REMEDIATION OF CONTAMINATED ENVIRONMENTS / 21 May 2019
Blog: Microbes in the Baltic Sea can be utilised in decommissioned oil cave remediation
Large, empty spaces can be a valuable resource in climate change mitigation. An example of a suitable space is the caves that were previously used in oil storage in Helsinki, Finland. What is the value of these caves, and what do they have to do with climate change?
Energy bound as heat in water can work like a battery. Heat energy can be stored when energy production exceeds consumption, and released in times of high energy demand. To store heat energy, Helen Ltd energy utility is planning to utilise decommissioned oil storage caves that lie underneath a new residential area in Helsinki. The energy stored in these caves will heat the new residential homes. The caves are to be filled with warm sea water from the nearby bay in July, and released for domestic heating in the cold winter months.
Oil caves need to be cleaned before use for energy
The two caves that have been excavated deep in the rock are huge, with a total of 300,000m3 in volume. They were used for diesel oil storage until 2008, after which they were emptied and mechanically cleaned. Since then, the caves are gradually filling with ground water that is mixed with residual oil seeping in from rock fractures. Before the caves can be used for sea water storage, the caves need to be remediated. A pilot project is currently underway to find the most cost-effective method.
Can microbes of sea water be used to clean the oil?
Microbes can use many organic and inorganic compounds for nutrition. A special delicacy for them is the alkane chains of oil hydrocarbons, which diesel oil is also mainly made of. Microbes are ubiquitous, and many oil degraders are known to inhabit the marine ecosystem. For example, microbial remediation was used in the remediation of the oil Exxon Valdez oil spill and the Deepwater Horizon accident.
Oil remediation in the decommissioned oil caves in Helsinki is a trailblazing project also in a global context, as the re-use of contaminated environments is dependent on cost control. In theory, the bacteria present in the sea water can reduce the oil concentration to a level where the costs from final-stage activated carbon filtering are reasonable. But how can a theory be put into practice in a project where failure could mean huge costs?
To estimate the ability of the sea water bacteria to degrade oil, information from experimental research can be utilised. Pöyry implemented, in collaboration with the Finnish Environment Institute, a pilot project in which the sea water of the nearby bay next to the caves, and oily water from the caves were studied in an aquarium mesocosm scale for six weeks. Oil concentrations, bacterial numbers and the genes needed for alkane and PAH compound degradation was quantified. Additionally, the changes in the microbial communities when the sea and cave waters are mixed were clarified by sequencing DNA directly from the environmental samples.
Results of the pilot project
The obtained results were promising enough to move on to planning the next phase of the project. Furthermore, the project provided scientific information on the oil degrading microbial communities of the Baltic Sea. This knowledge can be applied in marine oil accidents, or in remediating similar oil caves. The experimental results also provide additional information on the success potential for the public authority, which makes the required permit process run more smoothly.