Remediation of groundwater contaminated with MTBE and benzene: The potential of vertical-flow soil filter systems

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Remediation of groundwater contaminated with MTBE and benzene: The potential of vertical-flow soil filter systems Manfred van Afferden a,*, Khaja Z. Rahman a, Peter Mosig a, Cecilia De Biase b, Martin Thullner b, Sascha E. Oswald c, Roland A. Mu¨ller a a

Centre for Environmental Biotechnology (UBZ), UFZeHelmholtz Centre for Environmental Research, Permoserstrasse 15, 04318 Leipzig, Germany b Department of Environmental Microbiology, UFZeHelmholtz Centre for Environmental Research, Permoserstrasse 15, 04318 Leipzig, Germany c Institute for Earth and Environmental Sciences, University of Potsdam, Potsdam, Germany

article info

abstract

Article history:

Field investigations on the treatment of MTBE and benzene from contaminated ground-

Received 23 May 2011

water in pilot or full-scale constructed wetlands are lacking hugely. The aim of this study

Received in revised form

was to develop a biological treatment technology that can be operated in an economic,

4 July 2011

reliable and robust mode over a long period of time. Two pilot-scale vertical-flow soil filter

Accepted 5 July 2011

eco-technologies, a roughing filter (RF) and a polishing filter (PF) with plants (willows), were

Available online 14 July 2011

operated independently in a single-stage configuration and coupled together in a multistage (RF þ PF) configuration to investigate the MTBE and benzene removal perfor-

Keywords:

mances. Both filters were loaded with groundwater from a refinery site contaminated with

Benzene

MTBE and benzene as the main contaminants, with a mean concentration of 2970  816

Groundwater remediation

and 13,966  1998 mg L1, respectively. Four different hydraulic loading rates (HLRs) with

Hydraulic loading rate

a stepwise increment of 60, 120, 240 and 480 L m2 d1 were applied over a period of 388

MTBE

days in the single-stage operation. At the highest HLR of 480 L m2 d1, the mean

Pilot-scale constructed wetland

concentrations of MTBE and benzene were found to be 550  133 and 65  123 mg L1 in the

Vertical-flow soil filter

effluent of the RF. In the effluent of the PF system, respective mean MTBE and benzene

Willow tree

concentrations of 49  77 and 0.5  0.2 mg L1 were obtained, which were well below the relevant MTBE and benzene limit values of 200 and 1 mg L1 for drinking water quality. But a dynamic fluctuation in the effluent MTBE concentration showed a lack of stability in regards to the increase in the measured values by nearly 10%, which were higher than the limit value. Therefore, both (RF þ PF) filters were combined in a multi-stage configuration and the combined system proved to be more stable and effective with a highly efficient reduction of the MTBE and benzene concentrations in the effluent. Nearly 70% of MTBE and 98% of benzene were eliminated from the influent groundwater by the first vertical filter (RF) and the remaining amount was almost completely diminished (w100% reduction) after passing through the second filter (PF), with a mean MTBE and benzene concentration of 5  10 and 0.6  0.2 mg L1 in the final effluent. The emission rate of volatile organic compounds mass into the air from the systems was less than 1% of the inflow mass loading rate. The results obtained in this study not only demonstrate the feasibility of vertical-flow soil filter systems for treating groundwater contaminated with MTBE and benzene, but can

* Corresponding author. Tel.: þ49 341 235 1848; fax: þ49 341 235 1830. E-mail address: [email protected] (M. van Afferden). 0043-1354/$ e see front matter ª 2011 Elsevier Ltd. All rights reserved. doi:10.1016/j.watres.2011.07.010

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also be considered a major step forward towards their application under full-scale conditions for commercial purposes in the oil and gas industries. ª 2011 Elsevier Ltd. All rights reserved.

1.

Introduction

Due to the widespread use of fuels, BTEX compounds (benzene, toluene, ethylbenzene, m-, o-, and p-xylene) and MTBE (methyl tertiary-butyl ether) are frequently detected groundwater contaminants, with releases occurring during their production, transportation and storage (Baehr et al., 1999; Deeb et al., 2000; Squillace et al., 1996). MTBE has received considerable attention in recent times as it migrates much more quickly through the soil than most of the petroleum distillates due to its high water solubility (up to 51 g L1, USEPA, 2004). Its presence in the environment is considered as a health and drinking water problem and classifies MTBE as a possible human carcinogen (Johnson et al., 2000). MTBE is relatively resistant to biological degradation under anaerobic conditions (Moreels et al., 2006), but several studies have shown a biodegradability under aerobic conditions (Deeb et al., 2000; Ferreira et al., 2006; Schmidt et al., 2004). Benzene is carcinogenic and the most water soluble BTEX compound. It can also be degraded by many microorganisms under aerobic conditions (Yerushalmi et al., 2002). The present limit concentrations established by the United States Environmental Protection Agency and the German guideline value are 200 mg L1 for MTBE and 1 mg L1 for benzene in drinking water (USEPA, 2005; DVGW, 2001). The physico-chemical properties especially the high water solubility and the low carbon adsorption coefficient of MTBE make it difficult to treat these organic contaminants by using conventional groundwater treatment technologies and represent some unique remediation challenges. The active exsitu remedial methods include air stripping and removal with granular activated carbon, vapour extraction, advanced chemical oxidation and multiphase high-vacuum extraction (Davis and Powers, 2000; Deeb et al., 2003; Sutherland et al., 2004; Wilhelm et al., 2002). However, the cost associated with the construction, maintenance and operation of these treatments diminishes their feasibility. Constructed wetland (CW) systems represent an effective and inexpensive option for treating municipal wastewater and becoming available due to their wide range of applications (Cooper, 1999; Kadlec and Wallace, 2009). They are also accepted as an alternative method to the commonly used engineering-based treatment technologies for the removal of organic contaminants from surface water or groundwater (Rubin and Ramaswami, 2001; Kassenga et al., 2004; Lorah and Voytek, 2004). In general, the vertical-flow constructed wetlands or soil filters are gaining popularity due to their greater oxygen transfer capacity and smaller size as compared to the horizontal-flow wetland systems (Cooper, 1999; Kadlec and Wallace, 2009). The findings of Eke and Scholz (2008) suggested that intermittently flooded vertical-flow constructed wetlands are able to effectively treat benzene from hydrocarbon-contaminated wastewater streams in the

presence of sufficient oxygen and fertilizer. But very little is known about the technical use of vertical-flow constructed wetlands for the removal of both MTBE and benzene from heavily contaminated groundwater. The SAFIRA-project (remediation research in regionally contaminated aquifers) is an interdisciplinary research project focussing on innovative remediation technologies to treat complex groundwater contamination. Within the framework of this research project, a pilot plant was constructed at a refinery in Leuna, Germany, aiming at the investigation and development of eco-technologies for the removal of volatile organic compounds. Since the groundwater treatment technology currently used in Leuna (pumpand-treat system associated with an air stripping and adsorption unit) is very expensive and requires high maintenance efforts, the aim of this work was to develop an alternative biological treatment technology that can be operated in an economic, reliable and robust mode over a long period of time. Therefore, a specially designed pilot-scale subsurface vertical-flow constructed wetland system was installed and operated at the Leuna site for field investigations on the removal of MTBE and benzene as the main groundwater contaminants. In order to identify the potential factors influencing the treatment efficiencies, the dynamics of MTBE and benzene were investigated using pilot-scale single-stage and multi-stage single-pass vertical-flow soil filter ecotechnologies with different hydraulic loading rates (HLRs) in this study. As far as we are aware, no such biological treatment system has been explored to date in pilot-scale facilities for treating MTBE and benzene compounds from contaminated groundwater using the planted and unplanted verticalflow soil filter systems, nor has the effect of the different hydraulic loading conditions been directly compared. The main objectives of this study were: (i) to explore the treatment performances of pilot-scale single-stage and multistage single-pass vertical-flow soil filter systems for removing MTBE and benzene from contaminated groundwater; (ii) to evaluate the potential effects of the different hydraulic loading rates (HLR) on the treatment efficiencies in both systems; and finally (iii) to assess the feasibility of applying a vertical-flow soil filter eco-technology to treat MTBE and benzene contaminated groundwater under full-scale conditions for commercial purposes.

2.

Materials and methods

2.1.

Site location and groundwater composition

The pilot-scale treatment facility was built near the Leuna refinery in the North-East of Germany in 2007. Due to accidental spills, improper handling (leaking underground storage tanks, pipelines, etc.), and damages due to heavy bombing

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during World War II, the groundwater in this area is heavily contaminated with high concentrations of different gasoline components (Martienssen et al., 2006). The fuel additive MTBE and benzene are the predominant groundwater contaminants at the site, with mean concentrations of 2970  816 and 13,966  1998 mg L1, respectively. The mean concentrations and standard deviations of the main organic and inorganic compounds present in the water and the geochemical characteristics of the influent groundwater observed during the investigation period are given in Table 1.

2.2.

Filter design

The two vertical-flow soil filters used in this study, the Roughing Filter (RF) and Polishing Filter (PF), consisted of two identical stainless steel containers (length: 2.30 m, width: 1.75 m, depth: 1.75 m), with a surface area of 4.025 m2 and a total volume of 7.04 m3. Both filters were filled with a granular material of different grain sizes and arranged in layers of varying configurations (Fig. 1). The filters were part of a larger pilot plant with central maintenance facilities and operated outdoors at the site, with their surface exposed to the local climatic conditions. The Roughing Filter (RF) consisted of three successive layers of filter packing materials: a cover layer on the top (25 cm), a main filter layer (120 cm) in the middle and a bottom layer (10 cm), which served as the drainage layer. The bottom drainage layer was separated from a 20 cm deep sump by a perforated steel plate. The cover layer was composed of coarse expanded clay material (8e16 mm), facilitating water distribution over the entire filter surface area and protecting

Table 1 e Influent groundwater characteristics based on samples collected during the whole experimental operation period of 20 months (from September 2008 to May 2010, except where noted). Parameter

MTBE Benzene Cl NHþ 4 SO2 4 PO3 4 Fe2þ Ca2þ Fetot Ptot Kþ Naþ Mg2þ Mn2þ O2 Eh s pHa Ta

Unit

mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mg L1 mV mS cm1 e  C

Inflow groundwater composition Mean

Standard deviation

Number of samples

2970.18 13,965.62 116.85 51.04 11.09 1.20 6.73 205.73 6.69 0.84 12.36 132.38 58.02 1.63 0.10 432.25 2.32 7.45 12.20

816.25 1997.88 9.96 9.34 8.95 0.75 2.36 14 1.57 0.18 0.87 8.03 3.20 0.23 0.07 161.7 0.40 0.35 3.11

484 469 44 44 44 44 44 44 43 44 44 44 44 44 57,075 57,935 57,946 54,046 54,046

a Online measurement from September 2008 to April 2010.

Fig. 1 e Schematic diagram of the roughing filter (RF; on the top) and the polishing filter (PF; on the bottom): (1) Inflow feeding pipe; (2) Distribution pipe; (3) Layered filter material; (4) Sump; (5) Plant biomass; (6) Drainage outlet pipe.

the surface of the main layer from erosion. The 25-cm thick cover layer was designed to reduce the emission of volatile organic compounds. The underlying main layer consisted of expanded clay material with a grain size in the range of fine gravel (3e6 mm). One reason for using such a gravel material was to prevent clogging due to a potential precipitation of iron and carbonate within the filter bed. The advantage of the larger pore spaces within these gravel particles reduced the chances of filter clogging and increased the possibility of applying higher hydraulic loads, which eventually facilitated this filter system to serve as a potential first treatment step. Finally, the drainage layer at the bottom consisted of crushed gravel (8e16 mm), which prevented the washing out of fine particles into the sump. The Polishing Filter (PF) comprised four successive layers. The 15-cm cover layer on the top consisted of a coarse expanded clay material (8e16 mm). The underlying main filter layer of 120 cm was filled with zeolite material (zeosoil; grain size 0e5 mm). The reason for using a finer material was that the proportion of the finer particles caused a greater surface area. Moreover, a longer hydraulic retention time is associated with a higher degradation of organic pollutants and a homogeneous distribution of the contaminated groundwater within this main filter layer. Zeolites have a larger surface area, a special texture and inner structure, as compared to conventional sand, and were therefore used within this filter system. However, their smaller pore spaces are associated with the risk of filter clogging and hence this filter system was designed to serve as a potential second treatment step. To facilitate better water discharge, the PF was constructed of two drainage layers underlying the main layer. The upper 20cm drainage layer consisted of crushed gravel (8e16 mm)

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and IV). The duration of each experimental phase was preset to guarantee that a representative number of samples were taken from each system. Detailed information on the operational strategies and loading schedules of both systems is listed in Table 2. During operational period 2 (days 388e611), the filters (RF and PF) were connected to each other and operated in series as a multi-stage single-pass vertical-flow filter system (RF þ PF). The RF was receiving the contaminated groundwater from the inflow storage tank and served as a first treatment step with a hydraulic loading rate (HLR) of 960 L m2 d1. The pre-treated groundwater from the effluent of the RF was then pumped into the second filter (PF) and passed through the second system at a hydraulic loading rate of 480 L m2 d1. The remaining 50% of the RF-effluent was sent to the nearby technical groundwater remediation plant for further treatment (stripping coupled with activated carbon adsorption) and then re-injected into the aquifer. With the highest HLR of 960 L m2 d1 in the RF system, we were interested to see if any hydraulic or technical problems occur, such as clogging, overloading, etc. This experimental phase V was run over a period of 223 days (days 388e611). Similarly to operational period 1 (single-stage configuration), both filters (RF and PF) were intermittently loaded with repeated pulses of groundwater (Table 2). The experiment started with period 1 in September 2008 and continued until the end of period 2 in May 2010. Willow trees on the PF system showed an active growth of their biomass, densely covering the whole filter surface area with green and healthy shoots before the start of the experiment.

followed by another 20-cm layer packed with even coarser crushed gravel (16e32 mm) and placed at the bottom of the filter. The PF was planted with white willows (Salix alba) on the top, with a density of around 5 plants m2. Trees of almost equal biomass (average height of 50 cm) and strength were obtained from a local supplier and uniformly planted at the end of August 2007. Willow trees were used due to their high biomass productivity, their relatively high resistance to organic contaminants, their ability to adapt to a broad range of climatic and site specific conditions, their broad reaching root systems, and their common use for phytoremediation (Mleczek et al., 2010; Rentz et al., 2005). The RF was unplanted in this investigation. The contaminated groundwater was injected from the top of each filter through a uniform distribution system of perforated PVC pipes, which was laid horizontally under the cover layer. Water drained through the filter media to the bottom of each basin, from where it was collected and discharged at the outflow by a PVC drainage pipe.

2.3.

Experimental conditions for filter operation

Contaminated groundwater was pumped by a timercontrolled pump into an anaerobic storage plastic container (Volume: w3 m3). Another timer-controlled pump distributed the water as intermittent loads through distribution pipes onto the surface of the two filter systems. This intermittent dosing of water was chosen to provide good oxygen transfer to the water phase (Kadlec, 2001). The pulse frequencies for the two filters under different experimental conditions are presented in Table 2. The experimental strategy was divided into two distinctly different operation periods. During operational period 1 (days 0e388), both filters (RF and PF) were operated independently as single-stage single-pass vertical-flow filter systems and received the influent groundwater separately from the same storage tank in parallel. Four different hydraulic loading rates (HLRs) were applied to the systems and increased stepwise (60, 120, 240 and 480 L m2 d1) over the period of 388 days comprising four different experimental phases (phase I, II, III

2.4.

Sampling and analysis

Concentrations of dissolved MTBE and benzene at the influent and effluent of each system were analysed online using a completely automated gas chromatograph (GC) equipped with a photoionisation detector (PID) (META Water sampling and analysis system WSS3; type: meta 3 HE II/PID, META, Messtechnische Systeme GmbH, Dresden, Germany). An Ultimetal column with a length of 25 m was used and the carrier gas was synthetic air, set at 5 bar. The oven and

Table 2 e Operation strategies and different experimental conditions (hydraulic loading schedules) of the vertical-flow soil filter systems during the whole investigation period. Period 1

2

Stage Single

Multiple

Phase

Duration (day)

I

0e86

II

86e235

III

235e297

IV

297e388

V

388e611

a Multi-stage combined system (RF þ PF).

Vertical filter

Volume of water per load (L)

Loading pulses per day ()

Injection interval (min)

HLR (L m2 d1)

RF PF RF PF RF PF RF PF

10 12 20 24 40 48 80 80

24 20 24 20 24 20 24 24

60 72 60 72 60 72 60 60

60 60 120 120 240 240 480 480

RFa PFa

80 60

48 32

30 45

960 480

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injection/detection port temperatures were 60 and 80  C, respectively. The detection limit for MTBE and benzene was 0.37 and 0.18 mg L1, respectively. The determination of detection and quantification limits of the calibration procedure was carried out in accordance with DIN 32645 (1994). Process control and data storage were carried out using the installed software (metaControl) that stored all the measurements and optional external signals on the hard disk of the attached PC. Known intermediate degradation products of MTBE, such as tert-butyl alcohol (TBA), tert-butyl formate (TBF) and aromatic hydrocarbons such as toluene, ethylbenzene, m-pXylene, o-Xylene, 1,3,5-Trimethylbenzene, 1,2,4Trimethylbenzene and Naphthalene were analysed in both influent groundwater and effluents from the filters by headspace gas chromatography and mass spectroscopic detection (HS-GCeMS). For headspace analysis, aqueous samples (10 ml) were stirred for 60 min at 70  C in headspace vials (20 ml) containing 2.5 g NaCl. Gas from the headspace (1 ml) was injected into a GC/MS (GC: Agilent 6890, MS: Agilent 5973) equipped with a 60 m HP1 column (Split injection 1:25, injection time 2 min). The time program was: 35  C for 6 min, to 120  C with 4  C/min and to 280  C with 20  C/min, held at 280  C for 5 min. The measuring time is 65 min per sample. The detection limit for TBA was 1.56 mg L1 and for other substances specified above was <1 mg L1.

2.5.

Emission measurement

In principle, the contaminated groundwater comes in contact with the atmosphere in both filter systems, and hence emissions of volatile organic substances are expected in the air during the treatment operation period. The volatile organic compounds (VOCs) were measured in terms of total organic carbon in a range of 0e100 mg TOC m3 using a mobile flame ionisation detector, FID 3-100 (JUM Engineering GmbH, Karlsfeld, Germany). The continuous flame ionisation chamber was heated up to 190  C. The measurements were performed at different heights (10, 20, 50, and 100 cm) in the air just above the centre (middle point) of each filter surface and also at same height immediately above the line of the inflow distribution pipe (inlet point) installed below the top layer of the filters. Moreover, the measurements were taken approx. 1e2 m downstream of each filter segment in the direction of the out-flowing wind (at 40 cm height; downwind) and approx. 5 m away from the filters against the wind direction (at 40 cm height; upwind) as a background value. The emission of VOCs in terms of TOC in mg m3 air was measured at an HLR of 480 L m2 d1 in both filter systems (experimental phase IV, single-stage operation). Measurements were taken in different measuring cycles over the RF and the PF system. Duration of each cycle was 60 min, which included an inflow feeding pulse with duration of 4e8 min and a continuous measurement of emission in the air at different specified heights. The emission of VOCs in terms of TOC in mg m3 from each measuring heights and also the background values were recorded over one feeding pulse interval in one cycle. The net emission at each particular height (measuring points) was calculated by subtracting the background value from the measured emission value attained at

that particular height. Four cycles were carried out for the emission estimation over the RF and only two cycles for the PF in this experimental phase with a same HLR in both the filters. Since wind can have a strong influence on the measurements, mobile walls were built around the filters to limit the movement of the air above the filter beds to a wind speed range of 0.1e0.5 m s1. The emitted mass of VOCs in each feeding pulse was also calculated with the assumption that the certain volume of water feeding on the filter segment per pulse was displacing the same volume of air which was coming out over the filter surface. Based on this assumption as a preliminary emission estimation study, the rate of emitted mass from each filter surface in terms of mg TOC m2 d1 and percentage of emission (%) from the inflow loading mass that goes in the atmosphere (air) were calculated.

3.

Results

3.1. Dynamics of MTBE and benzene: single-stage systems The influent and effluent dynamics of MTBE and benzene in the RF and PF system within the different experimental phases are shown in Figs. 2 and 3. During experimental phase I (days 0e86) with an HLR of 60 L m2 d1, the mean MTBE concentration in the effluent of the RF was detected to be 139  69 mg L1, which was below the limit value of 200 mg L1 for MTBE. In contrast, a relatively higher and wide range of MTBE concentration with a mean value of 332  680 mg L1 was

A B

Fig. 2 e Influent and effluent concentrations of MTBE in the A) RF and B) PF system during different experimental phases (IeIV) of operational period 1.

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A B

Fig. 3 e Influent and effluent concentrations of benzene in the A) RF and B) PF system during different experimental phases (IeIV) of operational period 1.

measured in the effluent of the PF system. In this phase, a mean reduction in the MTBE concentration of 97 and 93% was obtained, when the RF and PF single-stage system was used, respectively. Consequently, a mean concentration of benzene of 64  76 and 0.3  0.2 mg L1 in the effluent of the RF and the PF, respectively, was measured. An extremely high reduction in the benzene concentration (w100%) was observed in the PF system. During experimental phase II (days 86e235) with an HLR of 120 L m2 d1, the mean effluent MTBE concentration in the RF increased at the beginning and then steadily slowed down to a mean value of 399  318 mg L1. A relatively sharp decreasing tendency within the effluent MTBE concentration of the PF was observed in the middle part of phase II and continued with a very low concentration until the end of this phase (see Fig. 2A and B). However, a mean value of 91 and 93% reduction in the MTBE concentration was achieved in the effluent of the RF and the PF, respectively. Similarly to the effluent dynamics of MTBE, the effluent benzene concentration in the RF was increased gradually and then lowered down to a mean effluent concentration of 413  736 mg L1 from a mean influent concentration of 15,126  2382 mg L1. In the PF, no particular trend was seen in the dynamics of the effluent benzene concentration and a relatively higher mean value of 11  53 mg L1 with a great deviation was detected, as compared to the previous phase I (see Fig. 3A and B). In experimental phase III (corresponding to days 235e297) with an HLR of 240 L m2 d1, both the systems RF and PF started to develop differently as it was observed in the MTBE and benzene effluent dynamics. A mean effluent value of 402  222 mg L1 resulted in a mean MTBE-concentration

reduction of 84% in the RF, whereas in the PF, the effluent MTBE concentration sharply decreased almost immediately after changing the experimental phase and maintained a low concentration until the end of the phase. A mean value of 43  90 mg L1 resulted in a remarkable reduction (w99%) of the mean MTBE concentration in the PF system (Fig. 2A and B). In the case of benzene, the effluent concentration varied drastically in the RF even though there was a relatively constant influent and a very high mean effluent value of 401  803 mg L1 at the end of this experimental phase. In contrast, a highly efficient reduction (w100%) in the benzene concentration was monitored in the effluent of the PF, with a mean value of 0.3  0.2 mg L1 (Fig. 3A and B). At a higher HLR of 480 L m2 d1 in the next experimental phase IV (corresponding to days 297e388), a relatively constant effluent MTBE concentration was observed in the RF, with a mean value of 550  133 mg L1, which contributed to a mean MTBE-concentration reduction of 75% from the influent. No particular trend in the reduction of the MTBEconcentration values was detected within the effluent dynamics of the RF and a continuous fluctuation in the MTBE concentration with a wide range of values was observed in the effluent of the PF. Nevertheless, the mean effluent MTBE concentration of 49  77 mg L1 in the PF was nearly 11-fold lower than the mean MTBE concentration of 550  133 mg L1 in the RF (Fig. 2A and B). Similarly to the previous experimental phase III, the dynamics of benzene in the effluent of the RF and the PF showed a completely opposite trend. In the RF system, a rapid fluctuation in the benzene concentration values showing no particular reduction trend resulted in a mean effluent benzene concentration of 65  123 mg L1, whereas a relatively constant trend in concentration reduction was observed in the effluent of the PF. The mean value of 0.5  0.2 mg L1 in the effluent contributed to a highly efficient (w100%) reduction in the benzene concentration of the PF system, as compared to the RF (see phase IV; Fig. 3A and B).

3.2. Dynamics of MTBE and benzene: multi-stage system The dynamics of MTBE and benzene in both the influent and effluent of the combined multi-stage vertical-flow soil filter system (RF þ PF) during operational period 2, in the experimental phase V (corresponding to days 388e611), are shown in Fig. 4. The RF system as the first filter received contaminated groundwater at an HLR of 960 L m2 d1. The mean influent MTBE-concentration value of 2760  594 mg L1 was reduced to a mean effluent value of 831  318 mg L1, which resulted in a mean MTBE-concentration reduction of 69% in this treatment step. This effluent of the RF system was pumped intermittently onto the surface of the second filter (PF) at an HLR of 480 L m2 d1. The results demonstrated a remarkable (w99%) reduction in the MTBE concentration of the effluent of the PF with a mean value of 5  10 mg L1. Although the dynamics of the MTBE concentration in the effluent of the PF showed a rapid fluctuation in the values during this experimental phase, all the effluent concentration values were well below the limit value of 200 mg L1 for MTBE.

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0.6  0.2 mg L1 in the effluent of the second filter contributed to an almost complete (w100%) removal of benzene in this combined multi-stage system. The overall treatment performances obtained in both filters (RF and PF) during the whole operational period of this study are summarized in Table 3. The mean concentrations of intermediate degradation products of MTBE (TBA, TBF) and other aromatic hydrocarbons such as toluene, ethylbenzene, m-p-Xylene, o-Xylene, 1,3,5-Trimethylbenzene, 1,2,4-Trimethylbenzene and Naphthalene in the influent groundwater and effluents of both the RF and the PF are given in Table 4. Both TBA and TBF were detected with a low mean concentration value in the effluent of the RF and the PF system during the single-stage operational phase, but their concentrations were almost diminished or went below the detection limit in the final effluent after passing the multi-stage system. Similarly, in the final effluent of the multi-stage operational phase, the other aromatic hydrocarbons could not be detected due to a very low concentration value at the end (see Table 4).

A

B

3.3. Fig. 4 e Influent and effluent concentration along with the limit value of A) MTBE and B) benzene in the multiphase combined (RF D PF) system during experimental phase (V) of operational period 2 (days 388e611).

Emission estimation

The emissions of the volatile organic compounds (VOCs) in the air phase at several specified heights over the vertical-flow soil filter systems at a given day in several measurement cycles were registered and plotted on curves in this study. The measurements were recorded during the single-stage operation period at the same HLR of 480 L m2 d1 in both filters (experimental phase IV). An example of emission calculation in one measurement cycle over the RF and PF is given in Fig. 5A and B. In the RF, the inflow feeding pulse with duration of nearly 4 min contributed to an immediate displacement of inside trapped air to the filter surface and over a period of approximately 17 min, the displaced air disappeared and the emission level came back to the concentration at the background value until the end of the 60 min cycle (Fig. 5A). Emissions of VOCs were measured in this 17 min time duration and the obtained results showed that the highest emission with a concentration of 12.27 mg TOC m3 was recorded

In the case of benzene, the first filter (RF) received the influent groundwater with a mean benzene concentration of 13,527  1638 mg L1 and a drastic fluctuation was observed in the effluent benzene concentration values of this filter system. The values were spread out over a large range but a mean effluent value of 291  573 mg L1 resulted in a mean reduction in the benzene concentration of 98% from the influent, which did not meet the allowable limit value of 1 mg L1. However, after passing through the second filter (PF), a remarkably low and stable benzene concentration was detected in the effluent of the PF system. The mean value of

Table 3 e Summary of the treatment performances in the RF and the PF during the whole operational period of 611 days. Stage

Single

Phase

I II III IV

Multiple

V

Filter

MTBE

Benzene

Influent (mg L1)

Effluent (mg L1)

Removal (%)

n

Influent (mg L1)

Effluent (mg L1)

Removal (%)

n

RF PF RF PF RF PF RF PF

3953  298 4337  338 3850  680 4207  456 2635  490 3104  587 2214  266 2204  301

139  69 332  680 399  318 289  370 402  222 43  90 550  133 49  77

97 93 91 93 84 99 75 98

40 50 92 99 47 51 83 84

15,574  2800 18,695  1578 15,126  2382 17,030  2664 13,046  1463 14,856  1115 13,649  1142 13,052  2462

64  76 0.3  0.2 413  736 11  53 401  803 0.3  0.2 65  123 0.5  0.2

99 100 98 100 97 100 99 100

36 26 77 61 45 46 83 46

RFa PFa

2760  594 831  318

831  318 5  10

69 99

154 140

13,527  1638 291  573

291  573 0.6  0.2

98 100

154 100

a Multi-stage combined system (RF þ PF), n: number of samples.

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Table 4 e Concentration of intermediate degradation products of MTBE and other aromatic compounds analysed in the influent groundwater and the effluents of the RF and the PF system during the whole operational period of 611 days. Substance

Single-stage (phase IeIV) Influent (mg L1)

TBA TBF Toluene Ethylbenzene m-p-Xylene o-Xylene 1,3,5-Trimethylbenzene 1,2,4-Trimethylbenzene Naphthalene

53 4.1 8 50 76 6.5 4.7 393 68

        

12 1 1.4 37 53 2.9 3.6 141 22

Effluent, RF (mg L1) 13  2 1.1  1.4  1.3  1.2  1.6  2.6  1.2 

8 1.5 0.1 0.1 1.0 0.6 1.0 2 0.6

Multi-stage (phase V)

Effluent, PF (mg L1)

n

Influent (mg L1)

43 2  1.5 n.d. 1.1  0.2 1.3  0.5 n.d. n.d. 1.2  0.6 n.d.

24 24 24 22 22 23 21 24 24

41 2.7 6.7 31 57 6.7 2.4 252 36

6  1.3  0.7  14 7  0.9  1.5  63 9

Effluent, RF (mg L1)

Effluent, PF (mg L1)

n

13  3 1.3  0.5 n.d. 1.1  0.3 n.d. n.d. n.d. 4.1  3.1 n.d.

21 n.d. n.d. n.d. n.d. n.d. n.d. n.d. n.d.

12 11 11 11 11 11 9 12 12

n.d.: below the limit of detection (<1 mg L1), n: number of samples.

at the height of 10 cm over the inlet pipe (buried underneath the top layer of the RF). The concentration of VOCs decreased down to background level after this 17 min time period. Therefore, the net emission at the highest concentration measurement point was estimated as 10.55 mg TOC m3, which was achieved by subtracting the mean background value of 1.72  0.06 mg TOC m3 from the highest measured emission value (H10 Inlet data set, Fig. 5A). For better estimation in the PF system, the inflow feeding pulse duration was adjusted to nearly 8 min and measurement values were recorded over the next 8e10 min after the dosing. The maximum concentration of 3.18 mg TOC m3 was registered at the height of 10 cm over the central middle point

A

B

of the filter surface. The net emission at that highest concentration measurement point was calculated as 0.72 mg TOC m3, by subtracting the measured mean background value of 1.4  0.02 mg TOC m3 from the highest emission value measured in this measuring cycle over the PF (H10 Middle data set, Fig. 5B). The regional background levels were consistent during the whole emission measurement experiment with values in the range of 1e2 mg TOC m3 measured in the air. After each feeding pulse, it was assumed that a total volume of 80 L water (Table 2) was flushing on the filter surface and the same volume of 80 L entrapped air was coming out from the filter over the surface within a short time (8e20 min) and then disappeared. Based on this assumption for a preliminary emission calculation, it was observed that a mass of 42.77 mg TOC m2 d1 was emitted over the segment of the RF system. This was calculated quantitatively by the triangular area under the curve (actual emission measuring zone by connecting the H10 Inlet data set, Fig. 5A) multiplied by the HLR of 480 L m2 d1. After this particular emission zone within the curve, the dynamics of measured air emission came back to the concentration at the background level until the cycle ends and hence they were not taken into account in this calculation. Comparing to the inflow TOC mass loading rate of 7566.94 mg TOC m2 d1 to the filter bed, it was observed that only 0.45% of the inflow TOC mass was emitted over the surface and went into the surrounding atmosphere. Similar calculation approach by using the H10 Middle data set (Fig. 5B) estimated that the emitted VOCs mass percentage value was even lower (0.04%) in the PF system, as compared to the RF system under the same HLR. The summary of emission measurement calculations in each cycle over the RF and the PF system is given in Table 5.

4.

Discussion

4.1. MTBE and benzene removal performances: singlestage systems Fig. 5 e Concentration of VOCs emission from the surface of the A) RF and B) PF measured overall sampling heights in cycle 1 with the same HLR in the single-stage operation.

During the stepwise increase of the HLR (60e480 L m2 d1) in the first operational period of the single-stage systems, the RF

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Table 5 e Summary of the emission measurement cycles during a particular day over the surface of the RF and the PF system with the same HLR obtained in single-stage operation. Stage

Filter

Single (HLR: 480 L m2 d1)

RF

PF

Measuring cycle

Inflow mass loading (mg TOC m2 d1)

Emitted mass over segment (mg TOC m2 d1)

Emission (%)

1 2 3 4 1 2

7566.94

42.77 50.76 30.23 21.41 2.98 1.37

0.45 0.67 0.40 0.28 0.04 0.02

7277.39

clearly showed a decreasing tendency in the mean MTBE removal (%), which means the lower the HLR, the better the MTBE removal from this system (Table 3). Moreover, the overall reductions in MTBE concentrations were not sufficient to reach the limit value of 200 mg L1 (USEPA, 2005; DVGW, 2001). In contrast, the PF showed a better performance of increasing the MTBE removal (%) associated with increases in the HLR. At the highest HLR of 480 L m2 d1 in the PF, the mean effluent MTBE concentration was recorded to be 49  77 mg L1, which was nearly 7 times less than the mean effluent MTBE concentration (332  680 mg L1) at the lowest HLR of 60 L m2 d1 and well below the limit value of 200 mg L1 for MTBE. However, although the mean MTBE concentration of 49  77 mg L1 was below the limit value, a rapid fluctuation in the dynamics of the effluent MTBE concentration was observed and nearly 10% of the measured values were higher than the limit value of 200 mg L1 (see Fig. 2B). This indicates that the MTBE removal performance obtained in the PF system was sufficient, but not stable. In the case of benzene, a mean removal within a range of 97e99% was achieved in the RF, but the overall effluent concentrations were never below the limit value of 1 mg L1. In contrast, the PF system exhibited a highly efficient benzene concentration reduction and the effluent concentrations were predominantly found to be below 1 mg L1 (see Fig. 3A and B). The differences in the MTBE and benzene treatment performance observed in the RF and PF might be due to the differences in filter designs mainly defined by the different filtering media used as the main filter materials (expanded clay in the RF and zeosoil in the PF; Fig. 1). The obtained results show that a filter loaded with a fine zeosoil (0e5 mm) filter material and plants is more efficient in MTBE and benzene removal than a filter loaded with a coarse expanded clay material (3e6 mm) without plants. This is probably due to a higher reactive surface area, a better oxygen transfer and a higher hydraulic retention time (data not shown) within the filter loaded with fine materials. But finer materials have the disadvantage of a possible filter clogging and also the problem associated with water saturation at high hydraulic loads. Additionally the differences in effective depth and different compactions of the filter bed, different gas exchange rates and planteroot activity in the case of the PF that provide oxygen to the rhizosphere (Scholz, 2006) might explain the observed differences in treatment performance. However, more investigations are needed before making any concluding remarks on these particular assumptions.

An effective benzene biodegradation could be expected in the two RF and PF filter systems, since this pollutant has been degraded in environmental systems even under hypoxic conditions and treatment efficiencies for aerobic bioreactors up to 100% have been described (Yerushalmi et al., 2002). In contrast, MTBE biodegradation is reported to be by far not as effective as benzene biodegradation. The possible reasons might be that MTBE is more resistant to enzymatic attacks due to its tertiary carbon atom and the ether bond (Davidson and Creek, 2000). Moreover, it is reported that the biodegradation of MTBE might be inhibited due to the presence of cocontaminants, such as benzene, ethylbenzene, toluene and xylene (BTEX), and the accumulation of by-products from the biodegradation of BTEX compounds (Raynal and Pruden, 2008). An inhibition of MTBE biodegradation in the presence of BTEX due to a potential production of by-products has also been suggested by others (Deeb et al., 2001; Sedran et al., 2002). These studies have focused mainly on substrate inhibition (Park, 1999), by-product inhibition (Wilson et al., 2002) or competitive inhibition (Sedran et al., 2002). Therefore, the presence of BTEX compounds in the groundwater of the refinery site was expected to inhibit MTBE biodegradation in both the RF and PF systems. But these inhibitory effects on MTBE biodegradation could not be observed in both the filters during this study. Table 4 shows the presence of BTEX and other aromatic hydrocarbons in the influent groundwater with a mean concentration value and still a highly efficient MTBE-concentration reduction (93e98%) can be seen especially in the effluent of the PF during singlestage operation (Table 3). However, very little is known about the microbial community structure during the aerobic MTBE degradation in the presence of BTEX. The disappearance of MTBE metabolites, such as tert-butyl alcohol (TBA) and tert-butyl formate (TBF) indicated the potential of complete biodegradation within the filter systems. As can be seen in Table 4, the mean concentrations of TBA and TBF were remarkably decreased in the effluent of the RF and the PF system during the single-stage operation period and almost completely diminished or biodegraded after passing through the second filter (PF) during the multistage operation period.

4.2. MTBE and benzene removal performances: multistage system The second operational period with a multi-staged combined (RF þ PF) vertical-flow soil filter system runs very well with

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a highly efficient MTBE and benzene removal. Both MTBE and benzene effluent concentrations were always stable and well below the allowable limit value after the second treatment step. Moreover, the high HLR in the first filter (RF) was not accompanied by hydraulic or technical problems, such as clogging, overloading, etc., and it was thus very encouraging to a further increase of HLR, which could reduce the cost of the required land for the filter construction. Therefore, it can be concluded that the multi-stage combined vertical-flow soil filter system is more stable, more effective and a better option for the removal of MTBE and benzene from contaminated groundwater, as compared to the single-stage system. In principle, the overall decrease in the MTBE and benzene concentrations obtained from the vertical-flow soil filter systems can be caused by microbial degradation, sorption onto solid filter packing materials and volatilization. Moreover, in the planted filter system, it might also be caused by plant uptake followed by transport, transformation and phyto-volatilization. A long-term operation of the verticalflow soil filter systems is leading to an established adsorption/desorption balance and therefore, a removal by sorption onto the filtering media can be assumed to be negligibly small in this investigation. Biodegradation of MTBE and benzene is expected to be the most dominant process for the removal of these contaminants from the groundwater. However, the extent of degradation cannot be estimated accurately without a long-term and complete set of data in terms of volatilization and plant uptake from pilot-scale vertical-flow constructed wetlands. Eke and Scholz (2008) also concluded that the impacts of volatilization, biodegradation and adsorption on the benzene removal are often difficult to separate quantitatively from each other. For a long-term operation of the vertical-flow soil filters, the designed hydraulic loads need to be achieved by optimising the volume of the water in each loading pulse and the associated frequency, in order to increase the dewatering efficiency of the filters in the period of time between the intermittent pulses, and thus promoting oxygenation and achieving treatments with the highest possible level of efficiency.

4.3.

Emissions

After estimating the emissions of volatile organic compounds from the vertical-flow soil filter systems, the overall results indicated that the emissions from the planted PF systems in the air were much lower than those from the RF system and were only slightly above the background value (Fig. 5). By comparing to the inflow TOC mass loading rate, only a negligible amount (<1%) was emitted from the surface of the both RF and the PF systems. Therefore, with a highly efficient mean MTBE and benzene concentration reduction in the effluent of the RF and the PF system and almost a negligible emission rate of VOCs mass, it can be concluded that the biodegradation is the predominant removal pathway of both MTBE and benzene within the vertical-flow soil filter system treating contaminated groundwater. Volatilization of toxic organic hydrocarbons may be increased by technological problems, such as clogging and subsequent flooding, and may lead to serious air pollution

(Braeckevelt et al., 2008). But both of our vertical-flow soil filter systems were almost free of technical problems such as filter clogging, overloading, surface flooding, etc. Experimental investigations have shown that phyto-volatilization is a potential emission path for MTBE and benzene along with the direct volatilization via the soil surface of a constructed wetland (Reiche et al., 2010). However, more improved technical equipment is necessary for measuring both the VOCs concentration in the air and the volume of air emitted from the surface of the filter beds. Future investigations should be carried out with the purpose of a final evaluation of the volatilization rate of MTBE and benzene per unit area (m2) of the filter surface and with the aim of achieving a complete mass balance of organic compounds and discovering the role of the cover layer for protecting volatilization.

5.

Conclusions

The following conclusions can be drawn from the current study: 1. The Polishing Filter (PF) with a finer material and plants is more efficient in removing MTBE and benzene from contaminated groundwater, as compared to the Roughing Filter (RF) with a coarse material and without plants. 2. Factors, such as filter packing material, particle size, filter depth and loading rate, are playing an important role in achieving a robust filter operation for the removal of organic contaminants by vertical-flow soil filter systems. 3. The MTBE removal performance decreases with an increasing HLR in the RF, whereas the PF system is characterized by a remarkable MTBE and benzene removal performance at an increasing HLR. 4. At a higher HLR, the MTBE removal performance of a single-stage vertical-flow soil filter system is often stable, but not sufficient. 5. In general, a continuous reduction in both the MTBE and benzene concentration of the effluent indicates that the maximum treatment capacity is yet to be reached in both the RF and the PF systems. 6. The multi-stage combined vertical-flow soil filter system (RF þ PF) produces the most stable and sufficient effluent concentrations to reach the limit concentrations of MTBE and benzene for drinking water. 7. Since a negligible amount of volatile organic compounds is going in the air from our filter systems, therefore they are not considered to be a potential source of air pollution affecting the surrounding environment. 8. Since the vertical-flow constructed wetlands are accumulative systems (biomass, organic matter, calcareous material, etc.), it is of great importance to assess the optimal operation/design load of the filters treating MTBE and benzene and to predict the cases in which hydraulic overloads might be problematic for the filter longevity. 9. Our systems are designed to minimize clogging and after treating groundwater by using our technology, the water will not need to be amended further and can be released into an aquifer or discharged into any conventional drainage system.

w a t e r r e s e a r c h 4 5 ( 2 0 1 1 ) 5 0 6 3 e5 0 7 4

10. This novel groundwater remediation technology promises to be a cost effective remediation approach for treating groundwater contaminated with MTBE and benzene on a full-scale. 11. To prove the long-term stability and process optimization as well as to reach a sound economical and ecological evaluation for this new approach, a pilot-scale or fullscale operation over an extended period of time is needed. Further studies are intended: i) to focus on identifying the major microbial processes that lead to an aerobic biodegradation of organic contaminants, ii) to quantify volatilization, adsorption, absorption, mineralization and other removal mechanisms in large-scale vertical-flow soil filter systems treating MTBE and benzene, iii) to characterize and quantify plant uptake, phyto-sorption and phyto-volatilization of MTBE and benzene, iv) to explore the effects of seasonal temperature changes on the removal of MTBE and benzene, v) to investigate the effects of iron and calcium precipitations on the filter performance in treating MTBE and benzene contaminated groundwater, vi) to define the design criteria for the remediation of contaminated groundwater using verticalflow soil filter eco-technologies.

Acknowledgements This work was supported by the Helmholtz Centre for Environmental Research e UFZ in the scope of the SAFIRA II Research Programme (Revitalization of Contaminated Land and Groundwater at Megasites, sub-project ‘‘Compartment Transfer e CoTra’’) and funded by a grant from the German Federal Ministry of Education and Research (BMBF). The authors would like to thank Francesca Lo¨per, Grit Weichert, Sibylle Mothes, Heidrun Paschke, and Christina Petzold for their assistance in the field and laboratory work.

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