPSS 17.0. Online sensors were used to monitor different points of the experimental plant. Redox potential probes (Digimed TH-404) were placed at the outlet of each treatment line (before the flowmeter) to measure redox potential every 10 s. A turbidimeter (Digimed TB-44 M) and ammonium probe (Digimed AI-NH3) were used to measure a sample per day of one settler, the effluent of the HUSB reactor and the outlet of each line. From each measuring spot a peristaltic pump diverted the water to the equipment. Data from online sensors were collected and stored automatically in a data-
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Table 1 Values of the main operational parameters of the HUSB reactor as a function of the HRT. Standard deviations in brackets, n = 10. HRT (h)
Flow rate (L/min)
OLRa (kg COD/m3 d)
SLRb (kg TSS/m3 d)
XR c (gVS/L)
SRTmin (d)
SRTmax (d)
3 5
0.42 0.25
0.45 (0.21) 0.23 (0.09)
0.25 (0.17) 0.18 (0.13)
12.25 (4.09) 7.78 (4.41)
3.64 (1.25) 3.40 (3.55)
24.46 (18.36) 16.46 (14.12)
a
Organic loading rate. Solids loading rate. Biomass concentration in the reactor.
b c
logger DT50. These data can be downloaded via a Wi-Fi Internet connection. Only a small part of the data from the online sensors is shown in this paper. All electrical devices of the plant (pumps, electrovalves, etc.) were controlled by an OMRON ZEN® programmable relay, programmed with ZEN Support 4.0® software. The configuration of the experimental plant (with three treatment lines) allows to address the effect of primary treatment and operation strategy by comparing the control line with alternative configurations (anaerobic and batch lines, respectively). However, anaerobic and batch lines are not comparable because these lines do not share any configuration element (neither primary treatment nor operation strategy). So even though the results are exposed all together, in the discussion section the effect of primary treatment and operation strategy on the removal efficiency will be treated separately. 3. Results
at the effluent of the HUSB reactor, indicative of a greater sulphate reduction activity. TSS clearly decreased from raw wastewater to the effluents of the primary treatments (55 and 57% of removal efficiency for the settler and HUSB reactor, respectively during the first period and 39 and 61% for the settler and HUSB reactor, respectively during the second period), although significant differences between primary treatments were only observed during the second period (P = 0.02). COD removal efficiency in the primary treatments was ranged between 20 and 30% with no significant differences between the treatments. Hydrolysis and solubilisation of particulate matter in the HUSB reactor was clearly favoured during the second period, when the dissolved COD was significantly higher than in raw wastewater and settler effluent (P = 0.04). BOD5 experienced very low decrease throughout the primary treatment (either HUSB or settler with removal efficiencies ranged from 3 to 12%), indicating that the particles removed in the primary treatment were not readily biodegradable.
3.1. Operation of HUSB reactor 3.3. Treatment lines efficiency Table 1 shows the main operational parameters of the reactor as a function of the HRT. From start-up to December 2008 (first period, with an HRT of 3 h), the HUSB reactor worked with at flow rate of 0.42 L/min. This created an upflow velocity of 0.46 L/h. During the second period (5 h HRT), the flow rate changed to 0.25 L/min, decreasing the upflow velocity into the digester to 0.27 L/h. The excess sludge of the HUSB reactor was purged every two weeks during the period of operation with an HRT of 3 h. From this point, the reactor was purged daily for the purpose of improving control of the biomass concentration (<10 g VS/L). The average biomass concentration in the sludge bed was slightly higher than 10 g VS/L during the first period (HRT of 3 h), while it was lower during the later period (5 h of HRT). Maximum solids retention time (SRTmax ) decreased by 30% during the period when HUSB was operated at 5 h of HRT. 3.2. Primary treatment efficiency The HUSB reactor produced effluents with significantly lower redox potentials (P = 0.00) than the settler (Table 2). This result concurs with the significantly lower sulphate concentration (P = 0.00)
During the study period the wetlands operated with a hydraulic loading rate of 28.5 mm/d (which corresponds to a theoretical HRT of 3.5 d per line, considering a gravel porosity of 40%). Average surface organic loads were 8.2 ± 3.3 g COD/m2 d and 4.7 ± 1.4 g BOD/m2 d for the control and batch lines, respectively (same primary treatment), and 8.8 ± 3.7 g COD/m2 d and 4.7 ± 1.5 g BOD/m2 d for the anaerobic line (with the HUSB reactor as primary treatment). Average surface ammonium load was approximately 1.1 g NH4 + –N/m2 d, irrespective of the experimental line analysed. All removal efficiencies were calculated in terms of mass removal as a consequence of considerable variations in evapotranspiration, which was estimated from the difference between influent and effluent flow rates. Because legal specifications for discharges are expressed in concentration, the results for the analysed contaminants are also presented in this way (Table 3). Evapotranspiration showed a typical trend, with higher values in summer (ranging from 25 to 30 mm/d) and lower in winter (ranging from 2 to 10 mm/d). In spite of the high rates of evapotranspiration during summer months (as high as the hydraulic loading rate) it was not observed a negative impact in treatment efficiency (Table 3).
Table 2 Average values and standard deviations (in brackets) of water quality parameters analysed in raw wastewater (from the outlet of the storage tank) and effluents of the primary treatments. Data sorted in two periods according to the HRT of the HUSB reactor. First period from March 2007 to December 2008 and second from January 2009 to July 2009. n changes depending on the frequency of each parameter analysed. Period
pH EH (mV) Turbidity (NTU) TSS (mg/L) COD (mg/L) COD dissolved (mg/L) BOD5 (mg/L) NH4 + –N (mg/L) SO4 = –S (mg/L)
n
58 58 57 52 46 6 18 55 44
First
n
Raw wastewater
Settler
HUSB (HRT = 3 h)
7.99 (0.22) 15.5 (103) 151 (61.1) 223 (154) 439 (243) 193 (107) 180 (50.2) 26.4 (9.81) 234 (53.6)
7.89 (0.25) 109 (148) 104 (40.8) 101 (47.5) 312 (169) 189 (98.3) 162 (50.7) 25.1 (9.67) 221 (55.9)
7.70 (0.23) -102 (43.7) 102 (46.7) 95.1 (56.9) 307 (157) 184 (72.2) 174 (52.1) 27.8 (9.73) 188 (52.9)
18 19 18 21 18 4 17 21 19
Second Raw wastewater
Settler
HUSB (HRT = 5 h)
7.79 (0.33) 180 (118) 103 (44.8) 161 (116) 293 (94.1) 146 (46.0) 170 (59.7) 25.2 (7.56) 165 (55.3)
7.74 (0.29) 172 (103) 93.3 (27.0) 98.8 (47.5) 235 (52.01) 136 (44.8) 165 (49.1) 20.7 (7.48) 162 (63.0)
7.45 (0.26) -103 (100) 74.0 (30.2) 62.1 (29.7) 234 (50.02) 176 (19.5) 149 (50.5) 26.4 (9.09) 133 (60.4)
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Table 3 Average values and standard deviations (in brackets) of physical and chemical water quality parameters of the effluents of each line. Concentrations and mass loads are shown for TSS, COD, BOD5 , and NH4 + –N.
pH EH (mV) Turbidity (NTU) TSS COD BOD5 +
NH4 –N TN NOx − –N TP (PO4 3− –P) SO4 2−
(mg/L) (g/m2 d) (mg/L) (g/m2 d) (mg/L) (g/m2 d) (mg/L) (g/m2 d) (mg/L) (mg/L) (mg/L) (mg/L)
n
Control
Batch
Anaerobic
76 70 75 75
7.1 (0.4) 3 (92.7) 7.3 (8.0) 11.4 (8.44) 0.3 (0.7) 60.3 (76.04) 2.0 (2.9) 20.3 (15.6) 0.3 (0.2) 6.07 (8.43) 0.15 (0.23) 7.45 (8.00) 0.09 (0.11) 2.53 (3.16) 374 (393)
7.1 (0.4) −5 (71) 7.0 (7.0) 10.5 (9.75) 0.2 (0.3) 55.8 (86.06) 1.6 (2.4) 15.0 (13.3) 0.2 (0.2) 4.11 (6.69) 0.10 (0.16) 4.84 (4.28) 0.14 (0.21) 1.87 (2.51) 453 (473)
7.2 (0.4) −45 (78) 8.6 (10.9) 10.6 (10.4) 0.2 (0.2) 63.3 (62.8) 2.4 (2.6) 35.4 (14.8) 0.6 (0.6) 7.36 (9.13) 0.20 (0.28) 5.58 (5.63) 0.10 (0.13) 3.47 (4.61) 262 (301)
65 35 76 10 28 23 63
The average effluent redox potential of the anaerobic line was significantly lower than for the other lines (P = 0.00) (Table 3). However, there were no significant differences in the average redox potential between batch and control lines (P = 0.21). All treatment lines removed TSS above 90%, and the average effluent mass loads were not significantly different (P = 0.147). The batch line had slightly higher COD mass removal efficiency (88%) than control (P = 0.084) and anaerobic lines (83% and 80%, respectively). There was no significant difference between anaerobic and control lines (P = 0.270). COD effluent mass loads presented a seasonal pattern with higher values in winter than in summer (Fig. 4). These higher values were less noticeable for the control and batch lines in winter 2008–2009. Batch and control lines had higher BOD5 mass removal efficiencies (96% and 94%, respectively) than the anaerobic line (87%); in fact, effluent mass loadings were significantly higher (P = 0.00) for the anaerobic line than the other two lines. Note that effluents of the anaerobic line had a higher organic matter content (measured as COD or BOD5 ) in accordance with the significantly lower redox potential values. The batch line had a higher ammonium mass removal efficiency (87%; P = 0.016) than the control line (80%) and the anaerobic line (73%). Lower ammonium removal efficiencies observed in the anaerobic line again concur with the significantly lower redox
potential. Ammonium effluent mass loads also presented a seasonal pattern, which was even more noticeable than the pattern observed for COD (Fig. 5). Note that the ammonium removal efficiencies ranged from almost zero in winter time to 100% during most of the summer. In fact, in summer the three lines were very efficient in ammonium removal. In addition, it is interesting to point out that the effluent ammonium mass was clearly lower in winter for the batch line (on average ± SD 0.24 g/m2 d ± 0.18) than for the other two lines (0.46 ± 0.13 and 0.53 ± 0.20 g/m2 d for control and anaerobic lines, respectively), which leads to a higher overall removal efficiency observed during the entire study for the batch line. Sulphate concentration increased from the influents to the effluents due to both, the overall driving evapotranspiration and by conversion of hydrogen sulphide to sulphate due to oxic conditions. Effluent concentrations were significantly different (P = 0.002) and lower in the anaerobic line (30% lower than the control line) in connection with the lower redox potential (Table 3). Sulphate concentrations were highest in the batch line. Comparable to the other parameters studied, total phosphorus mass removal efficiency was higher for the batch line (75%) than for the control (66%). And again, the anaerobic line presented the worst efficiencies (54%) (Table 3), although it was not statistically different (P = 0.273).
Fig. 4. Temporal changes of the influent COD mass loads (settler and HUSB reactor) and the effluent mass loads of each treatment line. Monthly averages are shown for the sake of clarity.
Fig. 5. Temporal changes of the influent ammonium mass loads (settler and HUSB reactor) and the effluent mass loads of each treatment line. Monthly averages are shown for the sake of clarity.
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4. Discussion An overview of the general performance of the plant will be discussed in this section. After that, the effect of primary treatment will be addressed by comparing control and anaerobic lines, whereas the effect of operating strategy will be examined by comparing control and batch lines. 4.1. Wetlands general performance The average organic loading rate in the wetlands of the three lines (4.7 ± 1.5 g BOD/m2 d) was slightly lower than the design load (6 g BOD/m2 d), but still in the range of 4–6 g BOD/m2 d generally recommended for horizontal subsurface flow treatment wetlands (Kadlec and Knight, 1996; García et al., 2005; Akratos and Tsihrintzis, 2007). In the 3 lines of the experimental plant the overall organic matter (COD) removal efficiency was in the commonly attained range (80% for anaerobic, 83% for control and 88% for batch, respectively). Ammonium mass removal efficiency, however, was higher than the values usually described for this type of systems (73% for anaerobic, 80% for control and 87% for batch) (Austin and Nivala, 2009; Vymazal and Kröpfelová, 2009). This is due to two characteristics observed previously and considered in the design: (1) low water depth (0.25 m) (García et al., 2005) and (2) intermittent feeding (Caselles-Osorio and García, 2007a,b). These shallow wetlands, fed intermittently, have a good capacity to carry out whole nitrogen removal and the results of the present study support this statement (low effluent concentrations of ammonium as well as oxidised nitrogen, see Table 3). A seasonal pattern for COD and ammonium was observed, with lower mass values in warm months (Fig. 4). Seasonal variations in COD are not generally seen in these types of systems (García et al., 2005; Ruíz et al., 2008; Serrano et al., 2009; Vymazal and Kröpfelová, 2009), although seasonal differences have been largely demonstrated for ammonium (Caselles-Osorio and García, 2007a; Kuschk et al., 2003). In this study, when COD removal was analysed in terms of concentration, higher removal in summer was not clearly observed. This could be due to the effect of evapotranspiration, which was very high in these months (25–30 mm/d). Accordingly, an increase of evapotranspiration leads to an increase in organic matter concentration, which could mask greater removal in summer. 4.2. Effect of primary treatment Settlers only provide physical wastewater treatment, in which removal efficiencies obtained for the main contaminants were in the common range of a primary settler with 2 h of HRT (Metcalf and Eddy, 2003), though efficiencies are related to raw wastewater characteristics (Mitchell and McNevin, 2001). The lower redox potentials and pH observed in the effluents of the HUSB during the entire study period are indicative of bacterial hydrolysis. Despite this biological activity in the HUSB reactor, differences in contaminant removal between the two types of primary treatment were only clearly observed in the period when the HUSB was operated at an HRT of 5 days. This is due to its operational parameters (Serrano et al., 2009; Álvarez et al., 2008a). Accordingly, higher removal efficiencies for TSS were recorded during the second operation period (5 h HRT), when the upflow velocity decreased from 0.46 to 0.27 L/h (Table 1). HUSB reactor operating at 5 h of HRT produced water with 20% fewer suspended solids and more biodegradable water than the settlers (Table 2). Accordingly, in this period the BOD5 /COD ratio changed from 41% in raw wastewater to 64% and 53% in HUSB and settler effluents, respectively. Similar values were recorded by Barros et al. (2008) for UASB reactors.
The anaerobic line had high removal efficiencies for selected pollutants (on average 80%, 87% and 73% for COD, BOD5 and ammonium, respectively), with results similar to those of previous studies (Singh et al., 2009; Ruíz et al., 2008), and effluent concentrations were lower than the standards set by Council Directive 91/271EEC. However, removal efficiencies for the anaerobic line were in all cases below those recorded for the control line (83%, 94% and 80% for COD, BOD5 and ammonium removal efficiency, respectively). These lower removal efficiencies concurred with lower average redox potentials observed in the effluents of the anaerobic line than in the control line (Table 3). The biological activity of the HUSB reactor produced more reduced primary effluents, which were eventually responsible for the more reduced environment of the wetlands in comparison to the wetlands of the control line. In winter ammonium removal efficiency was lower in the anaerobic line than in the control line (P = 0.000), whereas this difference was minimized during warm seasons (P = 0.165) (Fig. 5). It is generally accepted that in warm seasons the active macrophytes enhance pollutant removal (Tanner et al., 1998; Brix, 1997), probably due to changes in water level mediated by evapotranspiration (Breen, 1997; Tanner et al., 1999). Accordingly, active plants of the wetlands in the anaerobic line may have improved the treatment efficiency of the system during these months. However, for the whole period of study there was a significant difference between the two treatments (P = 0.007). The results of this study indicate that the implementation of a HUSB reactor coupled with treatment wetlands under the conditions tested in this study does not benefit contaminant removal in comparison of a settler-wetland system, though pollutants concentrations were generally below the law limits. However, the HUSB reactor has a higher degree of solids removal in comparison with a conventional settler and its implementation could prevent early clogging of the wetlands (Shepherd et al., 2001). Note that all treatment lines presented an excellent removal of TSS (>90%). If these solids are not readily biodegradable, then they will contribute to clogging. Therefore, improved primary treatments are of capital importance in order to reduce the clogging. 4.3. Effect of operation strategy In general, the batch line had slightly higher removal efficiencies (between 2% and 10%) than the control line for the main contaminants (Table 3), and similar to those generally described for vertical SSF TWs (Brix and Arias, 2005). Currently, the operation strategy of this line, alternating saturated and unsaturated phases, is comparable to the operation of vertical flow systems. Higher removal efficiencies for the batch line seem to be due to the prevalence of a less reduced environment of the wetlands in this line. This hypothesis is confirmed by 32% less ammonium and 21% more sulphate concentrations in the effluents of the batch line than in the control line (Table 3). However, the results of average redox potential (without significant differences between both lines, Table 3) contrast with the less reduced environment for the batch line stated above. Lack of differences in redox potential may have been an artefact, due to the operation of the batch line with cycles alternating batch-unsaturated and permanently saturated phases (note that redox potential was recorded in the effluent once per week and always at the same time). As a result of this type of operation, high daily variations in the flow rate and redox potential for the batch line are encountered. Fig. 6 shows temporal changes in redox potential and ammonium concentration obtained from the online sensors in four representative weeks during the cold season (Fig. 6a and c) and the warm season (Fig. 6b and d) in control and the batch lines. As stated above, daily redox potential variations in the batch line are con-
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Fig. 6. Temporal changes of redox potential and ammonium in control and batch lines during one month in winter (a, c) and another in spring (b, d). For redox potential 6 values per day are shown (recorded every 4 h, coinciding with influent discharges) while for ammonium 1 value per day was recorded with online sensors.
siderable, in relation to the type of operation. Redox values were generally lower in February (average −141 mV and −1 mV for the control and the batch lines, respectively) than in May (149 mV and 189 mV for the control and the batch lines). This behaviour concurred with ammonia concentrations, which were lower in May (0.4 ± 0.3 mg N/L for both lines, P = 0.05) than in February (26.9 ± 6.1 mg N/L and 10.7 ± 3.01 mg N/L for the control and the batch lines, P = 0.58). Caselles-Osorio and García (2007a) found a similar temporal opposite relationship between redox potential and ammonium concentration. It can therefore be concluded that differences between systems occur because the batch operation creates a more oxidised environment in winter, which improves its performance during these months (approximately 50% higher removal efficiency) and is eventually responsible for improved overall annual removal efficiencies (approximately 10% higher removal efficiency). In winter, when the reeds were dry and harvested, the plant aeration effect (Tanner et al., 1999, 2002; Davies et al., 2006) may have been negligible and the batch operation may have promoted a less reduced environment, subsequently enhancing ammonium removal. Note that in February redox potential in the control line was indicative of clearly anaerobic conditions while for batch line was in the range of anoxic environments (Fig. 6a), in which alternatives ways to remove ammonium (and not oxygen dependent) can be given (Tanner et al., 2002). This is in accordance with the lower ammonium concentration recorded for batch line (Fig. 6c). The results of this study confirm that aeration of the granular medium of the wetlands occurring after draining of the batch line in small wetlands helps to increase the removal efficiency of organic matter and ammonium in treatment wetland systems
(Tanner et al., 1999; Chazarenc et al., 2009). The difference in our study in comparison with previous reports is that this increased efficiency is only observed in winter months. 5. Conclusions This study demonstrates that shallow horizontal SSF TWs are good systems for wastewater treatment and can remove contaminants at high rates (around 80% for COD and ammonium, and 90% for TSS and BOD5 ) because of shallow beds and intermittent feeding. The implementation of a HUSB reactor as a primary treatment for horizontal subsurface-flow treatment wetlands does not improve contaminant removal efficiency in comparison with a conventional settler in the conditions tested here. On the contrary, the HUSB reactor slightly decreases the efficiency of the system due to the reduced nature of its effluents, though maintaining the effluent concentration under the law limits. Moreover, the application of a HUSB reactor (in this case operated at 5 h of hydraulic retention time) can significantly reduce the amount of solids entering a wetland (20% fewer suspended solids than conventional settlers) and therefore help prevent or delay clogging processes. A treatment wetland system composed of horizontal subsurface flow wetland operated with filling–resting–drain phases (as in the batch line) offers higher efficiencies in contaminant removal than a normally operated system (as in the control line) (between 5% and 10% more efficient in COD and ammonium removal, respectively). However, differences in contaminant removal (especially in terms of ammonium) are season-dependent. In this regard, differences in ammonium removal between a system operated under batch
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conditions and a system operated continuously are maximized in winter (up to 50% higher than a continuously fed system). The results obtained in this study should be taken in consideration in the design and operation of horizontal SSF TWs. After a long experimentation period, we have provided sufficient data and clearly demonstrated that the efficiency of shallow horizontal SSF TWs can be increased, particularly in cold months, using a batch operation strategy. Acknowledgements This study was made possible by funding from the Spanish Ministry of Innovation and Science for the NEWWET 2008 Project (CTM2008-06676-C05-01). Anna Pedescoll also acknowledges the fund provided by the Spanish Ministry of Innovation and Science. The authors are particularly grateful to Enrica Uggetti, Gemma Lesan, Gian Paolo Mottola and Fedro Tapia for their help during the construction of the experimental plant; to Toni Lara, Carlos Soriano, ˜ G. Admirable and Roger Samsó for their help with mainteBegona nance tasks; and finally, to Elif Bozdogan, Javier Carretero, Adriana Gallardo and Miriam Planas for their contribution to laboratory tasks. References Akratos, C.S., Tsihrintzis, V.A., 2007. Effect of temperature, HRT, vegetation and porous media on removal efficiency of pilot-scale horizontal subsurfaceflow constructed wetlands. Ecological Engineering 29, 173–191. Álvarez, J.A., Ruíz, I., Soto, M., 2008a. Anaerobic digesters as a pretreatment for constructed wetlands. Ecological Engineering 33, 54–67. Álvarez, J.A., Armstrong, E., Gómez, M., Soto, M., 2008b. Anaerobic treatment of lowstrength municipal wastewater by a two-stage pilot plant under psychrophilic conditions. Bioresource Technology 99, 7051–7062. APHA-AWWA-WPCF, 2001. Standard Methods for the Examination of Water and Wastewater, 20th ed. APHA-AWWA-WPCF, Washington DC, USA. Austin, D., Nivala, J., 2009. Energy requirements for nitrification and biological nitrogen removal in engineered wetlands. Ecological Engineering 35, 184–192. Barros, P., Ruiz, I., Soto, M., 2008. Performance of an anaerobic digester-constructed wetland system for a small community. Ecological Engineering 33, 142–149. Breen, P.F., 1997. The performance of vertical flow experimental wetlands under a range of operational formats and experimental conditions. Water Science and Technology 35 (5), 167–174. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Science and Technology 35 (5), 11–17. Brix, H., Arias, C., 2005. The use of vertical flow constructed wetlands for on-site treatment of domestic wastewater: New Danish guidelines. Ecological Engineering 25, 491–500. Calheiros, C.S.C., Duque, A.F., Moura, A., Henriques, I.S., Correia, A., Rangel, A.O.S.S., Castro, P.M.L., 2009. Substrate effect on bacterial communities from constructed wetlands planted with Typha latifolia treating industrial wastewater. Ecological Engineering 35, 744–753. Caselles-Osorio, A., García, J., 2007a. Impact of continuous and intermittent feeding strategies on the performance of shallow horizontal subsurface-flow constructed wetlands. Science of the Total Environment 378, 253–262. Caselles-Osorio, A., García, J., 2007b. Effect of physico-chemical pre-treatment on the removal efficiency of experimental horizontal subsurface-flow constructed wetlands. Environmental Pollution 146, 55–63. Chazarenc, F., Gagnon, V., Comeau, Y., Brisson, J., 2009. Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands. Ecological Engineering 35, 1005–1010. Davies, L.C., Pedro, I.S., Novais, J.M., Martins-Dias, S., 2006. Aerobic degradation of acid orange 7 in a vertical constructed wetland. Water Research 40, 2055– 2063.
Faulwetter, J.L., Gagnon, V., Sundberg, C., Chazarenc, F., Burr, M.D., Brisson, J., Camper, A.K., Stein, O.R., 2009. Microbial processes influencing performance of treatment wetlands: a review. Ecological Engineering 35, 987–1004. García, J., Aguirre, P., Barragán, J., Mujeriego, R., Matamoros, V., Bayona, J.M., 2005. Effect of key design parameters on the efficiency of horizontal subsurface flow constructed wetlands: long-term performance pilot study. Ecological Engineering 25, 405–418. Huang, Y., Latorre, A., Barceló, D., García, J., Aguirre, P., Mujeriego, R., Bayona, J.M., 2004. Factors affecting linear alkylbenzene removal in subsurface flow constructed wetlands. Environmental Science and Technology 38, 2657–2663. Kadlec, R.H., Knight, R.L., 1996. Treatment Wetlands. CRC Press, Boca Raton, 893 pp. Knowles, P., Nivala, J., Dotro, G., García, J., 2011. Clogging in subsurface-flow treatment wetlands: Occurrence and contributing factors. Ecological Engineering. doi:10.1016/j.ecoleng.2010.08.005. Kuschk, P., Wiener, A., Kappelmayer, U., Weibrodt, E., Kästner, M., Stottmeister, U., 2003. Annual cycle of nitrogen removal by a pilot-scale subsurface horizontal flow in a constructed wetland under moderate climate. Water Research 37, 4236–4242. Matamoros, V., García, J., Bayona, J.M., 2005. Behavior of selected pharmaceuticals in subsurface flow constructed wetlands. A pilot scale study. Environmental Science and Technology 39, 5449–5454. Metcalf, Eddy, 2003. Wastewater Engineering: Treatment and Reuse, 4th ed. McGraw-Hill, New York, 1819 pp. Mitchell, C., McNevin, D., 2001. Alternative analysis of BOD removal in subsurface flow constructed wetlands employing monod kinetics. Water Research 35 (5), 1295–1303. Molle, P., Liénard, A., Boutin, C., Merlin, G., Iwema, A., 2005. How to treat raw sewage with constructed wetlands: an overview of the French systems. Water Science and Technology 51 (9), 11–21. ˜ Puigagut, J., Villasenor, J., Salas, J.J., Bécares, E., García, J., 2007. Subsurface-flow constructed wetlands in Spain for the sanitation of small communities: a comparative study. Ecological Engineering 30, 312–319. Rousseau, D.P.L., Vanrolleghem, P.A., De Pauw, N., 2004. Model-based design of horizontal subsurface flow constructed treatment wetlands: a review. Water Research 38 (6), 1484–1493. Ruíz, I., Álvarez, J.A., Díaz, M.A., Serrano, L., Soto, M., 2008. Municipal wastewater treatment in an anaerobic digester-constructed wetland system. Environmental Technology 29, 1249–1256. Serrano, L., De La Varga, D., Díaz, M.A., Ruiz, I., Bondelle, R., Soto, M., 2009. HUSB-CW treatment system for winery wastewater. In: Proceedings of the 5th International Specialized Conference on Sustainable Viticulture: Winery Waste and Ecologic Impacts Management , March 30th to April 3rd, Trento and Verona, Italy, pp. 317–324. Shepherd, H., Grismer, M.E., Tchobanoglous, G., 2001. Treatment of high-strength winery wastewater using a subsurface-flow constructed wetland. Water Environment Research 73 (4), 394–403. Singh, S., Haberl, R., Moog, O., Shrestha, R.R., Shrestha, P., Shrestha, R., 2009. Performance of an anaerobic baffled reactor and hybrid constructed wetland treating high-strength wastewater in Nepal – a model for DEWATS. Ecological Engineering 35, 654–660. Song, H.-L., Nakano, K., Taniguchi, T., Nomura, M., Nishimura, O., 2009. Estrogen removal from treated municipal effluent in small-scale constructed wetland with different depth. Bioresource Technology 100, 945–2951. Stein, O.R., Hook, P.B., Biederman, J.A., Allen, W.C., Borden, D.J., 2003. Does batch operation enhance oxidation in subsurface constructed wetlands? Water Science and Technology 48 (5), 149–156. Tanner, C.C., Sukias, J.P.S., Upsdell, M.P., 1998. Organic matter accumulation and maturation of gravel bed constructed wetlands treating dairy farm wastewaters. Water Research 32 (10), 3046–3054. Tanner, C.C., D’Eugenio, J., McBride, G.B., Sukias, J.P.S., Thompson, K., 1999. Effect of water level fluctuation on nitrogen removal from constructed wetland mesocosms. Ecol. Eng. 12, 67–92. 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. Ecological Engineering 18, 499–520. Vymazal, J., Kröpfelová, L., 2009. Removal of organics in constructed wetlands with horizontal sub-surface flow: a review of the field experience. Science of the Total Environment 407, 3911–3922. Vymazal, J., Masa, M., 2003. Horizontal sub-surface flow constructed wetland with pulsing water level. Water Science and Technology 48 (5), 143–148.
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Contaminant removal efficiency depending on primary treatment and operational strategy in horizontal subsurface flow treatment wetlands Anna Pedescoll, Angélica Corzo, Eduardo Álvarez, Jaume Puigagut, Joan García ∗ Environmental Engineering Division, Department of Hydraulic, Maritime and Environmental Engineering. Technical University of Catalonia, c/Jordi Girona 1-3, Building D1, 08034 Barcelona, Spain
a r t i c l e
i n f o
Article history: Received 3 August 2010 Received in revised form 1 December 2010 Accepted 10 December 2010 Available online 11 January 2011 Keywords: Constructed wetlands Reed beds Design Anaerobic digester Intermittent feeding
a b s t r a c t This study aimed to evaluate the contaminant removal efficiency of shallow horizontal subsurface flow treatment wetlands (SSF TWs) as a function of (1) primary treatment (hydrolytic upflow sludge blanket (HUSB) reactor vs. conventional settling) and (2) operation strategy (alternation of saturated/unsaturated phases vs. permanently saturated). An experimental plant was constructed, operated and surveyed for the main water quality parameters over a period of 2.5 years. The plant had 3 treatment lines: a control line (settler-wetland permanently saturated), a batch line (settler-wetland operated with saturated/unsaturated phases) and an anaerobic line (HUSB reactor-wetland permanently saturated). In each line wetlands had a surface area of 2.80 m2 , a water depth of 25 cm and a granular medium D60 = 7.3 mm, and were planted with common reed. During the study period the wetlands were operated at a hydraulic and organic load of 28.5 mm/d and about 4.7 g BOD/m2 d, respectively. Effluent average redox potential was lower for the anaerobic line (−45 ± 78 mV) than for the other two lines (3 ± 92.7 and −5 ± 71 mV for control and batch, respectively). Overall, chemical oxygen demand (COD), biochemical oxygen demand (BOD5 ) and ammonium mass removal efficiencies were slightly greater for the batch line (88%, 96% and 87%, respectively) than for the control line (83%, 94% and 80%) and the anaerobic line (80%, 87% and 73%). During cold seasons, COD and ammonium removal in the batch line was around 30% and 50% higher than in the control line, respectively. The results of this study indicate that the implementation of a HUSB reactor as primary treatment did not enhance the treatment capacity of the system (in comparison with a conventional settler). The efficiency of treatment wetland systems with horizontal subsurface flow can be improved using a batch operation strategy. © 2010 Elsevier B.V. All rights reserved.
1. Introduction Subsurface flow treatment wetlands (SSF TWs) are extensive systems used for wastewater treatment that are particularly suitable for the sanitation of small communities (Rousseau et al., 2004; Puigagut et al., 2007; Vymazal and Kröpfelová, 2009). Contaminant removal efficiency attained by wetlands (such as ammonium removal) depends to a large extent on the redox status of the systems (Caselles-Osorio and García, 2007a). Accordingly, more oxidising conditions lead to greater contaminant removal efficiencies. Horizontal SSF TWs are complex bioreactors in which the removal of organic contaminants occurs mainly by means of anaerobic reactions and, to a lesser extent, by means of anoxic and
∗ Corresponding author. Tel.: +34 93 401 6464; fax: +34 93 401 7357. E-mail address: [email protected] (J. García). 0925-8574/$ – see front matter © 2010 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2010.12.011
aerobic pathways (Calheiros et al., 2009). Water depth is one of the main design factors of horizontal SSF TWs influencing the redox status of the system (Faulwetter et al., 2009). Accordingly, the lower the depth the higher the oxidising conditions within the bed (Singh et al., 2009). The higher efficiency of shallow horizontal SSF TWs (0.3 m) has been demonstrated for a wide variety of contaminants (Song et al., 2009; García et al., 2005; Matamoros et al., 2005; Huang et al., 2004). Overall, conventional horizontal SSF TWs with a water depth of about 0.5–0.7 m are considered to rarely have ammonium removal efficiencies above 40–55% because of their inability to provide enough oxidised conditions for nitrification (Austin and Nivala, 2009; Vymazal and Kröpfelová, 2009). Primary treatment is a previous key step to horizontal SSF TWs that is most often achieved using septic or Imhoff tanks (Knowles et al., 2011). However, other technologies, such as upflow anaerobic sludge blanket (UASB) reactors, have been recently investigated as a suitable primary treatment for SSF TWs, mainly because they can provide effluents with lower total suspended solids (TSS) and COD concentrations than standard primary treatments (Barros et al.,
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2008). Hydrolytic upflow sludge blanket (HUSB) reactors have been little investigated within the context of treatment wetland technology (Álvarez et al., 2008a,b). HUSB reactors are essentially UASB reactors operated at a lower hydraulic retention time (HRT) (from 2 to 5 h) in order to avoid methanogenesis reaction wherever possible. In general, solids retention time (SRT) in HUSB reactors is maintained for over 15 days in order to achieve high hydrolysis rates of wastewater solids. Field experience gained during the last few years has demonstrated that alternating unsaturated phases during the operation of SSF TWs is of capital importance to maintaining aerobic conditions within the wetlands (Molle et al., 2005). Accordingly, several studies have shown that wetlands operated under batch conditions perform better than those operated under continuous conditions (Caselles-Osorio and García, 2007a; Stein et al., 2003; Tanner et al., 1999). Higher contaminant removal efficiencies reported for batchoperated systems have also been linked to higher water level fluctuations (mediated by evapotranspiration) in comparison with continuous feeding systems. These fluctuations expose more granular medium to the atmosphere, thus promoting more oxidised conditions within the bed (Breen, 1997; Tanner et al., 1999) and with a positive effect on the removal of several contaminants, including COD and ammonium (Vymazal and Masa, 2003). This study aims to evaluate the contaminant removal efficiency of experimental shallow horizontal SSF TWs as a function of: (1) type of primary treatment (HUSB reactor or conventional settling) and (2) operating strategy (alternating batch-unsaturated phases and permanently saturated). For this purpose an experimental plant was constructed and operated over a period of two and half years after system start-up. To our knowledge, this is the first time that the effect of the type of primary treatment and operating strategy has been evaluated on rigorously controlled horizontal SSF TWs (in terms of hydraulic and organic loading rates) and over a sufficiently long period of time. Our aim was to obtain results useful for improving the design and operation of treatment wetlands.
2. Materials and methods 2.1. Experimental plant The plant used is set in the open at the experimental facility of the Department of Hydraulic, Maritime and Environmental Engineering of the Universitat Politècnica de Cataluya, Barcelona, Spain. Built in 2006, the plant became operational in February 2007 and treats urban wastewater pumped directly using 2 pumps from a nearby municipal sewer. Firstly, the wastewater is coarsely screened and subsequently stored in a 1.2 m3 plastic tank, which is continuously stirred in order to avoid sedimentation of solids. This tank is equipped with level buoys that control the operation of the feeding pumps. Accordingly, the pumps start automatically when the volume of wastewater in the tank is about 600 L and stop when the volume is about 1000 L. Wastewater retention time in this tank is approximately 12 h. From the storage tank, the wastewater is conveyed to 3 different treatment lines which, for ease of understanding, have been named batch, control and anaerobic lines (Fig. 1). Differences between treatment lines are related to the type of primary treatment and the operation strategy applied. The layout of the wetlands is the same in all three lines: three small wetlands in parallel (0.65 m2 each), two of them connected to a big wetland in series (1.65 m2 ) (Fig. 1). One of the three small wetlands was left unplanted and discharges directly to the sewer and, strictly speaking, does not belong to the treatment lines. These unplanted wetlands were constructed in order to study plant influence on clogging processes. However, clogging processes fall
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outside the scope of this paper and unplanted wetlands will therefore not be considered here. The two small parallel wetlands were necessary for the operation of the batch line. This system was also adopted in the other two lines for comparative purposes. These two small wetlands have a joint surface area (1.3 m2 ), which is approximately 45% of the total surface area of the treatment line (2.95 m2 ). Note that the appearance of clogging in this type of wetlands is more evident in the inlet zone of the wetlands (Caselles-Osorio and García, 2007b), and this is why the total surface of the wetland area was split in two (one big and two small wetlands), because this experimental set up was also designed to study clogging processes. Each of the three lines received a flow of 84 L/d (not including the unplanted wetlands). The wetlands were therefore operated at a hydraulic loading rate of 28.5 mm/d. The wetlands of each line were designed to have a maximum surface loading rate of approximately 6 g BOD/m2 d, as recommended by Kadlec and Knight (1996), García et al. (2005) or Akratos and Tsihrintzis (2007). 2.1.1. Primary treatment Batch and control lines have cylindrical PVC static settlers as primary treatment and are filled with screened wastewater pumped from the storage tank every 4 h. The control line has three settlers, one for each small wetland (planted and unplanted), with an internal diameter of 190 mm and an effective volume of 7 L (Fig. 2). The batch line has only two settlers because the small planted wetlands operate alternately. Settlers for the batch line have an internal diameter of 300 mm and an effective volume of 14 L (Fig. 2). In the control line, after 2 h of settling from each settler, 7 L of wastewater are discharged into the small wetlands (in total 14 L to the planted wetlands at each discharge). In the batch line, after 2 h of settling, 14 L of wastewater are discharged into one small wetland. Irrespective of the treatment line (control or batch), wastewater is discharged from settlers by means of electrovalves, while sludge wasting is realized by means of pumps that convey sludge back to the municipal sewer. The effect of differences in diameter and volume of the settlers between the control and the batch line was evaluated at the beginning of the study and there were no significant differences for TSS removal efficiency (results not shown). The anaerobic line has a cylindrical PVC HUSB reactor as primary treatment, which has an internal diameter of 300 mm, a total height of 1900 mm and an effective volume of 105 L (Fig. 2). The reactor is continuously fed with wastewater from the storage tank by means of a peristaltic pump that supplies a known flow. The HUSB reactor was operated at two different HRTs during the study period in order to assess optimum operational conditions. Specifically, the HUSB reactor was operated at 3 h of HRT from February 2007 to December 2008 and at 5 h of HRT from January 2009 to July 2009. The sludge blanket inside the HUSB reactor was kept as much as possible at a volatile solids (VS) concentration of <10 g/L by means of manual purges. HUSB reactor had 5 taps located in a vertical series starting from a height of 480 mm and each placed at a respective distance of 150 mm (Fig. 2). Overall, VS concentration was estimated every two weeks measuring concentrations at each tap. The minimum and maximum solids retention time (SRTmin and SRTmax ) were estimated according to the following equation (Álvarez et al., 2008b): SRTmax =
VXR QW XW
SRTmin =
VXR [(QW XW ) + (QXe )]
where V is the reactor volume (m3 ), XR is the weighted average biomass concentration in the reactor (kg VS/m3 ), Qw is the purged biomass flow rate (m3 /d), Xw is the concentration of purged
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Fig. 1. Schematic diagram of the experimental plant and side view of the batch line. The set-up of wetlands in the other lines was similar except for the presence of the electrovalve between big and small wetlands. The electrovalve in the batch line enabled draining of the small wetlands.
Fig. 2. Schematic side view of the primary treatments. Distances expressed in cm.
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Fig. 3. Schematic diagram of an operation cycle (4 days) of the batch line compared with the control and the anaerobic line (small wetlands). * The operation of one of the two small wetlands is shown for control and anaerobic lines only.
biomass (kg VS/m3 ), Q is the influent flow rate (m3 /d) and Xe is the effluent volatile solids concentration (kg VS/m3 ). Every 4 h the content of the upper part of the HUSB reactor was discharged into three distribution tanks (one per each small wetland) with an internal diameter of 300 mm and an effective volume of 7 L. Without time for particle settling (tanks were charged and discharged in 10 min), wastewater was discharged from these tanks into the small wetlands by means of electrovalves. This set-up ensured a flow of 7 L to each small wetland every 4 h. 2.1.2. Wetlands Big and small wetlands consist of plastic containers 1.5 m long, 1.1 m wide and 0.50 m high, and 0.95 m long, 0.70 m wide and 0.45 m high, respectively. Wastewater from the primary treatment was discharged by means of perforated pipes located along the width of the wetlands. Each container had a drainage pipe on the flat bottom for effluent discharge. The uniform gravel layer (D60 = 7.3 mm, Cu = 0.83, 40% initial porosity) was 0.3 m deep and the water level was kept 0.05 m below the gravel surface to give a water depth of 0.25 m. The theoretical hydraulic retention time (HRT) was 3.5 d per line. In April 2007 the wetlands were planted with developed rhizomes of common reed (Phragmites australis) and by July 2007 the plants were well-established and covered the entire surface of the wetlands. In anaerobic and control lines all the wetlands remained permanently saturated. The batch line, however, operated under a scheme of a four-day cycle and the small wetlands were not permanently saturated (Fig. 3). Accordingly, for the first two days of the cycle the small wetlands were fed in the same way as the control line, on the third day the wetland rested under saturated conditions but received no influent and on the fourth day the wetland was drained (by means of an electrovalve, see Fig. 1) and rested under unsaturated conditions. Note that the two small wetlands of the batch line are at different phases at any time (Fig. 3).
Effluent from each line was discharged into a tank (flow meter) with a water sensor level that was activated every 5 L of effluent. When this level was reached, the tank emptied. The total effluent volume was then calculated by multiplying the times that the sensor had been activated by 5 L. Flow measurements were recorded on a daily basis. 2.2. Samples and analyses In order to evaluate the removal efficiency of the experimental plant samples of the influent (storage tank), primary treatments (settlers and HUSB reactor) and effluent (big wetlands) were taken (always at 9:00 and 9:30 am) 3–4 times per month from April 2007 to July 2009. Samples were analysed for pH, redox potential, turbidity, COD, TSS, ammonium and sulphates. In addition, from October 2007 BOD5 , dissolved COD, nitrites, nitrates, TKN and total phosphorus were also analysed approximately once a month. Redox potential values were corrected for the potential of the hydrogen electrode. Analyses were carried out following the methods described in APHA-AWWA-WPCF (2001). The results will be presented in concentration and mass per area unit, though removal efficiencies were only calculated in terms of areal mass removal. Differences between lines were statistically evaluated through the two-way (time and treatment) ANOVA (without replication) test using the software package SPSS 17.0. Online sensors were used to monitor different points of the experimental plant. Redox potential probes (Digimed TH-404) were placed at the outlet of each treatment line (before the flowmeter) to measure redox potential every 10 s. A turbidimeter (Digimed TB-44 M) and ammonium probe (Digimed AI-NH3) were used to measure a sample per day of one settler, the effluent of the HUSB reactor and the outlet of each line. From each measuring spot a peristaltic pump diverted the water to the equipment. Data from online sensors were collected and stored automatically in a data-
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Table 1 Values of the main operational parameters of the HUSB reactor as a function of the HRT. Standard deviations in brackets, n = 10. HRT (h)
Flow rate (L/min)
OLRa (kg COD/m3 d)
SLRb (kg TSS/m3 d)
XR c (gVS/L)
SRTmin (d)
SRTmax (d)
3 5
0.42 0.25
0.45 (0.21) 0.23 (0.09)
0.25 (0.17) 0.18 (0.13)
12.25 (4.09) 7.78 (4.41)
3.64 (1.25) 3.40 (3.55)
24.46 (18.36) 16.46 (14.12)
a
Organic loading rate. Solids loading rate. Biomass concentration in the reactor.
b c
logger DT50. These data can be downloaded via a Wi-Fi Internet connection. Only a small part of the data from the online sensors is shown in this paper. All electrical devices of the plant (pumps, electrovalves, etc.) were controlled by an OMRON ZEN® programmable relay, programmed with ZEN Support 4.0® software. The configuration of the experimental plant (with three treatment lines) allows to address the effect of primary treatment and operation strategy by comparing the control line with alternative configurations (anaerobic and batch lines, respectively). However, anaerobic and batch lines are not comparable because these lines do not share any configuration element (neither primary treatment nor operation strategy). So even though the results are exposed all together, in the discussion section the effect of primary treatment and operation strategy on the removal efficiency will be treated separately. 3. Results
at the effluent of the HUSB reactor, indicative of a greater sulphate reduction activity. TSS clearly decreased from raw wastewater to the effluents of the primary treatments (55 and 57% of removal efficiency for the settler and HUSB reactor, respectively during the first period and 39 and 61% for the settler and HUSB reactor, respectively during the second period), although significant differences between primary treatments were only observed during the second period (P = 0.02). COD removal efficiency in the primary treatments was ranged between 20 and 30% with no significant differences between the treatments. Hydrolysis and solubilisation of particulate matter in the HUSB reactor was clearly favoured during the second period, when the dissolved COD was significantly higher than in raw wastewater and settler effluent (P = 0.04). BOD5 experienced very low decrease throughout the primary treatment (either HUSB or settler with removal efficiencies ranged from 3 to 12%), indicating that the particles removed in the primary treatment were not readily biodegradable.
3.1. Operation of HUSB reactor 3.3. Treatment lines efficiency Table 1 shows the main operational parameters of the reactor as a function of the HRT. From start-up to December 2008 (first period, with an HRT of 3 h), the HUSB reactor worked with at flow rate of 0.42 L/min. This created an upflow velocity of 0.46 L/h. During the second period (5 h HRT), the flow rate changed to 0.25 L/min, decreasing the upflow velocity into the digester to 0.27 L/h. The excess sludge of the HUSB reactor was purged every two weeks during the period of operation with an HRT of 3 h. From this point, the reactor was purged daily for the purpose of improving control of the biomass concentration (<10 g VS/L). The average biomass concentration in the sludge bed was slightly higher than 10 g VS/L during the first period (HRT of 3 h), while it was lower during the later period (5 h of HRT). Maximum solids retention time (SRTmax ) decreased by 30% during the period when HUSB was operated at 5 h of HRT. 3.2. Primary treatment efficiency The HUSB reactor produced effluents with significantly lower redox potentials (P = 0.00) than the settler (Table 2). This result concurs with the significantly lower sulphate concentration (P = 0.00)
During the study period the wetlands operated with a hydraulic loading rate of 28.5 mm/d (which corresponds to a theoretical HRT of 3.5 d per line, considering a gravel porosity of 40%). Average surface organic loads were 8.2 ± 3.3 g COD/m2 d and 4.7 ± 1.4 g BOD/m2 d for the control and batch lines, respectively (same primary treatment), and 8.8 ± 3.7 g COD/m2 d and 4.7 ± 1.5 g BOD/m2 d for the anaerobic line (with the HUSB reactor as primary treatment). Average surface ammonium load was approximately 1.1 g NH4 + –N/m2 d, irrespective of the experimental line analysed. All removal efficiencies were calculated in terms of mass removal as a consequence of considerable variations in evapotranspiration, which was estimated from the difference between influent and effluent flow rates. Because legal specifications for discharges are expressed in concentration, the results for the analysed contaminants are also presented in this way (Table 3). Evapotranspiration showed a typical trend, with higher values in summer (ranging from 25 to 30 mm/d) and lower in winter (ranging from 2 to 10 mm/d). In spite of the high rates of evapotranspiration during summer months (as high as the hydraulic loading rate) it was not observed a negative impact in treatment efficiency (Table 3).
Table 2 Average values and standard deviations (in brackets) of water quality parameters analysed in raw wastewater (from the outlet of the storage tank) and effluents of the primary treatments. Data sorted in two periods according to the HRT of the HUSB reactor. First period from March 2007 to December 2008 and second from January 2009 to July 2009. n changes depending on the frequency of each parameter analysed. Period
pH EH (mV) Turbidity (NTU) TSS (mg/L) COD (mg/L) COD dissolved (mg/L) BOD5 (mg/L) NH4 + –N (mg/L) SO4 = –S (mg/L)
n
58 58 57 52 46 6 18 55 44
First
n
Raw wastewater
Settler
HUSB (HRT = 3 h)
7.99 (0.22) 15.5 (103) 151 (61.1) 223 (154) 439 (243) 193 (107) 180 (50.2) 26.4 (9.81) 234 (53.6)
7.89 (0.25) 109 (148) 104 (40.8) 101 (47.5) 312 (169) 189 (98.3) 162 (50.7) 25.1 (9.67) 221 (55.9)
7.70 (0.23) -102 (43.7) 102 (46.7) 95.1 (56.9) 307 (157) 184 (72.2) 174 (52.1) 27.8 (9.73) 188 (52.9)
18 19 18 21 18 4 17 21 19
Second Raw wastewater
Settler
HUSB (HRT = 5 h)
7.79 (0.33) 180 (118) 103 (44.8) 161 (116) 293 (94.1) 146 (46.0) 170 (59.7) 25.2 (7.56) 165 (55.3)
7.74 (0.29) 172 (103) 93.3 (27.0) 98.8 (47.5) 235 (52.01) 136 (44.8) 165 (49.1) 20.7 (7.48) 162 (63.0)
7.45 (0.26) -103 (100) 74.0 (30.2) 62.1 (29.7) 234 (50.02) 176 (19.5) 149 (50.5) 26.4 (9.09) 133 (60.4)
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Table 3 Average values and standard deviations (in brackets) of physical and chemical water quality parameters of the effluents of each line. Concentrations and mass loads are shown for TSS, COD, BOD5 , and NH4 + –N.
pH EH (mV) Turbidity (NTU) TSS COD BOD5 +
NH4 –N TN NOx − –N TP (PO4 3− –P) SO4 2−
(mg/L) (g/m2 d) (mg/L) (g/m2 d) (mg/L) (g/m2 d) (mg/L) (g/m2 d) (mg/L) (mg/L) (mg/L) (mg/L)
n
Control
Batch
Anaerobic
76 70 75 75
7.1 (0.4) 3 (92.7) 7.3 (8.0) 11.4 (8.44) 0.3 (0.7) 60.3 (76.04) 2.0 (2.9) 20.3 (15.6) 0.3 (0.2) 6.07 (8.43) 0.15 (0.23) 7.45 (8.00) 0.09 (0.11) 2.53 (3.16) 374 (393)
7.1 (0.4) −5 (71) 7.0 (7.0) 10.5 (9.75) 0.2 (0.3) 55.8 (86.06) 1.6 (2.4) 15.0 (13.3) 0.2 (0.2) 4.11 (6.69) 0.10 (0.16) 4.84 (4.28) 0.14 (0.21) 1.87 (2.51) 453 (473)
7.2 (0.4) −45 (78) 8.6 (10.9) 10.6 (10.4) 0.2 (0.2) 63.3 (62.8) 2.4 (2.6) 35.4 (14.8) 0.6 (0.6) 7.36 (9.13) 0.20 (0.28) 5.58 (5.63) 0.10 (0.13) 3.47 (4.61) 262 (301)
65 35 76 10 28 23 63
The average effluent redox potential of the anaerobic line was significantly lower than for the other lines (P = 0.00) (Table 3). However, there were no significant differences in the average redox potential between batch and control lines (P = 0.21). All treatment lines removed TSS above 90%, and the average effluent mass loads were not significantly different (P = 0.147). The batch line had slightly higher COD mass removal efficiency (88%) than control (P = 0.084) and anaerobic lines (83% and 80%, respectively). There was no significant difference between anaerobic and control lines (P = 0.270). COD effluent mass loads presented a seasonal pattern with higher values in winter than in summer (Fig. 4). These higher values were less noticeable for the control and batch lines in winter 2008–2009. Batch and control lines had higher BOD5 mass removal efficiencies (96% and 94%, respectively) than the anaerobic line (87%); in fact, effluent mass loadings were significantly higher (P = 0.00) for the anaerobic line than the other two lines. Note that effluents of the anaerobic line had a higher organic matter content (measured as COD or BOD5 ) in accordance with the significantly lower redox potential values. The batch line had a higher ammonium mass removal efficiency (87%; P = 0.016) than the control line (80%) and the anaerobic line (73%). Lower ammonium removal efficiencies observed in the anaerobic line again concur with the significantly lower redox
potential. Ammonium effluent mass loads also presented a seasonal pattern, which was even more noticeable than the pattern observed for COD (Fig. 5). Note that the ammonium removal efficiencies ranged from almost zero in winter time to 100% during most of the summer. In fact, in summer the three lines were very efficient in ammonium removal. In addition, it is interesting to point out that the effluent ammonium mass was clearly lower in winter for the batch line (on average ± SD 0.24 g/m2 d ± 0.18) than for the other two lines (0.46 ± 0.13 and 0.53 ± 0.20 g/m2 d for control and anaerobic lines, respectively), which leads to a higher overall removal efficiency observed during the entire study for the batch line. Sulphate concentration increased from the influents to the effluents due to both, the overall driving evapotranspiration and by conversion of hydrogen sulphide to sulphate due to oxic conditions. Effluent concentrations were significantly different (P = 0.002) and lower in the anaerobic line (30% lower than the control line) in connection with the lower redox potential (Table 3). Sulphate concentrations were highest in the batch line. Comparable to the other parameters studied, total phosphorus mass removal efficiency was higher for the batch line (75%) than for the control (66%). And again, the anaerobic line presented the worst efficiencies (54%) (Table 3), although it was not statistically different (P = 0.273).
Fig. 4. Temporal changes of the influent COD mass loads (settler and HUSB reactor) and the effluent mass loads of each treatment line. Monthly averages are shown for the sake of clarity.
Fig. 5. Temporal changes of the influent ammonium mass loads (settler and HUSB reactor) and the effluent mass loads of each treatment line. Monthly averages are shown for the sake of clarity.
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4. Discussion An overview of the general performance of the plant will be discussed in this section. After that, the effect of primary treatment will be addressed by comparing control and anaerobic lines, whereas the effect of operating strategy will be examined by comparing control and batch lines. 4.1. Wetlands general performance The average organic loading rate in the wetlands of the three lines (4.7 ± 1.5 g BOD/m2 d) was slightly lower than the design load (6 g BOD/m2 d), but still in the range of 4–6 g BOD/m2 d generally recommended for horizontal subsurface flow treatment wetlands (Kadlec and Knight, 1996; García et al., 2005; Akratos and Tsihrintzis, 2007). In the 3 lines of the experimental plant the overall organic matter (COD) removal efficiency was in the commonly attained range (80% for anaerobic, 83% for control and 88% for batch, respectively). Ammonium mass removal efficiency, however, was higher than the values usually described for this type of systems (73% for anaerobic, 80% for control and 87% for batch) (Austin and Nivala, 2009; Vymazal and Kröpfelová, 2009). This is due to two characteristics observed previously and considered in the design: (1) low water depth (0.25 m) (García et al., 2005) and (2) intermittent feeding (Caselles-Osorio and García, 2007a,b). These shallow wetlands, fed intermittently, have a good capacity to carry out whole nitrogen removal and the results of the present study support this statement (low effluent concentrations of ammonium as well as oxidised nitrogen, see Table 3). A seasonal pattern for COD and ammonium was observed, with lower mass values in warm months (Fig. 4). Seasonal variations in COD are not generally seen in these types of systems (García et al., 2005; Ruíz et al., 2008; Serrano et al., 2009; Vymazal and Kröpfelová, 2009), although seasonal differences have been largely demonstrated for ammonium (Caselles-Osorio and García, 2007a; Kuschk et al., 2003). In this study, when COD removal was analysed in terms of concentration, higher removal in summer was not clearly observed. This could be due to the effect of evapotranspiration, which was very high in these months (25–30 mm/d). Accordingly, an increase of evapotranspiration leads to an increase in organic matter concentration, which could mask greater removal in summer. 4.2. Effect of primary treatment Settlers only provide physical wastewater treatment, in which removal efficiencies obtained for the main contaminants were in the common range of a primary settler with 2 h of HRT (Metcalf and Eddy, 2003), though efficiencies are related to raw wastewater characteristics (Mitchell and McNevin, 2001). The lower redox potentials and pH observed in the effluents of the HUSB during the entire study period are indicative of bacterial hydrolysis. Despite this biological activity in the HUSB reactor, differences in contaminant removal between the two types of primary treatment were only clearly observed in the period when the HUSB was operated at an HRT of 5 days. This is due to its operational parameters (Serrano et al., 2009; Álvarez et al., 2008a). Accordingly, higher removal efficiencies for TSS were recorded during the second operation period (5 h HRT), when the upflow velocity decreased from 0.46 to 0.27 L/h (Table 1). HUSB reactor operating at 5 h of HRT produced water with 20% fewer suspended solids and more biodegradable water than the settlers (Table 2). Accordingly, in this period the BOD5 /COD ratio changed from 41% in raw wastewater to 64% and 53% in HUSB and settler effluents, respectively. Similar values were recorded by Barros et al. (2008) for UASB reactors.
The anaerobic line had high removal efficiencies for selected pollutants (on average 80%, 87% and 73% for COD, BOD5 and ammonium, respectively), with results similar to those of previous studies (Singh et al., 2009; Ruíz et al., 2008), and effluent concentrations were lower than the standards set by Council Directive 91/271EEC. However, removal efficiencies for the anaerobic line were in all cases below those recorded for the control line (83%, 94% and 80% for COD, BOD5 and ammonium removal efficiency, respectively). These lower removal efficiencies concurred with lower average redox potentials observed in the effluents of the anaerobic line than in the control line (Table 3). The biological activity of the HUSB reactor produced more reduced primary effluents, which were eventually responsible for the more reduced environment of the wetlands in comparison to the wetlands of the control line. In winter ammonium removal efficiency was lower in the anaerobic line than in the control line (P = 0.000), whereas this difference was minimized during warm seasons (P = 0.165) (Fig. 5). It is generally accepted that in warm seasons the active macrophytes enhance pollutant removal (Tanner et al., 1998; Brix, 1997), probably due to changes in water level mediated by evapotranspiration (Breen, 1997; Tanner et al., 1999). Accordingly, active plants of the wetlands in the anaerobic line may have improved the treatment efficiency of the system during these months. However, for the whole period of study there was a significant difference between the two treatments (P = 0.007). The results of this study indicate that the implementation of a HUSB reactor coupled with treatment wetlands under the conditions tested in this study does not benefit contaminant removal in comparison of a settler-wetland system, though pollutants concentrations were generally below the law limits. However, the HUSB reactor has a higher degree of solids removal in comparison with a conventional settler and its implementation could prevent early clogging of the wetlands (Shepherd et al., 2001). Note that all treatment lines presented an excellent removal of TSS (>90%). If these solids are not readily biodegradable, then they will contribute to clogging. Therefore, improved primary treatments are of capital importance in order to reduce the clogging. 4.3. Effect of operation strategy In general, the batch line had slightly higher removal efficiencies (between 2% and 10%) than the control line for the main contaminants (Table 3), and similar to those generally described for vertical SSF TWs (Brix and Arias, 2005). Currently, the operation strategy of this line, alternating saturated and unsaturated phases, is comparable to the operation of vertical flow systems. Higher removal efficiencies for the batch line seem to be due to the prevalence of a less reduced environment of the wetlands in this line. This hypothesis is confirmed by 32% less ammonium and 21% more sulphate concentrations in the effluents of the batch line than in the control line (Table 3). However, the results of average redox potential (without significant differences between both lines, Table 3) contrast with the less reduced environment for the batch line stated above. Lack of differences in redox potential may have been an artefact, due to the operation of the batch line with cycles alternating batch-unsaturated and permanently saturated phases (note that redox potential was recorded in the effluent once per week and always at the same time). As a result of this type of operation, high daily variations in the flow rate and redox potential for the batch line are encountered. Fig. 6 shows temporal changes in redox potential and ammonium concentration obtained from the online sensors in four representative weeks during the cold season (Fig. 6a and c) and the warm season (Fig. 6b and d) in control and the batch lines. As stated above, daily redox potential variations in the batch line are con-
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Fig. 6. Temporal changes of redox potential and ammonium in control and batch lines during one month in winter (a, c) and another in spring (b, d). For redox potential 6 values per day are shown (recorded every 4 h, coinciding with influent discharges) while for ammonium 1 value per day was recorded with online sensors.
siderable, in relation to the type of operation. Redox values were generally lower in February (average −141 mV and −1 mV for the control and the batch lines, respectively) than in May (149 mV and 189 mV for the control and the batch lines). This behaviour concurred with ammonia concentrations, which were lower in May (0.4 ± 0.3 mg N/L for both lines, P = 0.05) than in February (26.9 ± 6.1 mg N/L and 10.7 ± 3.01 mg N/L for the control and the batch lines, P = 0.58). Caselles-Osorio and García (2007a) found a similar temporal opposite relationship between redox potential and ammonium concentration. It can therefore be concluded that differences between systems occur because the batch operation creates a more oxidised environment in winter, which improves its performance during these months (approximately 50% higher removal efficiency) and is eventually responsible for improved overall annual removal efficiencies (approximately 10% higher removal efficiency). In winter, when the reeds were dry and harvested, the plant aeration effect (Tanner et al., 1999, 2002; Davies et al., 2006) may have been negligible and the batch operation may have promoted a less reduced environment, subsequently enhancing ammonium removal. Note that in February redox potential in the control line was indicative of clearly anaerobic conditions while for batch line was in the range of anoxic environments (Fig. 6a), in which alternatives ways to remove ammonium (and not oxygen dependent) can be given (Tanner et al., 2002). This is in accordance with the lower ammonium concentration recorded for batch line (Fig. 6c). The results of this study confirm that aeration of the granular medium of the wetlands occurring after draining of the batch line in small wetlands helps to increase the removal efficiency of organic matter and ammonium in treatment wetland systems
(Tanner et al., 1999; Chazarenc et al., 2009). The difference in our study in comparison with previous reports is that this increased efficiency is only observed in winter months. 5. Conclusions This study demonstrates that shallow horizontal SSF TWs are good systems for wastewater treatment and can remove contaminants at high rates (around 80% for COD and ammonium, and 90% for TSS and BOD5 ) because of shallow beds and intermittent feeding. The implementation of a HUSB reactor as a primary treatment for horizontal subsurface-flow treatment wetlands does not improve contaminant removal efficiency in comparison with a conventional settler in the conditions tested here. On the contrary, the HUSB reactor slightly decreases the efficiency of the system due to the reduced nature of its effluents, though maintaining the effluent concentration under the law limits. Moreover, the application of a HUSB reactor (in this case operated at 5 h of hydraulic retention time) can significantly reduce the amount of solids entering a wetland (20% fewer suspended solids than conventional settlers) and therefore help prevent or delay clogging processes. A treatment wetland system composed of horizontal subsurface flow wetland operated with filling–resting–drain phases (as in the batch line) offers higher efficiencies in contaminant removal than a normally operated system (as in the control line) (between 5% and 10% more efficient in COD and ammonium removal, respectively). However, differences in contaminant removal (especially in terms of ammonium) are season-dependent. In this regard, differences in ammonium removal between a system operated under batch
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conditions and a system operated continuously are maximized in winter (up to 50% higher than a continuously fed system). The results obtained in this study should be taken in consideration in the design and operation of horizontal SSF TWs. After a long experimentation period, we have provided sufficient data and clearly demonstrated that the efficiency of shallow horizontal SSF TWs can be increased, particularly in cold months, using a batch operation strategy. Acknowledgements This study was made possible by funding from the Spanish Ministry of Innovation and Science for the NEWWET 2008 Project (CTM2008-06676-C05-01). Anna Pedescoll also acknowledges the fund provided by the Spanish Ministry of Innovation and Science. The authors are particularly grateful to Enrica Uggetti, Gemma Lesan, Gian Paolo Mottola and Fedro Tapia for their help during the construction of the experimental plant; to Toni Lara, Carlos Soriano, ˜ G. Admirable and Roger Samsó for their help with mainteBegona nance tasks; and finally, to Elif Bozdogan, Javier Carretero, Adriana Gallardo and Miriam Planas for their contribution to laboratory tasks. References Akratos, C.S., Tsihrintzis, V.A., 2007. Effect of temperature, HRT, vegetation and porous media on removal efficiency of pilot-scale horizontal subsurfaceflow constructed wetlands. Ecological Engineering 29, 173–191. Álvarez, J.A., Ruíz, I., Soto, M., 2008a. Anaerobic digesters as a pretreatment for constructed wetlands. Ecological Engineering 33, 54–67. Álvarez, J.A., Armstrong, E., Gómez, M., Soto, M., 2008b. Anaerobic treatment of lowstrength municipal wastewater by a two-stage pilot plant under psychrophilic conditions. Bioresource Technology 99, 7051–7062. APHA-AWWA-WPCF, 2001. Standard Methods for the Examination of Water and Wastewater, 20th ed. APHA-AWWA-WPCF, Washington DC, USA. Austin, D., Nivala, J., 2009. Energy requirements for nitrification and biological nitrogen removal in engineered wetlands. Ecological Engineering 35, 184–192. Barros, P., Ruiz, I., Soto, M., 2008. Performance of an anaerobic digester-constructed wetland system for a small community. Ecological Engineering 33, 142–149. Breen, P.F., 1997. The performance of vertical flow experimental wetlands under a range of operational formats and experimental conditions. Water Science and Technology 35 (5), 167–174. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Science and Technology 35 (5), 11–17. Brix, H., Arias, C., 2005. The use of vertical flow constructed wetlands for on-site treatment of domestic wastewater: New Danish guidelines. Ecological Engineering 25, 491–500. Calheiros, C.S.C., Duque, A.F., Moura, A., Henriques, I.S., Correia, A., Rangel, A.O.S.S., Castro, P.M.L., 2009. Substrate effect on bacterial communities from constructed wetlands planted with Typha latifolia treating industrial wastewater. Ecological Engineering 35, 744–753. Caselles-Osorio, A., García, J., 2007a. Impact of continuous and intermittent feeding strategies on the performance of shallow horizontal subsurface-flow constructed wetlands. Science of the Total Environment 378, 253–262. Caselles-Osorio, A., García, J., 2007b. Effect of physico-chemical pre-treatment on the removal efficiency of experimental horizontal subsurface-flow constructed wetlands. Environmental Pollution 146, 55–63. Chazarenc, F., Gagnon, V., Comeau, Y., Brisson, J., 2009. Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands. Ecological Engineering 35, 1005–1010. Davies, L.C., Pedro, I.S., Novais, J.M., Martins-Dias, S., 2006. Aerobic degradation of acid orange 7 in a vertical constructed wetland. Water Research 40, 2055– 2063.
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