An operational petrochemical site in Belgium is contaminated with a variety of contaminants (MTBE, BTEX, mineral oil and MCB). Greensoil started a successful pilot test in 2017 demonstrating the aerobic biological degradation of the contamination evolving to the installation of a biobarrier enclosing the south border of the chemical plant in 2022.
The site conditions are complex due to the high COD and strong reducing conditions. The source contamination is difficult to reach due to infrastructure in ATEX zones and the presence of storage tanks. From the active site a large mixed groundwater plume leaves the site towards the nearby river receptor.
At the site previous feasibility tests (MPE and ISCO)both failed due to the strong reducing conditions and the presence of very high levels of methane in the groundwater and soil vapor. An alternative remedial approach was suggested by GreenSoil Group based on aerobic biological degradation. Despite the very strong reducing conditions in the soil prior to start, Greensoil was convinced geochemistry could be turned around to aerobic conditions within reasonable time.
In general groundwater is extracted, treated above-ground in a patented (MTBE) bioreactor and recirculated in the aquifer to distribute nutrients and bacteria in the soil. The groundwater recirculation system is combined with a biosparging and soil vapor extraction system to aerate the soil and provide oxygen for the bacteria.
The pilot field tests were performed at the site in different areas. Pilot 1 was conducted in an uncontaminated area to test the radius of influence of the biosparging system. A radius of influence in between 7-10 m was observed.
Pilot 2 was conducted to test the feasibility of stimulated aerobic biodegradation in a contaminated area. The groundwater was extracted from a well located downstream, and after a retention time in the bioreactor, reinfiltrated in another well located about upstream.
Above-ground bioreactor:
In the first 9 months, the flow of the MTBE bioreactor was limited due to the high COD of the groundwater and the rather low influent concentrations (± 1000 µg/l). It was found that the high COD/BOD was caused by a formerly unknown organic acid. Both MTBE and TBA were degraded with >99% efficiency in the bioreactor to concentrations below detection limit (<40 µg/l).
In situ aerobic biodegradation:
At the start, the redox conditions remained strongly reduced as the natural oxygen demand of both soil and groundwater was extremely high. After 2 months, high methane concentrations were no longer present allowing to upscale the in situ aeration. After 5-6 months, geochemistry was turned over from anaerobic to aerobic. After 7 months, hydrocarbons and monochlorobenzene (MCB) were removed with an efficiency of >99%, while the in-situ MTBE removal efficiency was in the range of 56-93%.
It was observed that in-situ MTBE degradation was significantly higher (93 %) closer to the infiltration well where the effluent from the MTBE bioreactor was infiltrated than in the wells further away, indicating the additional effect of bioaugmentation with our patented VITO bioreactor and specific bacteria cultures for bioaugmentation.
The outcome of the pilot tests demonstrated that the aerobic biodegradation could be strongly enhanced resulting in a strong reduction of contaminants. Based on these results full scale remediation applications were engineered of which the first was the installation and operation of an aerobic biobarrier to mitigate further off site migration of the large groundwater plume.
In addition to the contaminants detected in the pilot area, Tetrahydrofuran (THF) and Dicyclopentadiene (DCPD) are also present within the biobarrier site. Concentrations of THF surpass 100 mg/l in the central area.
The start of installation works was preceded by the careful uncovering and mapping of existing high-pressure piping in cooperation with the network operators. Followed by the individual drilling of the system wells and biosparging points.
The biobarrier has a total length of 325 meters up to 8 m-bgl and is setup to create a very active biological area that acts as a screen to prevent off site spreading of the contaminants. THF is mainly present in the central part of the biobarrier, to effectively address the high mass of THF, the aerobic biobarrier was adjusted to a dual operation system to separate the high THF area (in the center) from the outside areas of the barrier with mainly MCB, TPH, BTEX and MTBE but little to no THF. Both operation streams rely on separate wells and lead to two separate above-ground groundwater treatment systems.
The R&D department of Greensoil is conducting further lab research on the biodegradability and degradation rates of THF and DCPD for the central part of the biobarrier. THF and DCPD degradation has been proven to go on at high efficiencies. The positive lab results on THF and DCPD are currently being assessed in the field with respect to the general functioning of the biobarrier.
· 17 km of tubing
· 67 infiltration wells (depth 8 m-bgl)
· 33 extraction wells (depth 9 m-bgl)
· 85 biosparging points (depth 7 m-bgl)
· 85 biosparging points (depth 9 m-bgl)
· 15 Soil vapor extraction wells
Bioreactors: Approximately 100 kg of cumulative contaminant mass was removed by the two bioreactors in a 6-month period (water phase).
SVE: Approximately 45 kg of cumulative contaminant mass was removed by the soil vapor extraction system in a 6-month period (vapor phase).
Data are collected for one year to date by intensive monitoring of multiple parameters:
· Groundwater recirculation: Injected and extracted flow rates, system pressures, contaminant influent/effluent bioreactor; monitoring well sampling
· Biosparging process: extracted and injected air flows, humidity, temperature
· SVE: extracted soil vapor (PID and LEL) and analytical monitoring
Geochemistry: the change from anaerobic to aerobic geochemistry is shown by increased oxygen and redox
Contaminants: strong reductions (>95%) during the first 6 months of operation were reported for BTEX and MCB. MTBE shows a decreasing trend but at slower pace.
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