Aerated treatment pond technology with biofilm promoting mats for the bioremediation of benzene, MTBE and ammonium contaminated groundwater
Introduction
Fuel, fuel additives and ammonium are frequently detected water pollutants worldwide (Christensen et al., 2001, Squillace et al., 1996). Benzene, toluene, ethylbenzene, the three xylene isomers (m-, o-, and p-xylene, BTEX compounds) and methyl tertiary-butyl ether (MTBE) are highly soluble and therefore extremely mobile in groundwater systems (Squillace et al., 1996). Therefore, these compounds are of environmental concern and represent suitable organic contaminants for testing groundwater remediation system effectiveness. Benzene, the most toxic and water soluble BTEX compound, can be degraded by many microorganisms under oxic (Agteren et al., 1998) and even hypoxic conditions (Yerushalmi et al., 2002). MTBE biodegradation is slow and difficult due to steric effects within the molecule, but has been shown by several studies to be degraded under aerobic conditions (Ferreira et al., 2006, Schmidt et al., 2004). Notably, growth rates and biomass yields for aerobic MTBE degraders have been shown to be much lower than aerobic benzene degraders (Fortin et al., 2001). Both MTBE and benzene are highly recalcitrant under anoxic conditions (Foght, 2008, Häggblom et al., 2007). Ammonium can be aerobically oxidized to nitrate by slow growing microorganisms in a two-step process known as nitrification.
Groundwater fuel contamination is generally characterized by a large chemical oxygen demand (COD). Therefore, increased oxygen concentrations are positively correlated with biodegradation rates. However, it is a challenge to supply sufficient amounts of oxygen into contaminated aquifers due to its low solubility. Enhanced biodegradation of BTEX (Borden et al., 1997) and MTBE (Salanitro et al., 2000, Wilson et al., 2002) has been achieved by active measures such as direct oxygen injection, or passive measures like introduction of oxygen-releasing compounds into the system. However, the long-term efficiency of reactive barriers may be impacted by biofilm clogging or precipitate formation (Scherer et al., 2000).
Aerobic ponds have found worldwide application in municipal wastewater treatment; however, degradation of fuel related contaminants in combination with ammonium is poorly investigated in these systems (Thorneby et al., 2006). Photosynthetic algae and bacteria as well physical aeration are often used to support oxic degradation processes. Retaining biomass in the system is a prerequisite for efficient oxidation of contaminants and has been shown to enhance biodegradation potential in benchtop scale experiments (Korkut et al., 2006). Direct implementation of aerated trenches in contaminated shallow aquifers could represent a cost effective treatment technique, and has not yet been described to our knowledge.
The aim of this study was to evaluate the effectiveness of aerated trench systems for the reduction of BTEX and MTBE at a field scale. Our model system simulates a scenario where contaminated groundwater is passed through a treatment facility at a constant recharge rate, directly from the aquifer, before being released into the environment. The treatment aims to reduce COD and contaminant levels in the effluent by promoting aerobic biodegradation carried out by organisms contained in contaminant-degrading biofilms. We test the effectiveness of two different geotextiles, a polypropylene fleece and a natural coconut fibre. Polypropylene fibres are relatively inert with regard to extreme pH, salinity and temperature conditions in comparison to polyester (Mathur et al., 1994) and have been shown to support biofilm formation of nitrifying bacteria (Korkut et al., 2006, McLean et al., 2000, Takamizawa et al., 1993), whereas coconut textile is natural and cost efficient. Here, the implementation of two aerated trench systems after fourteen months of continuous operation is discussed in terms of contaminant degradation rates and performance of the different textiles in the system.
Section snippets
Site location and groundwater composition
The model treatment facility was set up next to a refinery plant in Leuna, Germany. Due to spills, improper handling, and war damage, the groundwater in this area is heavily contaminated with high concentrations of ammonium, the fuel additive MTBE, benzene, and considerable amounts of iron (Table 1). Groundwater for processing was obtained from a well located downstream from the refinery.
Setup of the aerobic pond system
The system consists of two parallel basins (basin 1 and basin 2), each 5 m long, 1.15 m wide and 2.2 m deep (
Physico-chemical conditions within the pond
Dissolved oxygen levels at 1.5 m from the inflow reached concentrations up to 0.6 mg/L (Table 2), even when the aeration was switched off in this zone of the basin. Additionally, diurnal fluctuations in oxygen levels (not shown) indicate oxygen production by algae during the day and consumption over night. This diurnal pattern was also observed on the textiles (see below). The pH of the inflow groundwater was constant at 7.1 ± 0.9, measured for the duration of the experiment at 15 min
Contaminant removal
Biodegradation of contaminants in aquifers is often rate-limited by the availability of oxygen. In this study, we investigated the direct implementation of an aerated treatment system in a shallow aquifer to stimulate aerobic degradation processes. A transition zone between an anoxic compartment in the inflow and an oxic compartment in the outflow area was simulated by adjusting an oxygen gradient from 0 mg/L in the inflow, 0.5 mg/L in the middle, to 1 mg/L in the outflow areas of the systems.
Conclusion
The following conclusions can be drawn from the current study:
- 1.
Biofilms developed on both textile materials within two months after initiating system operation. Results from microbial community analysis and laboratory microcosm experiments indicate that the development of a distinct microbial community, adapted to contaminant degradation, on the surfaces of both tested geotextile materials was achieved.
- 2.
Benzene was effectively biodegraded from 20 mg/L inflow concentration to less than 2 μg/L
Acknowledgements
We thank Ute Kuhlicke for operating the CLSM, Krista Versteeg for technical assistance in the MPN series, Marcell Nikolausz for help and advice with DGGE analysis, Peter Mosig, Stefan Kukla and Francesca Löper for technical assistance in the system operation and sampling, and the department of Analytical Chemistry as well as Grit Weichert of the Centre for Environmental Biotechnology for analytical support. We acknowledge the suggestions and recommendations of two unknown reviewers, Brandon E.
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