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Treatment of water contaminated by volatile organic compounds in hydroponic root mats
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Treatment of water contaminated by volatile organic compounds in hydroponic root mats Zhongbing Chen a,b,∗ , Nils Reiche c , Jan Vymazal b , Peter Kuschk c a b c
College of Resources and Environment, Huazhong Agricultural University, Shizishan 1, Wuhan, 430070, China Department of Applied Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kam´ ycká 129, 16521, Prague, Czech Republic Department of Environmental Biotechnology, Helmholtz Centre for Environmental Research−UFZ, Permoserstrasse 15, 04318 Leipzig, Germany
a r t i c l e
i n f o
Article history: Received 29 January 2016 Received in revised form 23 July 2016 Accepted 28 August 2016 Available online xxx Keywords: Benzene Floating hydroponic root mat Groundwater MTBE Hydroponic root mat filter Volatile organic compounds
a b s t r a c t Hydroponic root mats (HRMs) are relatively new ecological water treatment technology with the aquatic vegetation forming buoyant root mats by their dense interwoven roots and rhizomes, either floating or sitting on the bottom of the water body. The aim of this study was to investigate the treatment of water contaminated by volatile organic compounds (VOCs) in HRMs under pilot-scale conditions. Floating hydroponic root mats (FHRM) and a hydroponic root mat filter (HRMF) mesocosms were established near a former industrial area in Leuna. The mesocosms received the contaminated groundwater with the main VOCs were benzene and methyl tert-butyl ether (MTBE). The results revealed that both systems exhibited a similar removal performance of MTBE during the two years operation. Seasonal variation was observed for benzene removal in both systems, with almost complete removal of benzene during summer period. The FHRM system exhibited higher benzene removal efficiency than the HRMF system, especially during the second year of operation. MTBE is more difficult to be removed than benzene in both systems, with overall MTBE mean removal efficiency of 32% as compare to 61% for benzene removal. Both benzene and MTBE removals are shown to be positively correlated with water loss in both systems, while air temperature only had significant influence on VOCs removal in the HRMF system. The emission rates of benzene and MTBE in the FHRM system are 8.4 and 6.9 mg m−2 d−1 , respectively. This removal accounted for 3.0% and 30.8% of the total benzene and MTBE removal. The emission rates of benzene and MTBE in the HRMF system were 6.0 and 1.4 mg m−2 d−1 , respectively, accounting for 2.3% and 8.3% of total removal of benzene and MTBE. In conclusion, HRMs can be an efficient approach to treat water contaminated by benzene in summer time, and the volatilization of benzene will decrease in the HRMs with the development of the root mat. © 2016 Elsevier B.V. All rights reserved.
1. Introduction A new variant of constructed wetlands (CWs) has been developed that employs emergent water plants, similar to those used in surface and subsurface flow CWs, growing as a floating root mat on the water surface or touching to the bottom of the water body where the root mat can function as a filter for the contaminated water. In general, this system called floating hydroponic root mats (FHRMs) (Chen et al., 2016), or floating treatment wetland (Headley and Tanner, 2012), in which the mats of helophytes are floating on
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Chen).
the water surface. It can be called as hydroponic root mat filters (HRMFs) when the root mat touches to the rooting-proof bottom of the water body and the water is forced to flow directly through the root mat filter (Chen et al., 2016). The FHRM forms a dense floating mat of roots and rhizomes, whereby a preferential hydraulic flow of the water zone between the root mat and the non-rooted bottom can be expected. A diagrammatic lay-out of FHRM and HRMF are shown in Fig. 1. Because of their specific structure, FHRMs combines benefits of ponds and CWs, and, therefore they are used for different types of wastewater such as eutrophicated lakes and rivers (Hoeger, 1988; Li et al., 2009, 2007; Nakamura et al., 1995; Song et al., 2009; Wu et al., 2006), mine drainage (Smith and Kalin, 2000), stormwater (Kerr-Upal et al., 2000; Revitt et al., 1997; Tanner and Headley, 2008), poultry processing wastewater (Todd et al., 2003), piggery effluent (Hubbard et al., 2004) or domestic wastewater (Ayaz and
http://dx.doi.org/10.1016/j.ecoleng.2016.08.012 0925-8574/© 2016 Elsevier B.V. All rights reserved.
Please cite this article in press as: Chen, Z., et al., Treatment of water contaminated by volatile organic compounds in hydroponic root mats. Ecol. Eng. (2016), http://dx.doi.org/10.1016/j.ecoleng.2016.08.012
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Fig. 1. Schematic lay-out of FHRM (floating hydroponic root mat) and HRMF (hydroponic root mat filter) systems.
Saygin, 1996; Cubillos et al., 2011; Hijosa-Valsero et al., 2010; Van de Moortel et al., 2010, 2011). Recently, FHRM and HRMF have been used to treat groundwater contaminated by chemical industry. The results showed that FHRM provided better treatment performance than unplanted horizontal subsurface flow (HSSF) CW and similar like planted HSSF CW for the removal of benzene, methyl tert-butyl ether (MTBE) (Chen et al., 2012), and better treatment performance for low chlorinated benzenes than in planted HSSF CW (Chen et al., 2015). When treating water contaminated by perchloroethene, carcinogenic metabolite vinyl chloride was not detected in the HRMF system but in the HSSF CW (Chen et al., 2014). It’s shown that the main process for benzene removal in CW is the aerobic microbial degradation with the proof of carbon isotope fractionation (Seeger et al., 2011a). Using the balancing model, Seeger et al. (2011b) proved that the degradation in the rhizosphere and plant uptake accounted for 83% and 11% of benzene removal, respectively, in the HRMF after three years of operation. In this present study, HRMF and FHRM systems have been investigated for the treatment of water contaminated by benzene and MTBE for two years. The objectives of this study were to: (1) compare the overall removal efficiency of benzene and MTBE between the HRMF and the FHRM systems; (2) evaluate the volatilization of benzene and MTBE between the HRMF and the FHRM systems with respect of the root mats maturity.
Table 1 Main inflow water composition during the investigation period.
2. Materials and methods
2.2. Water sampling and analyze
2.1. Setup of the pilot-scale wetlands
Water samples at different distances (1, 2.5 and 4 m) from the inlet and at different depths (15 and 30 cm in the HRMF, 15 cm in the FHRM) as well as the inflow and outflow water samples were taken at both systems. The temperature and redox potential were measured on-site using a flow-through cell equipped with redox electrodes (Pt/Ag + /AgCl/Cl-type Sentix ORP, WTW, Germany). For measuring the organic contaminants, 5 mL of water sample was transferred into 20 mL headspace vials, at the same time added 50 L bromobenzene-d5 (with a final concentration of 250 g L−1 ) as an internal standard and 5 mL solution of NaN3 (with a final concentration of 0.65 g L−1 ) in order to inhibit microbial activity. The samples for the organic analysis were transported to the laboratory using ice bags and stored in a cooling storage room under 4 ◦ C before analysis. The VOCs were analyzed by means of a headspace GC-FID (Agilent 6890 Gas Chromatograph) with a capillary column (30 m × 0.45 mm × 2.55 m, Agilent DB-MTBE). The following temperature program was performed: 35 ◦ C (6 min), 4 ◦ C/min to 120 ◦ C, 20 ◦ C/min to 280 ◦ C (5 min), nitrogen gas was used as the carrier
The pilot-scale treatment plant was established nearby an industrial area in Leuna, Germany. At this site, the groundwater was contaminated with different gasoline components, the main ones being benzene and MTBE with mean concentrations of 10.5 ± 2.6 mg L−1 and 2.1 ± 0.4 mg L−1 , respectively, during the investigation period (year 2010 and 2011). The main inflow water composition is shown in Table 1. All the systems consisting of a basin with a dimension of 5.0 m × 1.1 m × 0.6 m, and planted with the root mats of common reed (Phragmites australis). The HRMF system was started in April 2008 with the water level of 15 cm, and increased to 30 cm in September 2009, with the root mat reaching at the bottom of the basin. The FHRM system was established by the roots and rhizomes of P. australis in March 2010, the water level was set at 15 cm. The mean density in the HRMF and the FHRM systems were 83 and 450 shoots per m2 , respectively. The respective shoot heights were 1.7 m and 1.1 m, respectively, measured on 9th
Compounds
Unit
Average concentration ± standard deviation
Benzene MTBE TOC COD BOD5 NO3 − NO2 − Cl− SO4 2− PO4 3− NH4 + Fe2+ Ca K Na Mg Mn pH
mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1
10.5 ± 2.6 2.1 ± 0.4 40 ± 7 106 ± 12 59 ± 6 5.3 ± 2.9 <0.01 105 ± 17 22.4 ± 25.3 1.3 ± 0.7 42.7 ± 3.2 5.7 ± 1.2 220 ± 21 11.8 ± 0.9 120 ± 11 55 ± 3 1.6 ± 0.1 6.8–7.7
August, 2010. Both systems were fed with the same inflow water and same inflow rate of 6.0 L h−1 . Both the inflow and outflow were measured using flow meters. Weather data such as precipitation and air temperature were collected every day.
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gas. The samples were equilibrated at 80 ◦ C for 30 min prior to the analysis. 2.3. Volatilization measurement One-week continuous gas sampling was carried out in April–May 2010 and 2011 (mean air temperature was 13 ◦ C and 14.5 ◦ C, respectively) using a dynamic air chamber. The whole system was covered by the chamber during the one-week continuous experiment. The air flow inside the chamber was set opposite to the water flow direction because the highest volatilization is expected in the first part of the water flow path through the system. Inflow gas samples were taken before the gas came into the chamber, the outflow gas samples were taken after the gas left the chamber. Inflow and outflow water samples were taken during the volatilization measurement to calculate the percentage of volatilization for overall pollutants removal. Detail information about sampling and analysis procedures are described elsewhere (Reiche et al., 2010). In brevity, gases analyses were trapped in two replicate sorbent tubes (MARKES, self-packed containing 150 mg Tenax TA and 100 mg Chromosorb 106), analyzed directly after arriving at the laboratory, followed by thermal desorption (using a MARKES Unity thermal desorber) and quantification by gas chromatography with mass selective detection. Fig. 2. Inflow and outflow load of benzene and MTBE in the two surveyed systems during the investigation (18th January, 2010–12th October, 2011).
2.4. Data analysis In total, 44 (HRMF) and 39 (FHRM) inflow and outflow samples were taken from January 2010 to November 2011. In addition, from April 2010 to November 2011, 23 and 19 samples were taken along the flow path in the HRMF and FHRM systems, respectively. The mean of two duplicates was used for concentration evaluation of MTBE and benzene concentrations for each sampling day. The treatment performance was compared using the inflow and outflow loads which were calculated by multiplying the concentration of the pollutants with the water inflow and outflow flow rates. The removal percentage by volatilization was calculated by the mass loss due to volatilization and the pollutant total mass loss (difference between inflow and outflow mass). Spearman rank order
correlation was carried out for VOCs removal efficiency with water loss and air temperature, the significant difference was regarded at p < 0.05. 3. Results and discussion 3.1. Treatment performance Both the FHRM and the HRMF systems showed a clear seasonal variation on benzene removal, with almost complete removal of benzene (>99%) during the summer period (Fig. 2). The removal rate of benzene may amount to 464 and 578 mg d−1 m−2 in the HRMF
Fig. 3. Loads of benzene and MTBE along the flow path in the two systems in years 2010 and 2011.
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Fig. 4. Emission rate and percentage of volatilization from the total removal of MTBE and benzene in the (A) FHRM (first year), (B) FHRM (second years), (C) HRMF.
and in the FHRM systems, respectively. These rates are higher than those reported previously for the FHRM system (325 mg d−1 m−2 ) (Chen et al., 2012). This means that benzene removal efficiency will increase with the development of root mats or when the FHRM operated with shallow water level. In general, the benzene removal efficiency was higher in the FHRM system than in the HRMF system. The mean removal efficiency amounted to 74% and 58% in the FHRM system and to 62% and 51% in the HRMF system for years 2010 and 2011, respectively. These values are comparable to values (87% with aeration, 63% without aeration) reported from a pilot scale CWs at Wyoming, USA, treating petroleum hydrocarbons with benzene inflow concentration about 0.3 mg L−1 (Haberl et al., 2003). However, benzene removal efficiency of 31% was reported in a full scale wetland near Phoenix, USA, this might be due to the low benzene inflow concentration about 0.01 g L−1 (Keefe et al., 2004). MTBE is a more resistant compound than benzene due to its structure (Schmidt et al., 2004), and the low growth rate of MTBEdegradation microbes (Salanitro et al., 2000), or even the inhibition by BTEX compounds (Deeb et al., 2001). During the two years of operation, both the HRMF and the FHRM systems showed a similar treatment pattern for MTBE removal, with the highest removal efficiency observed during summer in the first year (Fig. 2). In general, the HRMF system exhibited higher MTBE removal efficiency than in the FHRM system, with the mean removal efficiencies of 48 and 24% in the HRMF system for year 2010 and 2011, 40 and 16% in the FHRM system. These values are comparable with values reported from a vertical flow constructed wetland (31% with aeration, 15% without aeration) with MTBE inflow concentration of 1.26 mg L−1 (Haberl et al., 2003). The MTBE removal rate can reach up to 60.1 and 58.6 mg d−1 m−2 in the HRMF and in the FHRM system, respectively. These values are slightly higher than those reported from unplanted (24 mg d−1 m−2 ) and planted (48 mg d−1 m−2 ) HSSF CWs with similar inflow load (Chen et al., 2012). The mass of benzene and MTBE decreased gradually along the flow path in both systems (Fig. 3), and the tendency for benzene reduction was slightly higher than the reduction of MTBE. The FHRM system can reach lower residual mass of both benzene and MTBE than the HRMF system in the first year, but the trend was reversed during the second year. This phenomenon can be explained by the lower emission rate during the second year in the FHRM system (Fig. 4).
Fig. 5. Redox potential in the two systems along flow path.
of the total removal of benzene and MTBE, respectively. In the FHRM system, benzene and MTBE emission rates in the first year (2010) amounted to 8.4 and 6.9 mg d−1 m−2 , respectively. The emissions formed 3.0% and 30.8% of total removal of benzene and MTBE. Benzene emission rate decreased to 1.8 mg d−1 m−2 in the second year in the FHRM system, and the contribution via volatilization also decreased to 2.2%. However, the MTBE emission rate decreased to 4.0 mg d−1 m−2 in the second year in the FHRM system with the contribution via volatilization increased to 43.0% (Fig. 4). This is because the total MTBE removal rate decreased in the second year of operation compared to the first year (Fig. 2). One explanation for the higher volatilization rate of MTBE in the FHRM than the HRMF is the lower water level in the FHRM (15 cm) than in the HRMF (30 cm) as the MTBE half-life increases with the increase of water depths (Stocking and Kavanaugh, 2000). The contribution of MTBE volatilization to the total removal in our study is comparable to the aerated trench system (53%) with the same inflow concentration at the same study site (Jechalke et al., 2010), this means the FHRM can reach similar MTBE removal efficiency like a more energy consuming system. Moreover, the volatilization of MTBE is also influenced by the wind speed, surface area, and water temperature (Stocking and Kavanaugh, 2000). The mentioned environmental parameters can explain the lower MTBE volatilization in the HRMF system than in the FHRM system due to the lower temperature in the HRMF than in the FHRM (difference of 1.3 ◦ C). MTBE and benzene have different volatilization potential because of their physicochemical properties, and the volatilization fluxes are related to vapor pressure (Burken and Schnoor, 1999). Therefore, higher volatilization of MTBE than benzene was detected due to the higher vapor pressure of MTBE (251 mm Hg) than benzene (86 mm Hg) (Moyer and Kostecki, 2003). 3.3. Factors affecting treatment performance
3.2. Volatilization The mean emission rates of benzene and MTBE in the HRMF system were 6.0 and 1.4 mg d−1 m−2 , which accounts for 2.3% and 8.3%
The seasonal variation of benzene removal in both systems is driven by the plant activity that changes with season, and further have effects on plant oxygenation and root exudation, which
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Fig. 6. Temperature and water loss of the two surveyed systems during the investigation period.
Table 2 Spearman rank order correlation of benzene and MTBE removal efficiency with water loss and air temperature in the HRMF and the FHRM. HRMF Number of samples
Water loss Correlation coefficient P value Air temperature Correlation coefficient P value
FHRM
23
17
Benzene
MTBE
Benzene
MTBE
0.708 <0.001
0.695 <0.001
0.509 0.036
0.567 0.018
0.616 <0.001
0.733 <0.001
0.172 0.502
0.179 0.484
influencing microbial activity. Water loss (mainly from evapotranspiration) can indicate the plant activity to some extent, and it has been shown that the removal efficiency of benzene and MTBE are significantly correlated (p < 0.05) with water loss in both systems (Table 2). In general, treatment performance in constructed wetlands can be effected by evapotranspiration (ET) which enhancing constituent transportation. It was shown that the removal efficiency of the conservative constituent has negative correlation with ET, while removal efficiency of the readily treatable constituent has positive correlation with ET (Beebe et al., 2014). Our results have shown a significant (p < 0.001) correlation between benzene removal efficiency with water loss. The lower water loss rate in the second year than in the first year may provide an explanation for the lower treatment performance in the second year than in the first one. Moreover, the presence of worms (Archanara geminipuncta) in the second year should be noted because they can decrease the plant activity when they destroy the plant stems. As a result, the decreased water loss was observed in the second year of operation. On the other hand, the elevated water loss will affect pollutants concentration (aqueous). In our study, stable water loss rates can
reach up to 95 L d−1 (66% of the inflow rate) which will concentrate the pollutants concentration up to 3 times. Redox potential is an indicator for the environmental condition in CWs. Oxidized conditions are followed by high redox potential which enhances aerobic processes. In contrast, reduced conditions are associated with lower redox potentials which promote anaerobic processes. Higher redox potential was observed in 2010 (>50 mV) than in 2011 (<50 mV) (Fig. 5). We suspect that more oxidized conditions were responsible for higher removal efficiency for both benzene and MTBE in 2010 than in 2011. Higher redox potential can be generated by longer hydraulic retention time (HRT), further increasing the pollutants removal (Faulwetter et al., 2009). The theoretical HRT was 8 and 5 days in the HRMF and FHRM system, respectively. The difference in a real hydraulic retention time in the two systems might even be big due to the higher water loss rate in the HRMF than in the FHRM system (Fig. 6). This gives another explanation for the lower residual loads observed in the HRMF system than in the FHRM system. In a FHRM system planted with vetiveria zizanioides and treating domestic wastewater under HRT of 3, 5, and 7 days, the results showed that the highest treatment efficieny was obtained with HRT of 7 days (Boonsong and Chansiri, 2008). Yang et al. (2008) also found that treatment efficiency correlated positively with HRT in a FHRM system (planted with Oenanthe javanica) treating agricultural runoff under HRT of 1, 2, and 3 days. In general, microbial activity is linked to temperature – the increased temperature favors the microbes’ growth and increasing the metabolic rates (Atlas, 1981; Kadlec and Reddy, 2001). Our results revealed that the treatment performance in the HMRF system was more sensitive to the air temperature than in the FHRM. The removal efficency of benzene and MTBE in the HMRF was significantly (p < 0.001) positively correlated with air temperature (Table 2). In agreement with our results, Tang et al. (2009) found that the effluent benzene concentration were negatively correlated
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with the redox potential and temperature. Furthermore, in the case of VOC removal, the temperature can also influence indirectly the surface volatilization and phytovolatilization (Imfeld et al., 2009). In order to obtain stable treatment performance under cold climate condition, heat preservation or heating measures for the systems are needed. This could be realized by additional surface protection against freezing using straw, rock wool, polystyrene, etc. Moreover, the systems design with larger and deeper beds will be effective to prevent freezing, especially for the FHRMs. 4. Conclusions Both the HRMF and the FHRM systems can reach high benzene removal in the summer period (>99%). The mean benzene and MTBE removal efficiency range from 51% to 74%, and from 16% to 48% in HRMF and FHRM systems, respectively. Plant evapotranspiration is a very important factor which influencing the removal of benzene and MTBE in both the HRMF and the FHRM system. The HRMF system is more sensitive to air temperature than the FHRM system, with the removal efficiencies of MTBE and benzene being positively correlated with the air temperature in the HRMF rather than in the FHRM system. Volatilization is not a dominant pathway for benzene removal in both systems, but it is a dominant pathway for MTBE removal in the FHRM. With a better root mat development in the HRMF, which provide a more surface area for the attachment of biofilms and coverage of the water body, higher MTBE removal and lower volatilization was achieved in the HRMF than in the FHRM. Therefore, for better understanding of the treatment processes in HRM systems, more research about the root mat development has to be done in the future. Acknowledgements This work was supported by the International Science and Technology Cooperation Program from Hubei Province of China (Grant No. 2015BHE010), and the Fundamental Research Funds for the Central Universities (Grant No. 2662015QC004). It also supported by the Helmholtz Centre for Environmental Research – UFZ within the scope of the SAFIRA II Research Program. References Atlas, R.M., 1981. Microbial degradation of petroleum hydrocarbons: an environmental perspective. Microbiol. Rev. 45 (1), 180–209. Ayaz, S.C., Saygin, O., 1996. Hydroponic tertiary treatment. Water Res. 30 (5), 1295–1298. Beebe, D.A., Castle, J.W., Molz, F.J., Rodgers Jr., J.H., 2014. Effects of evapotranspiration on treatment performance in constructed wetlands: experimental studies and modeling. Ecol. Eng. 71, 394–400. Boonsong, K., Chansiri, M., 2008. Domestic wastewater treatment using vetiver grass cultivated with floating platform technique. AU J. T. 12 (2), 73–80. Burken, J.G., Schnoor, J.L., 1999. Distribution and volatilization of organic compounds following uptake by hybrid poplar trees. Int. J. Phytorem. 1 (2), 139–151. Chen, Z., Kuschk, P., Reiche, N., Borsdorf, H., Kästner, M., Köser, H., 2012. Comparative evaluation of pilot scale horizontal subsurface-flow constructed wetlands and plant root mats for treating groundwater contaminated with benzene and MTBE. J. Hazard. Mater. 209–210, 510–515. Chen, Z., Kuschk, P., Paschke, H., Kästner, M., Müller, J.A., Köser, H., 2014. Treatment of a sulfate-rich groundwater contaminated with perchloroethene in a hydroponic plant root mat filter and a horizontal subsurface flow constructed wetland at pilot-scale. Chemosphere 117, 178–184. Chen, Z., Kuschk, P., Paschke, H., Kastner, M., Koser, H., 2015. The dynamics of low-chlorinated benzenes in a pilot-scale constructed wetland and a hydroponic plant root mat treating sulfate-rich groundwater. Environ. Sci. Pollut. Res. 22 (5), 3886–3894. Chen, Z., Cuervo, D.P., Müller, J.A., Wiessner, A., Köser, H., Vymazal, J., Kästner, M., Kuschk, P., 2016. Hydroponic root mats for wastewater treatment—a review. Environ. Sci. Pollut. Res., http://dx.doi.org/10.1007/s11356-016-6801-3. Cubillos, J., Paredes, D., Kuschk, P., 2011. Comparison between floating plant mat and horizontal subsurface flow CW for the treatment of a low strength domestic wastewater. In: Joint Meeting of Society of Wetland Scientists,
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Treatment of water contaminated by volatile organic compounds in hydroponic root mats Zhongbing Chen a,b,∗ , Nils Reiche c , Jan Vymazal b , Peter Kuschk c a b c
College of Resources and Environment, Huazhong Agricultural University, Shizishan 1, Wuhan, 430070, China Department of Applied Ecology, Faculty of Environmental Sciences, Czech University of Life Sciences Prague, Kam´ ycká 129, 16521, Prague, Czech Republic Department of Environmental Biotechnology, Helmholtz Centre for Environmental Research−UFZ, Permoserstrasse 15, 04318 Leipzig, Germany
a r t i c l e
i n f o
Article history: Received 29 January 2016 Received in revised form 23 July 2016 Accepted 28 August 2016 Available online xxx Keywords: Benzene Floating hydroponic root mat Groundwater MTBE Hydroponic root mat filter Volatile organic compounds
a b s t r a c t Hydroponic root mats (HRMs) are relatively new ecological water treatment technology with the aquatic vegetation forming buoyant root mats by their dense interwoven roots and rhizomes, either floating or sitting on the bottom of the water body. The aim of this study was to investigate the treatment of water contaminated by volatile organic compounds (VOCs) in HRMs under pilot-scale conditions. Floating hydroponic root mats (FHRM) and a hydroponic root mat filter (HRMF) mesocosms were established near a former industrial area in Leuna. The mesocosms received the contaminated groundwater with the main VOCs were benzene and methyl tert-butyl ether (MTBE). The results revealed that both systems exhibited a similar removal performance of MTBE during the two years operation. Seasonal variation was observed for benzene removal in both systems, with almost complete removal of benzene during summer period. The FHRM system exhibited higher benzene removal efficiency than the HRMF system, especially during the second year of operation. MTBE is more difficult to be removed than benzene in both systems, with overall MTBE mean removal efficiency of 32% as compare to 61% for benzene removal. Both benzene and MTBE removals are shown to be positively correlated with water loss in both systems, while air temperature only had significant influence on VOCs removal in the HRMF system. The emission rates of benzene and MTBE in the FHRM system are 8.4 and 6.9 mg m−2 d−1 , respectively. This removal accounted for 3.0% and 30.8% of the total benzene and MTBE removal. The emission rates of benzene and MTBE in the HRMF system were 6.0 and 1.4 mg m−2 d−1 , respectively, accounting for 2.3% and 8.3% of total removal of benzene and MTBE. In conclusion, HRMs can be an efficient approach to treat water contaminated by benzene in summer time, and the volatilization of benzene will decrease in the HRMs with the development of the root mat. © 2016 Elsevier B.V. All rights reserved.
1. Introduction A new variant of constructed wetlands (CWs) has been developed that employs emergent water plants, similar to those used in surface and subsurface flow CWs, growing as a floating root mat on the water surface or touching to the bottom of the water body where the root mat can function as a filter for the contaminated water. In general, this system called floating hydroponic root mats (FHRMs) (Chen et al., 2016), or floating treatment wetland (Headley and Tanner, 2012), in which the mats of helophytes are floating on
∗ Corresponding author. E-mail addresses: [email protected], [email protected] (Z. Chen).
the water surface. It can be called as hydroponic root mat filters (HRMFs) when the root mat touches to the rooting-proof bottom of the water body and the water is forced to flow directly through the root mat filter (Chen et al., 2016). The FHRM forms a dense floating mat of roots and rhizomes, whereby a preferential hydraulic flow of the water zone between the root mat and the non-rooted bottom can be expected. A diagrammatic lay-out of FHRM and HRMF are shown in Fig. 1. Because of their specific structure, FHRMs combines benefits of ponds and CWs, and, therefore they are used for different types of wastewater such as eutrophicated lakes and rivers (Hoeger, 1988; Li et al., 2009, 2007; Nakamura et al., 1995; Song et al., 2009; Wu et al., 2006), mine drainage (Smith and Kalin, 2000), stormwater (Kerr-Upal et al., 2000; Revitt et al., 1997; Tanner and Headley, 2008), poultry processing wastewater (Todd et al., 2003), piggery effluent (Hubbard et al., 2004) or domestic wastewater (Ayaz and
http://dx.doi.org/10.1016/j.ecoleng.2016.08.012 0925-8574/© 2016 Elsevier B.V. All rights reserved.
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Fig. 1. Schematic lay-out of FHRM (floating hydroponic root mat) and HRMF (hydroponic root mat filter) systems.
Saygin, 1996; Cubillos et al., 2011; Hijosa-Valsero et al., 2010; Van de Moortel et al., 2010, 2011). Recently, FHRM and HRMF have been used to treat groundwater contaminated by chemical industry. The results showed that FHRM provided better treatment performance than unplanted horizontal subsurface flow (HSSF) CW and similar like planted HSSF CW for the removal of benzene, methyl tert-butyl ether (MTBE) (Chen et al., 2012), and better treatment performance for low chlorinated benzenes than in planted HSSF CW (Chen et al., 2015). When treating water contaminated by perchloroethene, carcinogenic metabolite vinyl chloride was not detected in the HRMF system but in the HSSF CW (Chen et al., 2014). It’s shown that the main process for benzene removal in CW is the aerobic microbial degradation with the proof of carbon isotope fractionation (Seeger et al., 2011a). Using the balancing model, Seeger et al. (2011b) proved that the degradation in the rhizosphere and plant uptake accounted for 83% and 11% of benzene removal, respectively, in the HRMF after three years of operation. In this present study, HRMF and FHRM systems have been investigated for the treatment of water contaminated by benzene and MTBE for two years. The objectives of this study were to: (1) compare the overall removal efficiency of benzene and MTBE between the HRMF and the FHRM systems; (2) evaluate the volatilization of benzene and MTBE between the HRMF and the FHRM systems with respect of the root mats maturity.
Table 1 Main inflow water composition during the investigation period.
2. Materials and methods
2.2. Water sampling and analyze
2.1. Setup of the pilot-scale wetlands
Water samples at different distances (1, 2.5 and 4 m) from the inlet and at different depths (15 and 30 cm in the HRMF, 15 cm in the FHRM) as well as the inflow and outflow water samples were taken at both systems. The temperature and redox potential were measured on-site using a flow-through cell equipped with redox electrodes (Pt/Ag + /AgCl/Cl-type Sentix ORP, WTW, Germany). For measuring the organic contaminants, 5 mL of water sample was transferred into 20 mL headspace vials, at the same time added 50 L bromobenzene-d5 (with a final concentration of 250 g L−1 ) as an internal standard and 5 mL solution of NaN3 (with a final concentration of 0.65 g L−1 ) in order to inhibit microbial activity. The samples for the organic analysis were transported to the laboratory using ice bags and stored in a cooling storage room under 4 ◦ C before analysis. The VOCs were analyzed by means of a headspace GC-FID (Agilent 6890 Gas Chromatograph) with a capillary column (30 m × 0.45 mm × 2.55 m, Agilent DB-MTBE). The following temperature program was performed: 35 ◦ C (6 min), 4 ◦ C/min to 120 ◦ C, 20 ◦ C/min to 280 ◦ C (5 min), nitrogen gas was used as the carrier
The pilot-scale treatment plant was established nearby an industrial area in Leuna, Germany. At this site, the groundwater was contaminated with different gasoline components, the main ones being benzene and MTBE with mean concentrations of 10.5 ± 2.6 mg L−1 and 2.1 ± 0.4 mg L−1 , respectively, during the investigation period (year 2010 and 2011). The main inflow water composition is shown in Table 1. All the systems consisting of a basin with a dimension of 5.0 m × 1.1 m × 0.6 m, and planted with the root mats of common reed (Phragmites australis). The HRMF system was started in April 2008 with the water level of 15 cm, and increased to 30 cm in September 2009, with the root mat reaching at the bottom of the basin. The FHRM system was established by the roots and rhizomes of P. australis in March 2010, the water level was set at 15 cm. The mean density in the HRMF and the FHRM systems were 83 and 450 shoots per m2 , respectively. The respective shoot heights were 1.7 m and 1.1 m, respectively, measured on 9th
Compounds
Unit
Average concentration ± standard deviation
Benzene MTBE TOC COD BOD5 NO3 − NO2 − Cl− SO4 2− PO4 3− NH4 + Fe2+ Ca K Na Mg Mn pH
mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1 mg L−1
10.5 ± 2.6 2.1 ± 0.4 40 ± 7 106 ± 12 59 ± 6 5.3 ± 2.9 <0.01 105 ± 17 22.4 ± 25.3 1.3 ± 0.7 42.7 ± 3.2 5.7 ± 1.2 220 ± 21 11.8 ± 0.9 120 ± 11 55 ± 3 1.6 ± 0.1 6.8–7.7
August, 2010. Both systems were fed with the same inflow water and same inflow rate of 6.0 L h−1 . Both the inflow and outflow were measured using flow meters. Weather data such as precipitation and air temperature were collected every day.
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gas. The samples were equilibrated at 80 ◦ C for 30 min prior to the analysis. 2.3. Volatilization measurement One-week continuous gas sampling was carried out in April–May 2010 and 2011 (mean air temperature was 13 ◦ C and 14.5 ◦ C, respectively) using a dynamic air chamber. The whole system was covered by the chamber during the one-week continuous experiment. The air flow inside the chamber was set opposite to the water flow direction because the highest volatilization is expected in the first part of the water flow path through the system. Inflow gas samples were taken before the gas came into the chamber, the outflow gas samples were taken after the gas left the chamber. Inflow and outflow water samples were taken during the volatilization measurement to calculate the percentage of volatilization for overall pollutants removal. Detail information about sampling and analysis procedures are described elsewhere (Reiche et al., 2010). In brevity, gases analyses were trapped in two replicate sorbent tubes (MARKES, self-packed containing 150 mg Tenax TA and 100 mg Chromosorb 106), analyzed directly after arriving at the laboratory, followed by thermal desorption (using a MARKES Unity thermal desorber) and quantification by gas chromatography with mass selective detection. Fig. 2. Inflow and outflow load of benzene and MTBE in the two surveyed systems during the investigation (18th January, 2010–12th October, 2011).
2.4. Data analysis In total, 44 (HRMF) and 39 (FHRM) inflow and outflow samples were taken from January 2010 to November 2011. In addition, from April 2010 to November 2011, 23 and 19 samples were taken along the flow path in the HRMF and FHRM systems, respectively. The mean of two duplicates was used for concentration evaluation of MTBE and benzene concentrations for each sampling day. The treatment performance was compared using the inflow and outflow loads which were calculated by multiplying the concentration of the pollutants with the water inflow and outflow flow rates. The removal percentage by volatilization was calculated by the mass loss due to volatilization and the pollutant total mass loss (difference between inflow and outflow mass). Spearman rank order
correlation was carried out for VOCs removal efficiency with water loss and air temperature, the significant difference was regarded at p < 0.05. 3. Results and discussion 3.1. Treatment performance Both the FHRM and the HRMF systems showed a clear seasonal variation on benzene removal, with almost complete removal of benzene (>99%) during the summer period (Fig. 2). The removal rate of benzene may amount to 464 and 578 mg d−1 m−2 in the HRMF
Fig. 3. Loads of benzene and MTBE along the flow path in the two systems in years 2010 and 2011.
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Fig. 4. Emission rate and percentage of volatilization from the total removal of MTBE and benzene in the (A) FHRM (first year), (B) FHRM (second years), (C) HRMF.
and in the FHRM systems, respectively. These rates are higher than those reported previously for the FHRM system (325 mg d−1 m−2 ) (Chen et al., 2012). This means that benzene removal efficiency will increase with the development of root mats or when the FHRM operated with shallow water level. In general, the benzene removal efficiency was higher in the FHRM system than in the HRMF system. The mean removal efficiency amounted to 74% and 58% in the FHRM system and to 62% and 51% in the HRMF system for years 2010 and 2011, respectively. These values are comparable to values (87% with aeration, 63% without aeration) reported from a pilot scale CWs at Wyoming, USA, treating petroleum hydrocarbons with benzene inflow concentration about 0.3 mg L−1 (Haberl et al., 2003). However, benzene removal efficiency of 31% was reported in a full scale wetland near Phoenix, USA, this might be due to the low benzene inflow concentration about 0.01 g L−1 (Keefe et al., 2004). MTBE is a more resistant compound than benzene due to its structure (Schmidt et al., 2004), and the low growth rate of MTBEdegradation microbes (Salanitro et al., 2000), or even the inhibition by BTEX compounds (Deeb et al., 2001). During the two years of operation, both the HRMF and the FHRM systems showed a similar treatment pattern for MTBE removal, with the highest removal efficiency observed during summer in the first year (Fig. 2). In general, the HRMF system exhibited higher MTBE removal efficiency than in the FHRM system, with the mean removal efficiencies of 48 and 24% in the HRMF system for year 2010 and 2011, 40 and 16% in the FHRM system. These values are comparable with values reported from a vertical flow constructed wetland (31% with aeration, 15% without aeration) with MTBE inflow concentration of 1.26 mg L−1 (Haberl et al., 2003). The MTBE removal rate can reach up to 60.1 and 58.6 mg d−1 m−2 in the HRMF and in the FHRM system, respectively. These values are slightly higher than those reported from unplanted (24 mg d−1 m−2 ) and planted (48 mg d−1 m−2 ) HSSF CWs with similar inflow load (Chen et al., 2012). The mass of benzene and MTBE decreased gradually along the flow path in both systems (Fig. 3), and the tendency for benzene reduction was slightly higher than the reduction of MTBE. The FHRM system can reach lower residual mass of both benzene and MTBE than the HRMF system in the first year, but the trend was reversed during the second year. This phenomenon can be explained by the lower emission rate during the second year in the FHRM system (Fig. 4).
Fig. 5. Redox potential in the two systems along flow path.
of the total removal of benzene and MTBE, respectively. In the FHRM system, benzene and MTBE emission rates in the first year (2010) amounted to 8.4 and 6.9 mg d−1 m−2 , respectively. The emissions formed 3.0% and 30.8% of total removal of benzene and MTBE. Benzene emission rate decreased to 1.8 mg d−1 m−2 in the second year in the FHRM system, and the contribution via volatilization also decreased to 2.2%. However, the MTBE emission rate decreased to 4.0 mg d−1 m−2 in the second year in the FHRM system with the contribution via volatilization increased to 43.0% (Fig. 4). This is because the total MTBE removal rate decreased in the second year of operation compared to the first year (Fig. 2). One explanation for the higher volatilization rate of MTBE in the FHRM than the HRMF is the lower water level in the FHRM (15 cm) than in the HRMF (30 cm) as the MTBE half-life increases with the increase of water depths (Stocking and Kavanaugh, 2000). The contribution of MTBE volatilization to the total removal in our study is comparable to the aerated trench system (53%) with the same inflow concentration at the same study site (Jechalke et al., 2010), this means the FHRM can reach similar MTBE removal efficiency like a more energy consuming system. Moreover, the volatilization of MTBE is also influenced by the wind speed, surface area, and water temperature (Stocking and Kavanaugh, 2000). The mentioned environmental parameters can explain the lower MTBE volatilization in the HRMF system than in the FHRM system due to the lower temperature in the HRMF than in the FHRM (difference of 1.3 ◦ C). MTBE and benzene have different volatilization potential because of their physicochemical properties, and the volatilization fluxes are related to vapor pressure (Burken and Schnoor, 1999). Therefore, higher volatilization of MTBE than benzene was detected due to the higher vapor pressure of MTBE (251 mm Hg) than benzene (86 mm Hg) (Moyer and Kostecki, 2003). 3.3. Factors affecting treatment performance
3.2. Volatilization The mean emission rates of benzene and MTBE in the HRMF system were 6.0 and 1.4 mg d−1 m−2 , which accounts for 2.3% and 8.3%
The seasonal variation of benzene removal in both systems is driven by the plant activity that changes with season, and further have effects on plant oxygenation and root exudation, which
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Fig. 6. Temperature and water loss of the two surveyed systems during the investigation period.
Table 2 Spearman rank order correlation of benzene and MTBE removal efficiency with water loss and air temperature in the HRMF and the FHRM. HRMF Number of samples
Water loss Correlation coefficient P value Air temperature Correlation coefficient P value
FHRM
23
17
Benzene
MTBE
Benzene
MTBE
0.708 <0.001
0.695 <0.001
0.509 0.036
0.567 0.018
0.616 <0.001
0.733 <0.001
0.172 0.502
0.179 0.484
influencing microbial activity. Water loss (mainly from evapotranspiration) can indicate the plant activity to some extent, and it has been shown that the removal efficiency of benzene and MTBE are significantly correlated (p < 0.05) with water loss in both systems (Table 2). In general, treatment performance in constructed wetlands can be effected by evapotranspiration (ET) which enhancing constituent transportation. It was shown that the removal efficiency of the conservative constituent has negative correlation with ET, while removal efficiency of the readily treatable constituent has positive correlation with ET (Beebe et al., 2014). Our results have shown a significant (p < 0.001) correlation between benzene removal efficiency with water loss. The lower water loss rate in the second year than in the first year may provide an explanation for the lower treatment performance in the second year than in the first one. Moreover, the presence of worms (Archanara geminipuncta) in the second year should be noted because they can decrease the plant activity when they destroy the plant stems. As a result, the decreased water loss was observed in the second year of operation. On the other hand, the elevated water loss will affect pollutants concentration (aqueous). In our study, stable water loss rates can
reach up to 95 L d−1 (66% of the inflow rate) which will concentrate the pollutants concentration up to 3 times. Redox potential is an indicator for the environmental condition in CWs. Oxidized conditions are followed by high redox potential which enhances aerobic processes. In contrast, reduced conditions are associated with lower redox potentials which promote anaerobic processes. Higher redox potential was observed in 2010 (>50 mV) than in 2011 (<50 mV) (Fig. 5). We suspect that more oxidized conditions were responsible for higher removal efficiency for both benzene and MTBE in 2010 than in 2011. Higher redox potential can be generated by longer hydraulic retention time (HRT), further increasing the pollutants removal (Faulwetter et al., 2009). The theoretical HRT was 8 and 5 days in the HRMF and FHRM system, respectively. The difference in a real hydraulic retention time in the two systems might even be big due to the higher water loss rate in the HRMF than in the FHRM system (Fig. 6). This gives another explanation for the lower residual loads observed in the HRMF system than in the FHRM system. In a FHRM system planted with vetiveria zizanioides and treating domestic wastewater under HRT of 3, 5, and 7 days, the results showed that the highest treatment efficieny was obtained with HRT of 7 days (Boonsong and Chansiri, 2008). Yang et al. (2008) also found that treatment efficiency correlated positively with HRT in a FHRM system (planted with Oenanthe javanica) treating agricultural runoff under HRT of 1, 2, and 3 days. In general, microbial activity is linked to temperature – the increased temperature favors the microbes’ growth and increasing the metabolic rates (Atlas, 1981; Kadlec and Reddy, 2001). Our results revealed that the treatment performance in the HMRF system was more sensitive to the air temperature than in the FHRM. The removal efficency of benzene and MTBE in the HMRF was significantly (p < 0.001) positively correlated with air temperature (Table 2). In agreement with our results, Tang et al. (2009) found that the effluent benzene concentration were negatively correlated
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with the redox potential and temperature. Furthermore, in the case of VOC removal, the temperature can also influence indirectly the surface volatilization and phytovolatilization (Imfeld et al., 2009). In order to obtain stable treatment performance under cold climate condition, heat preservation or heating measures for the systems are needed. This could be realized by additional surface protection against freezing using straw, rock wool, polystyrene, etc. Moreover, the systems design with larger and deeper beds will be effective to prevent freezing, especially for the FHRMs. 4. Conclusions Both the HRMF and the FHRM systems can reach high benzene removal in the summer period (>99%). The mean benzene and MTBE removal efficiency range from 51% to 74%, and from 16% to 48% in HRMF and FHRM systems, respectively. Plant evapotranspiration is a very important factor which influencing the removal of benzene and MTBE in both the HRMF and the FHRM system. The HRMF system is more sensitive to air temperature than the FHRM system, with the removal efficiencies of MTBE and benzene being positively correlated with the air temperature in the HRMF rather than in the FHRM system. Volatilization is not a dominant pathway for benzene removal in both systems, but it is a dominant pathway for MTBE removal in the FHRM. With a better root mat development in the HRMF, which provide a more surface area for the attachment of biofilms and coverage of the water body, higher MTBE removal and lower volatilization was achieved in the HRMF than in the FHRM. Therefore, for better understanding of the treatment processes in HRM systems, more research about the root mat development has to be done in the future. Acknowledgements This work was supported by the International Science and Technology Cooperation Program from Hubei Province of China (Grant No. 2015BHE010), and the Fundamental Research Funds for the Central Universities (Grant No. 2662015QC004). It also supported by the Helmholtz Centre for Environmental Research – UFZ within the scope of the SAFIRA II Research Program. References Atlas, R.M., 1981. Microbial degradation of petroleum hydrocarbons: an environmental perspective. Microbiol. Rev. 45 (1), 180–209. Ayaz, S.C., Saygin, O., 1996. Hydroponic tertiary treatment. Water Res. 30 (5), 1295–1298. Beebe, D.A., Castle, J.W., Molz, F.J., Rodgers Jr., J.H., 2014. Effects of evapotranspiration on treatment performance in constructed wetlands: experimental studies and modeling. Ecol. Eng. 71, 394–400. Boonsong, K., Chansiri, M., 2008. Domestic wastewater treatment using vetiver grass cultivated with floating platform technique. AU J. T. 12 (2), 73–80. Burken, J.G., Schnoor, J.L., 1999. Distribution and volatilization of organic compounds following uptake by hybrid poplar trees. Int. J. Phytorem. 1 (2), 139–151. Chen, Z., Kuschk, P., Reiche, N., Borsdorf, H., Kästner, M., Köser, H., 2012. Comparative evaluation of pilot scale horizontal subsurface-flow constructed wetlands and plant root mats for treating groundwater contaminated with benzene and MTBE. J. Hazard. Mater. 209–210, 510–515. Chen, Z., Kuschk, P., Paschke, H., Kästner, M., Müller, J.A., Köser, H., 2014. Treatment of a sulfate-rich groundwater contaminated with perchloroethene in a hydroponic plant root mat filter and a horizontal subsurface flow constructed wetland at pilot-scale. Chemosphere 117, 178–184. Chen, Z., Kuschk, P., Paschke, H., Kastner, M., Koser, H., 2015. The dynamics of low-chlorinated benzenes in a pilot-scale constructed wetland and a hydroponic plant root mat treating sulfate-rich groundwater. Environ. Sci. Pollut. Res. 22 (5), 3886–3894. Chen, Z., Cuervo, D.P., Müller, J.A., Wiessner, A., Köser, H., Vymazal, J., Kästner, M., Kuschk, P., 2016. Hydroponic root mats for wastewater treatment—a review. Environ. Sci. Pollut. Res., http://dx.doi.org/10.1007/s11356-016-6801-3. Cubillos, J., Paredes, D., Kuschk, P., 2011. Comparison between floating plant mat and horizontal subsurface flow CW for the treatment of a low strength domestic wastewater. In: Joint Meeting of Society of Wetland Scientists,
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