Over the years, extensive research on In-situ Bioremediation of groundwater has come up with new techniques using accelerated biodegradation process that made it possible to enhance and expedite the clean-up rate of aquifers contaminated with organic pollutants. This is especially important for those contaminated underground water mixed with contaminants coming from landfill leachate or accidental spillages such as gasoline, oil residues and other organic solvents especially chlorinated hydrocarbon. What is more serious are those polluted sources containing toxic substances such as pesticides which has even more serious implications towards public health concerns.
Most of these problems actually occur due to ignorance by business owners with the objective in mind to maximize profit or simply due to lack of understanding on the consequences. In the past, law & regulations were not that strict and these were often overlooked until it becomes a serious problem. Partly to blame also is the old technology especially during the time when underground gasoline storage tank is made up of a single-wall type prone to corrosion and pitting that leads to the gasoline getting leaked out and contaminate the underground water source. Past remediation activities involve extracting out the volume of water using vacuum and then sent it for conventional treatment process in a wastewater treatment plant. This method requires extensive clean-up efforts and is very time consuming and furthermore, there is a big question mark on its effectiveness because the contaminants can easily leach underground together with the water and get trapped in between the layers of soil with relatively low permeability, thus making the whole process complicated and very expensive to execute. To complicate the whole matter, some contaminants can be highly miscible with water while others are having higher density making the whole approach become less feasible. Because of these issues, new developments coupled with detailed studies have been carried out to develop the specific strain of bacteria/microorganisms suited for the type of contaminants and these offer new possibilities that even non-organic contaminants can be removed using similar method.
In-situ bioremediation is looked upon as the best approach to deal with the underground water contamination. In order to increase microbial activity and to ensure the continuous growth of the bacteria in the Biologically Active Zone (BAZ), the limiting substances such as macro nutrient (nitrogen or phosphorus), electron acceptor (such as oxygen, nitrate) or carbon source has to be continuously supplied and injected to the system. Success of the whole program is largely dependent on the soil properties, selection of the right bacteria to use matching the type of contaminants present, hydrogeology and other factors. BAZ offers the advantage of accelerated biodegradation reaction by the bacteria because the idea is to have the microorganisms to be present at the closest possible location very near to the contaminants. These includes not only those present in the water but also those that get absorbed to the aquifer.
In-situ biodegradation typically involves setting up injection wells to allow the seeding of the microorganisms and the transfer of nutrients, carbon source and electron acceptor to begin the process of breakdown of the organic compound. Before the situ-bioremediation is started, the flow field must be defined so that the treatment site can marked and identified. The water in the flow field must pass through the BAZ made up of microorganisms attached to the solid surface of the soil particles and the treatment will take place as it passes through the area close to the wells before the water is extracted out from the system. As mentioned earlier, since growth is caused by the limiting substances and in order to ensure that this doesn’t become a restriction to the development of biomass in the aquifer, continuous supply of limiting substrate needs to be introduced to the system.
Some of the problems faced when it comes to successful implementation are mostly due to confined localized biological activity taking place only at the area close to the injection wells. Second, aquifer clogging is also another potential problem that needs to be addressed because this can prevent the limiting substrate from reaching the microorganisms and cause groundwater to flow around the BAZ effectively bypassing the treatment process. The first problem can be solved by setting up multiple injection wells along the flow path to ensure that there is sufficient exposure to breakdown the contaminants. As shown in the diagram above, a series of injection wells in placed along the flow path. As for the clogging issue, introduction of Protozoan can help to solve the problem but a better approach is to feed the nutrients and limiting substances in pulses so that this can control the biomass growth. Addition of hydrogen peroxide also seems to help prevent the excessive growth. Apart from the operational issues, the biochemical reaction itself must not produce other harmful by-products in the biodegradation process. This can be prevented by conducting a detailed study to understand the contaminant properties before selecting the type of bacteria for it.
Some other topic of discussion includes using natural occurring biosurfactant to enhance solubility and biodegradation activity of the bacteria