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Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 1005–1010
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands F. Chazarenc a,b,∗ , V. Gagnon a , Y. Comeau b , J. Brisson a a
Institut de Recherche en Biologie Végétale, Département de Sciences biologiques, Université de Montréal, 4101 rue Sherbrooke Est, Montreal, Québec, Canada H1X 2B2 b Department of Civil, Geological and Mining Engineering, Ecole Polytechnique 2900, Edouard-Montpetit, Montreal, Québec, Canada H3T 1J4
a r t i c l e
i n f o
a b s t r a c t
Article history:
In constructed wetlands, solids accumulation may have two consequences with opposing
Received 16 January 2008
effects on treatment efficiency: it decreases the longevity by reducing void space and it
Received in revised form 1 July 2008
enhances biological activity by favoring biofilm development. The goal of our study was to
Accepted 11 July 2008
estimate the effect of plants (presence and species) and artificial aeration on solids accumulation (volatile and inorganic). The horizontal and vertical distribution of solids was sampled using solids traps in 12 constructed wetland mesocosms (5 years old). Microbial density and
Keywords:
activity were estimated in the biological fraction of the sampled solids. The effect of plant
Water treatment
presence reduced accumulated solids by 26% and sulphide content by 50% sulphide content.
Typha angustifolia
There was more solids accumulation in Typha angustifolia units than in Phragmites australis.
Phragmites australis
Also, T. angustifolia generated more biological activities at the surface and close to the inlet
Mesocosms
while conditions were more homogeneous throughout P. australis units. Aeration (1) stimu-
Microbial activity
lated biofilm development at the inlet of planted beds, (2) seemed to reduce mineral matter
Sulphide
accumulation and (3) generated the same pattern of activities in planted beds enabling to reach a total nitrogen removal rate of up to 0.65 g N m−2 d−1 . © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
In constructed wetland systems (CWs) for wastewater treatment, solids accumulation may have two consequences with opposing effects on treatment efficiency: it decreases the longevity by reducing void space and it enhances biological activity by favoring biofilm development. The goal of our study was first to estimate the effect of plant (presence and species) and artificial aeration on solids accumulation (volatile and inorganic), and then to measure the accumulated solids composition and biological activities.
In horizontal flow CWs, solids accumulation has been recognized as a major factor decreasing CW longevity (Cooper et al., 2005). The origin of solids accumulated in void space of CWs is attributed to a fraction of TSS and VSS contained in the influent (Tanner and Sukias, 1995; Tanner et al., 1998; Caselles-Osorio et al., 2007), to biological growth within the CWs (Kadlec and Watson, 1993; Suliman et al., 2006), and to plant litter deposition (Tanner et al., 1998; Nguyen, 2000). The presence of sulphide accumulation is linked to overloading conditions or a lack of aerobic conditions (Wiessner et al., 2005; Stein et al., 2007).
∗ Corresponding author. Current address: École des Mines de Nantes, Département Systèmes Energétiques et Environnement, 4 rue Alfred Kastler, BP 20722, F-44307 Nantes Cedex 3, France. Tel.: +33 2 51 85 86 93; fax: +33 2 51 85 82 99. E-mail address: fl[email protected] (F. Chazarenc). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.07.008
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e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 1005–1010
Plant presence reduces clogging (Brix, 1997) and increases biological activity (Stottmeister et al., 2003), but at the same time favors solids accumulation by mean of plant litter deposition (Tanner and Sukias, 1995). We did not find studies dealing with the effect of plant species on solids accumulation. However, the effect of plants on micro-organism activity was found to vary according to plant species, and this effect may be caused by species differences in root morphology (Gagnon et al., 2007). Thus, we hypothesize that the difference between plant belowground morphology may be a factor generating differences in solids accumulation related to biological growth. The use of artificial aeration in CWs was initially developed to treat recalcitrant wastewaters in cold climate (Higgins et al., 1999). It increases biological activities and stimulates nitrifying/denitrifying mechanisms (Davies and Hart, 1990; Cottingham et al., 1999). While artificial aeration is thought to increase CWs’ life span (Ouellet-Plamondon et al., 2006), its direct effect on solids accumulation has not been measured. Artificial aeration may have a significant effect on solids accumulation since it can both increase biological growth and reduce VSS accumulation. The horizontal and vertical distribution of solids, the biological fraction accumulated and its activity were sampled using plastic solids traps inserted in the units during 6 months of operation. The goal of this study was to determine the influence of plant presence and species, and artificial aeration on solids accumulation using a well established pilot system (5 years old) composed of 12 CWs mesocosm units. Three plant conditions (Phragmites australis, Typha angustifolia and unplanted control) were tested in the presence and absence of artificial aeration.
2.
Materials and methods
2.1.
Wetland system
Four CW mesocosm units (1 m2 ) were planted with P. australis, four with T. angustifolia, and four were left unplanted. Half of the units in each treatment were aerated continuously with a diaphragm air pump using a perforated horizontal 20-cm diameter circular tubing at the bottom entrance of the beds, at an airflow of 2 ± 1 L/min. All units were intermittently fed with a reconstituted fish farm effluent using a hydraulic loading rate of 6 cm/d and an organic loading (in g m−2 d−1 ) of 3.2 TSS, 9.1 COD and 0.8 TKN, resulting in a concentration of (in mg/L): TSS: 50, COD: 150, TN: 15, TP: 15. More details on the experimental system are presented in Chazarenc et al. (2007).
2.2.
Solids sampling and extraction
In early May 2005, cores (D = 4.6 cm, L = 30 cm) containing clean gravel (2–6 mm, the same diameter as the one used in the bed matrix) were inserted in 2 piezometers located near the inlet (I) and outlet (O) of each unit (Fig. 1). The cores were made of polyester mesh with a pore diameter of 5 mm. The cores were removed after 6 months of operation, in early November 2005. The gravel in each core was divided in two parts according to depth (0–15 and 15–30 cm). The solids accumulated in each part were extracted by shaking vigorously using 400 mL of a
Fig. 1 – Schematic view of an aerated unit planted with Typha.
phosphorus buffer (K2 HPO4 9.3 g/L; KH2 PO4 1.8 g/L) leading to an accumulated solids solution (AS solution).
2.3.
Solids analysis
Total matter and volatile matter were determined, respectively, by drying 6 mL of the AS solution at 105 ◦ C during 24 h and burning the dry residue at 550 ◦ C during 15 min. Sulphide concentration was determined by diluting 0.5 mL of AS solution in 12 mL of distilled water and by using the methylene blue method (8131) provided by Hach Company. Bacteria were counted by flow cytometry (FACScan, Becton and Dickinson, San Jose, USA) using the protocol adapted by Gagnon et al. (2007). Potential aerobic respiration was measured with a respirometer (AER-200, Challenge Environmental Systems) on 100 mL of AS solution, under continuous agitation for 5 h. Enzyme activity was measured using fluorescein diacetate (FDA, Sigma–Aldrich) with a protocol adapted by Gagnon et al. (2007). Dehydrogenase activity was measured by the reduction rate of p-iodonitro-tetrazolium violet (INT, Molecular Probes) to INT-Formazan (INT-F). Dehydrogenase was assessed using 0.25 mL of INT (4 mM) and 1 mL of AS solution incubated during 1 h at room temperature. Reaction was stopped with 0.25 mL of formaldehyde (37%) and the Formazan precipitate was extracted using methanol for 30 min. Supernatant absorbance (480 nm) was measured and INT-F concentration was calculated using Beer–Lambert’s law (ε = 15,500 M−1 cm−1 ).
2.4.
Water sampling and analysis
From May 2005 to November 2005, influent/effluent were collected at different intervals following a 24-h sampling (total 15 days of sampling). The following parameters were measured according to APHA et al. (1998): total suspended solids (TSS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammonium (NH4 ), and nitrate (NO3 ). Evapotranspiration (ET) was estimated by measuring the total
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 1005–1010
inflow and outflow. Removal efficiencies were calculated on a mass balance basis. All statistical analyses were performed with SPSS 16.0 statistical software using a two-way repeated measures analysis of variance (ANOVA) followed by a test of multiple comparisons (Tukey HSD) to assess the differences among treatments (aeration, macrophytes). Statistical tests were considered significant at the 0.05 level. All variables met normality assumptions and had homogeneous variance.
to a better N-NH4 removal compared to unplanted aerated units, which is in agreement with previous studies using the same units (Ouellet-Plamondon et al., 2006; Maltais-Landry et al., 2007). In aerated units, a higher nitrate residual concentration was observed especially in planted beds (0.13 and 0.11 g N NO3 m−2 d−1 for Phragmites and Typha, respectively), probably as a consequence of the extra oxygen supplied that stimulated the nitrification step by providing an aerobic environment.
3.2.
3.
Results and discussion
3.1.
Treatment performances
TSS and COD removal rates averaged respectively 75% and 60% for all units (results not shown). These removal rates did not vary according to plant conditions, but they were slightly enhanced by artificial aeration, as observed by Ouellet-Plamondon et al. (2006) using the same experimental system. Nitrogen removal was greatly influenced both by plant presence and artificial aeration: N-TKN removal rates averaged from 0.31 g N m−2 d−1 in unplanted mesocosms to 0.5 g N m−2 d−1 in planted mesocosms and up to 0.65 g N m−2 d−1 in planted mesocosms with artificial aeration. The TKN load was mainly in the form of urea and artificial aeration probably promoted ammonification since all the N-TKN was under ammonium form at the outlet of planted and aerated mesocosms (Fig. 2). In unplanted mesocosms, additional aeration did not fully compensate for the absence of plants as tot-N removal was greater in planted mesocosms (55.8 and 63.8% for Phragmites and Typha, respectively) compare to unplanted aerated (55.7%). No effect of plant species was observed on treatment performances. There was a combined effect of plant and artificial aeration that led
Solids accumulation
Solids (dry matter, DM) accumulation rates ranged from 7.2 to 13.2 kg DM m−2 year−1 (Fig. 3). Although our experimental system has been operating for more than 5 years, this accumulation rate is closer to those observed in young CWs having less than 2 years of operation (from 1.3 to 3.0 kg DM m−2 year−1 Tanner et al., 1998; from 0.6 to 14.3 kg DM m−2 year−1 in Caselles-Osorio et al., 2007). This elevated accumulation rate could be a consequence of the large void space available since we used clean gravel in the traps. Also, the amount of solids accumulated was probably increased by the exportable solids present in the vicinity of the trap core. On average, the rate of solids accumulation was 12% greater near the inlet of all units (Fig. 3). Hydraulic conductivity was reduced in unplanted units without aeration where presence of surface water was occasionally observed during feeding operation. The effect of artificial aeration on solids accumulation depended on plant treatment, with a clear reduction (−26%) in unplanted units, a slight increase (11%) in Typha units, and no apparent effect (−1%) in Phragmites units (Fig. 3). Under our experimental conditions, artificial aeration did not reduce solids accumulation in planted units. However, it must be noted that our study was conducted in summer, so that in winter conditions, when plants are dormant, artificial aeration could play a greater role in reducing solids accumulation.
3.3.
Fig. 2 – Average nitrogen mass flow measured between inlet and outlet of the mesocosms (n = 15). Other N forms include particulate TKN, urea, and every other form except N-NH4 . Repeated measures ANOVA was used to test differences between the aerated and non-aerated CWs (F(1, 6) = 46.4, p < 0.001) and among macrophyte treatments (F(2, 6) = 25.8, p < 0.001). Capital letters denote significant differences between aerated and non-aerated CWs, and lower case letters denote significant differences between macrophyte treatments within aerated or non-aerated conditions, using a Tukey HSD test.
1007
Volatile matter accumulation
On average, volatile matter (VM) represented 23% of total accumulated solids, which is comparable to results obtained by Caselles-Osorio et al. (2007) with VM < 20% in samples from full scale CWs. The effect of Phragmites and Typha was similar and led to a net decrease (45%) of accumulated VM in non-aerated units (Fig. 4A). Differences between plant species in others studies was attributed to differences in plant litter deposition (Tanner and Sukias, 1995; Tanner et al., 1998). The fact that all aerial biomass was removed by hand each fall in our experimental setup reduced the possible contribution of plant litter to solids accumulation, especially on the surface of the units. Artificial aeration generated a decrease of 33% VM in unplanted beds and led to an increase in planted units (11 and 66% for Phragmites and Typha, respectively), especially near the inlet, in the vicinity of the aeration system. In terms of solids and VM accumulation, our results suggest that artificial aeration may have both positive and negative effects, first in favoring aerobic degradation of pollutants (thus reducing solids accumulation) and second in mobilizing more VM (probably under biofilm form), espe-
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e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 1005–1010
Fig. 3 – Annual solids accumulation rate (in dry matter) in the CWs units (In and Out: inlet and outlet cores), average of two replicates.
cially in planted units. However, the studied period was too short to be conclusive regarding the possible long-term effect of artificial aeration in reducing inlet or outlet clogging.
3.4.
Sulphide accumulation
As observed by Stein et al. (2007), the mean amount of sulphide accumulated was dramatically reduced by plant and/or artificial aeration (from 1 g S/m2 in unplanted to 0.3 g S/m2 in planted mesocosms with aeration, Fig. 4B). Diakova et al. (2006) measured a large proportion of iron under Fe2+ form in CW’s which mainly precipitated with S2− . It was most probably the case in our experiment since the sulphides created a typical black layer at the bottom of the cores. The use of artificial aeration decreased sulphide content even in planted beds (especially in Phragmites: from 0.42 to 0.26 g S/m2 ). N removal rates were probably positively influenced by low sulphide content since S2− is considered a nitrification inhibitor in CWs (Wiessner et al., 2005). However, sulphide accumulation and thus SO4 2− removal are preferred in CWs designed to retain metals: in these cases, artificial aeration and even plant presence are not recommended (Stein et al., 2007). Our sampling method enabled us to collect all the accumulated S2− that were at the very bottom of the core and that can be omitted when using partial sampling (Caselles-Osorio et al., 2007). In CWs, sulphide accumulation is often related to the presence/dominance of sulphato-reducing vibrio bacteria. In CWs designed to treat domestic wastewater, sulphide accumulation is rather a sign of malfunction caused by COD overloading and excessive solids accumulation (Stein et al., 2007; Wiessner et al., 2005). Thus, the use of artificial aeration can help to decrease sulphide formation by increasing aerobic COD removal.
3.5.
Biological activities
Plant presence multiplied potential respiration rate recorded in samples by 5 for Phragmites and by 1.3 for Typha, but seemed to have no effect on potential dehydrogenase activity (Fig. 4). Potential enzymatic activity was even greater in inlet cores of unplanted units, which seems to be in accordance with results from McHenry and Werker (2005) who measured no influence of plants in mesocosms using FDA as enzymatic tracer in real conditions. Summer aerobic activities may be limited in CWs (Stein and Hook, 2005) although our results suggest that potential respiration remained significant in planted beds especially with Phragmites. There was a plant species effect on biological activity in non-aerated units (Fig. 4), although removal rates of TKN and COD were similar for Phragmites and Typha (Fig. 2). Respiration rates appear to be higher at the inlet for Typha (17 g O2 d−1 m−2 at inlet and 6 g O2 d−1 m−2 at outlet) but were more homogeneous in cores sampled in Phragmites units (about 40 g O2 d−1 m−2 at inlet and outlet, Fig. 4D). The same pattern was observed to a lesser extent for bacteria count and potential enzymatic activity (Fig. 4C and F). These results are in agreement with those of Gagnon et al. (2007) who also measured a greater plant biomass in the upper layer for Typha but a more equally distributed biomass along bed depth for Phragmites. Typha was associated with more solids accumulation than Phragmites in non-aerated units. As roots and rootlets densities were observed to be proportional to root surface throughout depth (Gagnon et al., 2007), we hypothesized that Typha below ground biomass generated more organic compounds than Phragmites, but further analysis are required to confirm our results. Providing artificial aeration generated a net increase of bacteria count and biological activities in the inlet zone in both Phragmites and Typha units (Fig. 4). Combined with plant pres-
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 1005–1010
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Fig. 4 – Biological parameters and activities recorded in solids sampled from various locations (inlet, outlet; top, bottom) in the aerated and non-aerated CWs. Results are the average of two replicates for each mesocosm. In and Out: inlet and outlet zones.
ence, artificial aeration always increased microbial activities probably leading to a change of both microbial community structure and diversity. It could be interesting to determine microbial diversity in response to artificial aeration in future studies.
4.
Conclusions
The effect of plant presence was mainly observed on TKN removal and led to a decrease of accumulated solids and
sulphide. We found no effect of plant species on treatment performance but solids accumulation was greater in Typha units. Also, Typha generated more biological activities at the surface and close to the inlet while conditions were more homogeneous throughout the Phragmites units. Volatile matter accumulation was reduced by plant presence and by aeration in unplanted mesocosms. Aeration (1) stimulated biofilm development at the inlet of planted beds, (2) seemed to reduce mineral matter accumulation and (3) generated a similar pattern of activities in planted beds. In planted beds, aeration resulted in a greater biological activity
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(more bacteria, potential respiration rates, dehydrogenase and enzymatic activities). The combination of plants and aeration improved TN treatment efficiency. The influence of artificial aeration on microbial community should be investigated in terms of microbial diversity.
Acknowledgements The authors thank Carole Radix and Denis Bouchard for technical assistance. This research was funded by the Natural Science and Engineering Research Council of Canada (NSERC).
references
APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association/American Water Works Association/Water Environment Federation, Washington, DC, USA. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Sci. Technol. 35 (5), 11–17. Caselles-Osorio, A., Puigagut, J., Segú, E., Vaello, N., Granés, F., García, D., García, J., 2007. Solids accumulation in six full-scale subsurface flow constructed wetlands. Water Res. 41 (6), 1388–1398. Chazarenc, F., Maltais-Landry, G., Troesch, S., Comeau, Y., Brisson, J., 2007. Effect of loading rate on performance of constructed wetlands treating an anaerobic supernatant. Water Sci. Technol. 56 (3), 23–29. Cooper, D., Griffin, P., Cooper, P., 2005. Factors affecting the longevity of sub-surface horizontal flow systems operating as tertiary treatment for sewage effluent. Water Sci. Technol. 51 (9), 127–135. Cottingham, P.D., Davies, T.H., Hart, B.T., 1999. Aeration to promote nitrification in constructed wetland. Environ. Technol. 20, 69–75. Davies, H.T., Hart, B.T., 1990. Use of aeration to promote nitrification in reed beds treating wastewater. In: Cooper, P.F., Findlater, B.C. (Eds.), Constructed Wetlands in Water Pollution Unplanted. Pergamon Press, Oxford, UK, pp. 383–389. Diakova, K., Holcova, V., Sima, J., Dusek, J., 2006. The distribution of iron oxidation states in a constructed wetland as an indicator of its redox properties. Chem. Biodivers. 3, 1288–1300. Gagnon, V., Chazarenc, F., Comeau, Y., Brisson, J., 2007. Influence of macrophytes species on microbial density and activity in constructed wetlands. Water Sci. Technol. 56 (3), 249–254.
Higgins, J.P., Hurd, S., Weil, C., 1999. The use of engineered wetlands to treat recalcitrant wastewaters. In: Proceedings of the 4th International Conference on Ecological Engineering, Oslo, Norway, June. Kadlec, R.H., Watson, J.T., 1993. Hydraulics and solids accumulation in a gravel bed treatment wetland. In: Moshiri, G.A. (Ed.), Constructed Wetlands for Water Quality Improvement. Lewis, Boca Raton, MI, pp. 227–235. Maltais-Landry, G., Chazarenc, F., Comeau, Y., Troesch, S., Brisson, J., 2007. Effects of artificial aeration, macrophyte species and loading rate on removal efficiency in constructed wetland mesocosms treating fish farm wastewater. J. Environ. Eng. Sci. 6, 409–414. McHenry, J., Werker, A., 2005. In-situ monitoring of microbial biomass in wetland mesocosms. Water Sci. Technol. 51 (9), 233–241. Nguyen, L.M., 2000. Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters. Ecol. Eng. 16 (2), 199–221. Ouellet-Plamondon, C., Chazarenc, F., Comeau, Y., Brisson, J., 2006. Artificial aeration to increase pollutant removal of constructed wetlands in cold climate. Ecol. Eng. 27, 258–264. Stein, O.R., Hook, P.B., 2005. Temperature, plants and oxygen, how does season affect constructed wetland performance? J. Environ. Sci. Health, Part A, Toxic/Hazard. Subst. Environ. Eng. 40, 1331–1342. Stein, O.R., Borden-Stewart, D.J., Hook, D.J., Jones, W.L., 2007. Seasonal influence on sulfate reduction and zinc sequestration in subsurface treatment wetlands. Water Res. 41 (15), 3440–3448. Stottmeister, U., Wießner, A., Kuschk, P., Kappelmeyer, U., Kästner, M., Bederski, O., Müller, R.A., Moormann, H., 2003. Effects of plants and microorganisms in constructed wetlands for wastewater treatment. Biotechnol. Adv. 22, 93–117. Suliman, F., French, H.K., Haugen, L.E., Søvik, A.K., 2006. Change in flow and transport patterns in horizontal subsurface flow constructed wetlands as a result of biological growth. Ecol. Eng. 27 (2), 124–133. Tanner, C.C., Sukias, J.P., 1995. Accumulation of organic solids in gravel-bed constructed wetlands. Water Sci. Technol. 32 (3), 229–239. Tanner, C.C., Sukias, J.P.S., Upsdell, M.P., 1998. Organic matter accumulation during maturation of gravel-bed constructed wetlands treating farm dairy wastewaters. Water Res. 32 (10), 3046–3054. Wiessner, A., Kappelmeyer, U., Kuschk, P., Kästner, M., 2005. Sulphate reduction and the removal of carbon and ammonia in a laboratory-scale constructed wetland. Water Res. 39 (13), 4643–4650.
available at www.sciencedirect.com
journal homepage: www.elsevier.com/locate/ecoleng
Effect of plant and artificial aeration on solids accumulation and biological activities in constructed wetlands F. Chazarenc a,b,∗ , V. Gagnon a , Y. Comeau b , J. Brisson a a
Institut de Recherche en Biologie Végétale, Département de Sciences biologiques, Université de Montréal, 4101 rue Sherbrooke Est, Montreal, Québec, Canada H1X 2B2 b Department of Civil, Geological and Mining Engineering, Ecole Polytechnique 2900, Edouard-Montpetit, Montreal, Québec, Canada H3T 1J4
a r t i c l e
i n f o
a b s t r a c t
Article history:
In constructed wetlands, solids accumulation may have two consequences with opposing
Received 16 January 2008
effects on treatment efficiency: it decreases the longevity by reducing void space and it
Received in revised form 1 July 2008
enhances biological activity by favoring biofilm development. The goal of our study was to
Accepted 11 July 2008
estimate the effect of plants (presence and species) and artificial aeration on solids accumulation (volatile and inorganic). The horizontal and vertical distribution of solids was sampled using solids traps in 12 constructed wetland mesocosms (5 years old). Microbial density and
Keywords:
activity were estimated in the biological fraction of the sampled solids. The effect of plant
Water treatment
presence reduced accumulated solids by 26% and sulphide content by 50% sulphide content.
Typha angustifolia
There was more solids accumulation in Typha angustifolia units than in Phragmites australis.
Phragmites australis
Also, T. angustifolia generated more biological activities at the surface and close to the inlet
Mesocosms
while conditions were more homogeneous throughout P. australis units. Aeration (1) stimu-
Microbial activity
lated biofilm development at the inlet of planted beds, (2) seemed to reduce mineral matter
Sulphide
accumulation and (3) generated the same pattern of activities in planted beds enabling to reach a total nitrogen removal rate of up to 0.65 g N m−2 d−1 . © 2008 Elsevier B.V. All rights reserved.
1.
Introduction
In constructed wetland systems (CWs) for wastewater treatment, solids accumulation may have two consequences with opposing effects on treatment efficiency: it decreases the longevity by reducing void space and it enhances biological activity by favoring biofilm development. The goal of our study was first to estimate the effect of plant (presence and species) and artificial aeration on solids accumulation (volatile and inorganic), and then to measure the accumulated solids composition and biological activities.
In horizontal flow CWs, solids accumulation has been recognized as a major factor decreasing CW longevity (Cooper et al., 2005). The origin of solids accumulated in void space of CWs is attributed to a fraction of TSS and VSS contained in the influent (Tanner and Sukias, 1995; Tanner et al., 1998; Caselles-Osorio et al., 2007), to biological growth within the CWs (Kadlec and Watson, 1993; Suliman et al., 2006), and to plant litter deposition (Tanner et al., 1998; Nguyen, 2000). The presence of sulphide accumulation is linked to overloading conditions or a lack of aerobic conditions (Wiessner et al., 2005; Stein et al., 2007).
∗ Corresponding author. Current address: École des Mines de Nantes, Département Systèmes Energétiques et Environnement, 4 rue Alfred Kastler, BP 20722, F-44307 Nantes Cedex 3, France. Tel.: +33 2 51 85 86 93; fax: +33 2 51 85 82 99. E-mail address: fl[email protected] (F. Chazarenc). 0925-8574/$ – see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.ecoleng.2008.07.008
1006
e c o l o g i c a l e n g i n e e r i n g 3 5 ( 2 0 0 9 ) 1005–1010
Plant presence reduces clogging (Brix, 1997) and increases biological activity (Stottmeister et al., 2003), but at the same time favors solids accumulation by mean of plant litter deposition (Tanner and Sukias, 1995). We did not find studies dealing with the effect of plant species on solids accumulation. However, the effect of plants on micro-organism activity was found to vary according to plant species, and this effect may be caused by species differences in root morphology (Gagnon et al., 2007). Thus, we hypothesize that the difference between plant belowground morphology may be a factor generating differences in solids accumulation related to biological growth. The use of artificial aeration in CWs was initially developed to treat recalcitrant wastewaters in cold climate (Higgins et al., 1999). It increases biological activities and stimulates nitrifying/denitrifying mechanisms (Davies and Hart, 1990; Cottingham et al., 1999). While artificial aeration is thought to increase CWs’ life span (Ouellet-Plamondon et al., 2006), its direct effect on solids accumulation has not been measured. Artificial aeration may have a significant effect on solids accumulation since it can both increase biological growth and reduce VSS accumulation. The horizontal and vertical distribution of solids, the biological fraction accumulated and its activity were sampled using plastic solids traps inserted in the units during 6 months of operation. The goal of this study was to determine the influence of plant presence and species, and artificial aeration on solids accumulation using a well established pilot system (5 years old) composed of 12 CWs mesocosm units. Three plant conditions (Phragmites australis, Typha angustifolia and unplanted control) were tested in the presence and absence of artificial aeration.
2.
Materials and methods
2.1.
Wetland system
Four CW mesocosm units (1 m2 ) were planted with P. australis, four with T. angustifolia, and four were left unplanted. Half of the units in each treatment were aerated continuously with a diaphragm air pump using a perforated horizontal 20-cm diameter circular tubing at the bottom entrance of the beds, at an airflow of 2 ± 1 L/min. All units were intermittently fed with a reconstituted fish farm effluent using a hydraulic loading rate of 6 cm/d and an organic loading (in g m−2 d−1 ) of 3.2 TSS, 9.1 COD and 0.8 TKN, resulting in a concentration of (in mg/L): TSS: 50, COD: 150, TN: 15, TP: 15. More details on the experimental system are presented in Chazarenc et al. (2007).
2.2.
Solids sampling and extraction
In early May 2005, cores (D = 4.6 cm, L = 30 cm) containing clean gravel (2–6 mm, the same diameter as the one used in the bed matrix) were inserted in 2 piezometers located near the inlet (I) and outlet (O) of each unit (Fig. 1). The cores were made of polyester mesh with a pore diameter of 5 mm. The cores were removed after 6 months of operation, in early November 2005. The gravel in each core was divided in two parts according to depth (0–15 and 15–30 cm). The solids accumulated in each part were extracted by shaking vigorously using 400 mL of a
Fig. 1 – Schematic view of an aerated unit planted with Typha.
phosphorus buffer (K2 HPO4 9.3 g/L; KH2 PO4 1.8 g/L) leading to an accumulated solids solution (AS solution).
2.3.
Solids analysis
Total matter and volatile matter were determined, respectively, by drying 6 mL of the AS solution at 105 ◦ C during 24 h and burning the dry residue at 550 ◦ C during 15 min. Sulphide concentration was determined by diluting 0.5 mL of AS solution in 12 mL of distilled water and by using the methylene blue method (8131) provided by Hach Company. Bacteria were counted by flow cytometry (FACScan, Becton and Dickinson, San Jose, USA) using the protocol adapted by Gagnon et al. (2007). Potential aerobic respiration was measured with a respirometer (AER-200, Challenge Environmental Systems) on 100 mL of AS solution, under continuous agitation for 5 h. Enzyme activity was measured using fluorescein diacetate (FDA, Sigma–Aldrich) with a protocol adapted by Gagnon et al. (2007). Dehydrogenase activity was measured by the reduction rate of p-iodonitro-tetrazolium violet (INT, Molecular Probes) to INT-Formazan (INT-F). Dehydrogenase was assessed using 0.25 mL of INT (4 mM) and 1 mL of AS solution incubated during 1 h at room temperature. Reaction was stopped with 0.25 mL of formaldehyde (37%) and the Formazan precipitate was extracted using methanol for 30 min. Supernatant absorbance (480 nm) was measured and INT-F concentration was calculated using Beer–Lambert’s law (ε = 15,500 M−1 cm−1 ).
2.4.
Water sampling and analysis
From May 2005 to November 2005, influent/effluent were collected at different intervals following a 24-h sampling (total 15 days of sampling). The following parameters were measured according to APHA et al. (1998): total suspended solids (TSS), chemical oxygen demand (COD), total Kjeldahl nitrogen (TKN), ammonium (NH4 ), and nitrate (NO3 ). Evapotranspiration (ET) was estimated by measuring the total
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inflow and outflow. Removal efficiencies were calculated on a mass balance basis. All statistical analyses were performed with SPSS 16.0 statistical software using a two-way repeated measures analysis of variance (ANOVA) followed by a test of multiple comparisons (Tukey HSD) to assess the differences among treatments (aeration, macrophytes). Statistical tests were considered significant at the 0.05 level. All variables met normality assumptions and had homogeneous variance.
to a better N-NH4 removal compared to unplanted aerated units, which is in agreement with previous studies using the same units (Ouellet-Plamondon et al., 2006; Maltais-Landry et al., 2007). In aerated units, a higher nitrate residual concentration was observed especially in planted beds (0.13 and 0.11 g N NO3 m−2 d−1 for Phragmites and Typha, respectively), probably as a consequence of the extra oxygen supplied that stimulated the nitrification step by providing an aerobic environment.
3.2.
3.
Results and discussion
3.1.
Treatment performances
TSS and COD removal rates averaged respectively 75% and 60% for all units (results not shown). These removal rates did not vary according to plant conditions, but they were slightly enhanced by artificial aeration, as observed by Ouellet-Plamondon et al. (2006) using the same experimental system. Nitrogen removal was greatly influenced both by plant presence and artificial aeration: N-TKN removal rates averaged from 0.31 g N m−2 d−1 in unplanted mesocosms to 0.5 g N m−2 d−1 in planted mesocosms and up to 0.65 g N m−2 d−1 in planted mesocosms with artificial aeration. The TKN load was mainly in the form of urea and artificial aeration probably promoted ammonification since all the N-TKN was under ammonium form at the outlet of planted and aerated mesocosms (Fig. 2). In unplanted mesocosms, additional aeration did not fully compensate for the absence of plants as tot-N removal was greater in planted mesocosms (55.8 and 63.8% for Phragmites and Typha, respectively) compare to unplanted aerated (55.7%). No effect of plant species was observed on treatment performances. There was a combined effect of plant and artificial aeration that led
Solids accumulation
Solids (dry matter, DM) accumulation rates ranged from 7.2 to 13.2 kg DM m−2 year−1 (Fig. 3). Although our experimental system has been operating for more than 5 years, this accumulation rate is closer to those observed in young CWs having less than 2 years of operation (from 1.3 to 3.0 kg DM m−2 year−1 Tanner et al., 1998; from 0.6 to 14.3 kg DM m−2 year−1 in Caselles-Osorio et al., 2007). This elevated accumulation rate could be a consequence of the large void space available since we used clean gravel in the traps. Also, the amount of solids accumulated was probably increased by the exportable solids present in the vicinity of the trap core. On average, the rate of solids accumulation was 12% greater near the inlet of all units (Fig. 3). Hydraulic conductivity was reduced in unplanted units without aeration where presence of surface water was occasionally observed during feeding operation. The effect of artificial aeration on solids accumulation depended on plant treatment, with a clear reduction (−26%) in unplanted units, a slight increase (11%) in Typha units, and no apparent effect (−1%) in Phragmites units (Fig. 3). Under our experimental conditions, artificial aeration did not reduce solids accumulation in planted units. However, it must be noted that our study was conducted in summer, so that in winter conditions, when plants are dormant, artificial aeration could play a greater role in reducing solids accumulation.
3.3.
Fig. 2 – Average nitrogen mass flow measured between inlet and outlet of the mesocosms (n = 15). Other N forms include particulate TKN, urea, and every other form except N-NH4 . Repeated measures ANOVA was used to test differences between the aerated and non-aerated CWs (F(1, 6) = 46.4, p < 0.001) and among macrophyte treatments (F(2, 6) = 25.8, p < 0.001). Capital letters denote significant differences between aerated and non-aerated CWs, and lower case letters denote significant differences between macrophyte treatments within aerated or non-aerated conditions, using a Tukey HSD test.
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Volatile matter accumulation
On average, volatile matter (VM) represented 23% of total accumulated solids, which is comparable to results obtained by Caselles-Osorio et al. (2007) with VM < 20% in samples from full scale CWs. The effect of Phragmites and Typha was similar and led to a net decrease (45%) of accumulated VM in non-aerated units (Fig. 4A). Differences between plant species in others studies was attributed to differences in plant litter deposition (Tanner and Sukias, 1995; Tanner et al., 1998). The fact that all aerial biomass was removed by hand each fall in our experimental setup reduced the possible contribution of plant litter to solids accumulation, especially on the surface of the units. Artificial aeration generated a decrease of 33% VM in unplanted beds and led to an increase in planted units (11 and 66% for Phragmites and Typha, respectively), especially near the inlet, in the vicinity of the aeration system. In terms of solids and VM accumulation, our results suggest that artificial aeration may have both positive and negative effects, first in favoring aerobic degradation of pollutants (thus reducing solids accumulation) and second in mobilizing more VM (probably under biofilm form), espe-
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Fig. 3 – Annual solids accumulation rate (in dry matter) in the CWs units (In and Out: inlet and outlet cores), average of two replicates.
cially in planted units. However, the studied period was too short to be conclusive regarding the possible long-term effect of artificial aeration in reducing inlet or outlet clogging.
3.4.
Sulphide accumulation
As observed by Stein et al. (2007), the mean amount of sulphide accumulated was dramatically reduced by plant and/or artificial aeration (from 1 g S/m2 in unplanted to 0.3 g S/m2 in planted mesocosms with aeration, Fig. 4B). Diakova et al. (2006) measured a large proportion of iron under Fe2+ form in CW’s which mainly precipitated with S2− . It was most probably the case in our experiment since the sulphides created a typical black layer at the bottom of the cores. The use of artificial aeration decreased sulphide content even in planted beds (especially in Phragmites: from 0.42 to 0.26 g S/m2 ). N removal rates were probably positively influenced by low sulphide content since S2− is considered a nitrification inhibitor in CWs (Wiessner et al., 2005). However, sulphide accumulation and thus SO4 2− removal are preferred in CWs designed to retain metals: in these cases, artificial aeration and even plant presence are not recommended (Stein et al., 2007). Our sampling method enabled us to collect all the accumulated S2− that were at the very bottom of the core and that can be omitted when using partial sampling (Caselles-Osorio et al., 2007). In CWs, sulphide accumulation is often related to the presence/dominance of sulphato-reducing vibrio bacteria. In CWs designed to treat domestic wastewater, sulphide accumulation is rather a sign of malfunction caused by COD overloading and excessive solids accumulation (Stein et al., 2007; Wiessner et al., 2005). Thus, the use of artificial aeration can help to decrease sulphide formation by increasing aerobic COD removal.
3.5.
Biological activities
Plant presence multiplied potential respiration rate recorded in samples by 5 for Phragmites and by 1.3 for Typha, but seemed to have no effect on potential dehydrogenase activity (Fig. 4). Potential enzymatic activity was even greater in inlet cores of unplanted units, which seems to be in accordance with results from McHenry and Werker (2005) who measured no influence of plants in mesocosms using FDA as enzymatic tracer in real conditions. Summer aerobic activities may be limited in CWs (Stein and Hook, 2005) although our results suggest that potential respiration remained significant in planted beds especially with Phragmites. There was a plant species effect on biological activity in non-aerated units (Fig. 4), although removal rates of TKN and COD were similar for Phragmites and Typha (Fig. 2). Respiration rates appear to be higher at the inlet for Typha (17 g O2 d−1 m−2 at inlet and 6 g O2 d−1 m−2 at outlet) but were more homogeneous in cores sampled in Phragmites units (about 40 g O2 d−1 m−2 at inlet and outlet, Fig. 4D). The same pattern was observed to a lesser extent for bacteria count and potential enzymatic activity (Fig. 4C and F). These results are in agreement with those of Gagnon et al. (2007) who also measured a greater plant biomass in the upper layer for Typha but a more equally distributed biomass along bed depth for Phragmites. Typha was associated with more solids accumulation than Phragmites in non-aerated units. As roots and rootlets densities were observed to be proportional to root surface throughout depth (Gagnon et al., 2007), we hypothesized that Typha below ground biomass generated more organic compounds than Phragmites, but further analysis are required to confirm our results. Providing artificial aeration generated a net increase of bacteria count and biological activities in the inlet zone in both Phragmites and Typha units (Fig. 4). Combined with plant pres-
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Fig. 4 – Biological parameters and activities recorded in solids sampled from various locations (inlet, outlet; top, bottom) in the aerated and non-aerated CWs. Results are the average of two replicates for each mesocosm. In and Out: inlet and outlet zones.
ence, artificial aeration always increased microbial activities probably leading to a change of both microbial community structure and diversity. It could be interesting to determine microbial diversity in response to artificial aeration in future studies.
4.
Conclusions
The effect of plant presence was mainly observed on TKN removal and led to a decrease of accumulated solids and
sulphide. We found no effect of plant species on treatment performance but solids accumulation was greater in Typha units. Also, Typha generated more biological activities at the surface and close to the inlet while conditions were more homogeneous throughout the Phragmites units. Volatile matter accumulation was reduced by plant presence and by aeration in unplanted mesocosms. Aeration (1) stimulated biofilm development at the inlet of planted beds, (2) seemed to reduce mineral matter accumulation and (3) generated a similar pattern of activities in planted beds. In planted beds, aeration resulted in a greater biological activity
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(more bacteria, potential respiration rates, dehydrogenase and enzymatic activities). The combination of plants and aeration improved TN treatment efficiency. The influence of artificial aeration on microbial community should be investigated in terms of microbial diversity.
Acknowledgements The authors thank Carole Radix and Denis Bouchard for technical assistance. This research was funded by the Natural Science and Engineering Research Council of Canada (NSERC).
references
APHA, AWWA, WEF, 1998. Standard Methods for the Examination of Water and Wastewater, 20th ed. American Public Health Association/American Water Works Association/Water Environment Federation, Washington, DC, USA. Brix, H., 1997. Do macrophytes play a role in constructed treatment wetlands? Water Sci. Technol. 35 (5), 11–17. Caselles-Osorio, A., Puigagut, J., Segú, E., Vaello, N., Granés, F., García, D., García, J., 2007. Solids accumulation in six full-scale subsurface flow constructed wetlands. Water Res. 41 (6), 1388–1398. Chazarenc, F., Maltais-Landry, G., Troesch, S., Comeau, Y., Brisson, J., 2007. Effect of loading rate on performance of constructed wetlands treating an anaerobic supernatant. Water Sci. Technol. 56 (3), 23–29. Cooper, D., Griffin, P., Cooper, P., 2005. Factors affecting the longevity of sub-surface horizontal flow systems operating as tertiary treatment for sewage effluent. Water Sci. Technol. 51 (9), 127–135. Cottingham, P.D., Davies, T.H., Hart, B.T., 1999. Aeration to promote nitrification in constructed wetland. Environ. Technol. 20, 69–75. Davies, H.T., Hart, B.T., 1990. Use of aeration to promote nitrification in reed beds treating wastewater. In: Cooper, P.F., Findlater, B.C. (Eds.), Constructed Wetlands in Water Pollution Unplanted. Pergamon Press, Oxford, UK, pp. 383–389. Diakova, K., Holcova, V., Sima, J., Dusek, J., 2006. The distribution of iron oxidation states in a constructed wetland as an indicator of its redox properties. Chem. Biodivers. 3, 1288–1300. Gagnon, V., Chazarenc, F., Comeau, Y., Brisson, J., 2007. Influence of macrophytes species on microbial density and activity in constructed wetlands. Water Sci. Technol. 56 (3), 249–254.
Higgins, J.P., Hurd, S., Weil, C., 1999. The use of engineered wetlands to treat recalcitrant wastewaters. In: Proceedings of the 4th International Conference on Ecological Engineering, Oslo, Norway, June. Kadlec, R.H., Watson, J.T., 1993. Hydraulics and solids accumulation in a gravel bed treatment wetland. In: Moshiri, G.A. (Ed.), Constructed Wetlands for Water Quality Improvement. Lewis, Boca Raton, MI, pp. 227–235. Maltais-Landry, G., Chazarenc, F., Comeau, Y., Troesch, S., Brisson, J., 2007. Effects of artificial aeration, macrophyte species and loading rate on removal efficiency in constructed wetland mesocosms treating fish farm wastewater. J. Environ. Eng. Sci. 6, 409–414. McHenry, J., Werker, A., 2005. In-situ monitoring of microbial biomass in wetland mesocosms. Water Sci. Technol. 51 (9), 233–241. Nguyen, L.M., 2000. Organic matter composition, microbial biomass and microbial activity in gravel-bed constructed wetlands treating farm dairy wastewaters. Ecol. Eng. 16 (2), 199–221. Ouellet-Plamondon, C., Chazarenc, F., Comeau, Y., Brisson, J., 2006. Artificial aeration to increase pollutant removal of constructed wetlands in cold climate. Ecol. Eng. 27, 258–264. Stein, O.R., Hook, P.B., 2005. Temperature, plants and oxygen, how does season affect constructed wetland performance? J. Environ. Sci. Health, Part A, Toxic/Hazard. Subst. Environ. Eng. 40, 1331–1342. Stein, O.R., Borden-Stewart, D.J., Hook, D.J., Jones, W.L., 2007. Seasonal influence on sulfate reduction and zinc sequestration in subsurface treatment wetlands. Water Res. 41 (15), 3440–3448. Stottmeister, U., Wießner, A., Kuschk, P., Kappelmeyer, U., Kästner, M., Bederski, O., Müller, R.A., Moormann, H., 2003. Effects of plants and microorganisms in constructed wetlands for wastewater treatment. Biotechnol. Adv. 22, 93–117. Suliman, F., French, H.K., Haugen, L.E., Søvik, A.K., 2006. Change in flow and transport patterns in horizontal subsurface flow constructed wetlands as a result of biological growth. Ecol. Eng. 27 (2), 124–133. Tanner, C.C., Sukias, J.P., 1995. Accumulation of organic solids in gravel-bed constructed wetlands. Water Sci. Technol. 32 (3), 229–239. Tanner, C.C., Sukias, J.P.S., Upsdell, M.P., 1998. Organic matter accumulation during maturation of gravel-bed constructed wetlands treating farm dairy wastewaters. Water Res. 32 (10), 3046–3054. Wiessner, A., Kappelmeyer, U., Kuschk, P., Kästner, M., 2005. Sulphate reduction and the removal of carbon and ammonia in a laboratory-scale constructed wetland. Water Res. 39 (13), 4643–4650.