Cleaning Polluted Groundwater: Innovative Solutions For A Better Tomorrow

Groundwater contamination is a pressing issue that poses significant challenges to communities worldwide. It occurs when undesirable substances, such as bacteria, viruses, detergents, and industrial waste, infiltrate groundwater sources, rendering them unsuitable for human use. With the recognition that prevention is the most practical solution, effective groundwater management practices are essential to safeguard this vital resource. However, in cases where contamination has already occurred, various methods can be employed to remediate the damage. These techniques, including pump and treat systems, in situ treatment, and bioremediation, aim to contain, treat, and restore groundwater to its beneficial uses. While challenging and costly, addressing groundwater pollution is crucial to protect human health and the environment.

Bioremediation: Using bacteria to break down organic contaminants

Bioremediation is a process that uses microorganisms such as bacteria, fungi, and plants to break down, change, remove, immobilize, or detoxify pollutants. It is a cost-effective and eco-friendly solution for removing environmental contaminants.

The Process

Bioremediation involves stimulating the growth of certain microbes that use contaminants, including oil, solvents, and pesticides, as sources of food and energy. These microbes convert contaminants into small amounts of water and harmless gases, such as carbon dioxide.

Advantages

Bioremediation offers several advantages over other cleanup methods:

• It minimizes damage to ecosystems by relying solely on natural processes.
• It often takes place underground, where amendments and microbes can be pumped to clean up contaminants in groundwater and soil without disrupting nearby communities.
• It creates relatively few harmful byproducts, as contaminants are converted into water and harmless gases like carbon dioxide.
• It is cheaper than most cleanup methods because it doesn't require substantial equipment or labor.

Limitations

However, there are some limitations and challenges to bioremediation:

• It can be a slow process, taking anywhere from several months to several years, depending on various factors such as the size of the contaminated area and the concentration of contaminants.
• It may not be suitable for all types of pollutants or contamination levels.
• Physical and chemical methods may be required for complete removal of certain contaminants.
• It requires the right temperature, nutrients, and food sources for the microbes to thrive.

Types of Bioremediation

There are three main types of bioremediation:

• Biostimulation: This process involves stimulating the microbes responsible for remediation by adding chemicals or nutrients that activate them.
• Bioaugmentation: This process involves adding bacteria or other microorganisms to the surface of the affected area and allowing them to grow. It is mainly used for cleaning up soil contamination.
• Intrinsic Bioremediation: This process converts toxic materials into inert materials using the native microbiome on the affected area.

Applications

Bioremediation can be used to clean up a variety of environmental wastes and pollutants, including:

• Oil spills and oil-based paints
• Organic pollutants such as pesticides, herbicides, and polychlorinated biphenyls (PCBs)
• Heavy metals such as mercury, lead, cadmium, and chromium
• Plastics and other synthetic pollutants

Air stripping: Using air to remove contaminants from water

Air stripping is a method of water purification that uses air to remove contaminants from water. This process is particularly effective for volatile organic compounds (VOCs) that can quickly evaporate into the air at room temperature, making them challenging to control and remove from water sources. VOCs are present in water due to various industrial, agricultural, and domestic activities. Examples of VOCs include benzene, toluene, trichloroethylene (TCE), perchloroethylene (PCE), vinyl chloride, xylene, ethylbenzene, and methylene chloride. These compounds can contaminate water sources through industrial discharges, petroleum spills, improper waste disposal, leaking storage tanks, and natural seepage from underground sources.

The air stripping process involves passing air through contaminated water or spraying the water over packing material in a large chamber. The basic principle behind air stripping is mass transfer, governed by Henry's Law, which states that VOCs in water are transferred to the air phase due to the difference in vapour pressure between the water and the contaminant phases. Air strippers are designed to maximise the air-water surface contact area to achieve maximum efficiency in removing contaminants.

There are several types of air strippers, including packed tower strippers, tray tower strippers, bubble diffuser air strippers, spray air strippers, and rotating cylinder air strippers. Packed tower strippers use a distributor at the top of the tower to evenly distribute packing materials such as engineered plastic, ceramic, or metal, maximising air-water contact. Tray tower strippers, or sieve tray towers, use trays with holes that allow water to drip through while air is introduced from below. Bubble diffuser air strippers release air or stripping gas as fine bubbles at the bottom of a water-filled tank, allowing intimate contact between the gas and water phases. Spray air strippers create a fine mist or spray of water, increasing the surface area for contact with the air or stripping gas. Rotating cylinder air strippers consist of a slowly rotating cylindrical drum or vessel into which contaminated water is introduced, and air or stripping gas is injected into the rotating drum.

The choice of air stripper type depends on factors such as the variety of contaminants, flow rates, site conditions, and regulatory requirements. Air stripping is a versatile and scalable technology, accommodating varying flow rates and contaminant loads, and it can be customised to suit specific water treatment requirements. It is also cost-effective, with relatively low operational costs and minimal maintenance needs. Additionally, air strippers can be easily integrated into existing water treatment systems, reducing the need for extensive infrastructure modifications.

Air stripping is an effective method for removing hazardous compounds from water, ensuring its safety for consumption or reuse. It is widely used in groundwater remediation, drinking water treatment, and industrial wastewater treatment to address issues of VOCs, acidity due to dissolved carbon dioxide (CO2), and disinfection byproducts (DBPs).

Pump and treat: Extracting groundwater to an above-ground treatment system

Pump and treat is a common method for cleaning up groundwater contaminated with dissolved chemicals, including industrial solvents, metals, and fuel oil. This method involves pumping groundwater from wells to an above-ground treatment system that removes the contaminants.

The above-ground treatment of extracted groundwater often includes multiple technologies used in a treatment train. The treatment train components may include air strippers and/or granular activated carbon (GAC) filters, and the design of piping, instrumentation, and control systems, including a programmable logic controller for system operation. The treated groundwater can then be reinjected into the subsurface, discharged to a publicly owned treatment works (POTW), discharged to a receiving surface water body, or beneficially reused (e.g. as irrigation water).

Pump and treat systems are also used to contain contaminant plumes and prevent their spread by pumping contaminated water towards the wells. This helps to keep contaminants from reaching drinking water wells, wetlands, streams, and other natural resources. The system may involve installing one or more wells to extract the contaminated groundwater, which is then pumped from these "extraction wells" to the ground surface, either directly into a treatment system or into a holding tank until treatment can begin.

The treatment system may consist of a single cleanup method, such as activated carbon or air stripping, to clean the water. In the process, free-phase contaminants and/or contaminated groundwater are pumped directly out of the surface, with treatment occurring above ground. The cleaned groundwater is then either discharged into sewer systems or re-injected into the subsurface.

Pump and treat systems have been operated at numerous sites for many years, but data collected from these sites reveals that although this method may be successful during the initial stages of implementation, performance drastically decreases over time. As a result, significant amounts of residual contamination can remain unaffected by continued treatment. Due to these limitations, the pump and treat method is now primarily used for free product recovery and control of contaminant plume migration.

In situ treatment: Destroying, immobilising, or removing contaminants without extraction

In situ treatment is a method of groundwater remediation that treats the water in its existing location, without the need for extraction. This approach is more cost-effective than removing and treating water off-site, but it is important to note that it can also be used in conjunction with other techniques to enhance the overall effectiveness of the remediation process.

In situ treatment technologies can be employed to destroy, immobilise, or remove contaminants from groundwater. Here are some specific methods and their benefits:

In Situ Chemical Oxidation

This process involves injecting oxidants, such as hydrogen peroxide or potassium permanganate, into the groundwater through wells. The oxidants react with the contaminants, converting them into less harmful substances like water and carbon dioxide. This method is particularly effective for treating pollutants like fuels, solvents, and pesticides.

In Situ Chemical Reduction

In situ chemical reduction (ISCR) is a process that uses chemical reactions to reduce the toxicity, mobility, or volume of contaminants. This technique is often combined with other remediation technologies to address different types of contaminants or media.

Permeable Reactive Barriers (PRBs)

PRBs are in situ remediation technologies that use adsorption, precipitation, electrostatic attraction, biodegradation, and redox reactions to immobilise or remove heavy metals and petroleum hydrocarbons from groundwater. They are effective, affordable, and have a long service life.

Bioremediation

Bioremediation leverages the power of microorganisms to digest and convert contaminants into harmless substances like water and carbon dioxide. It is a natural, effective, and inexpensive treatment process that does not require the use of chemicals or disinfectants. Bioremediation can be enhanced through bioaugmentation and biostimulation, which involve adding specific microbes or stimulating the growth of indigenous microbes, respectively.

Monitored Natural Attenuation

Monitored natural attenuation relies on natural physical, chemical, and biological processes to achieve remediation goals. It includes various mechanisms such as dispersion, dilution, sorption, volatilisation, radioactive decay, and biodegradation. While this method can be effective, it typically takes a long time to achieve the desired level of remediation.

Containment: Using vertical, impermeable barriers to prevent plume migration

Containment methods are essential to prevent the spread of groundwater pollution and protect drinking water sources. Vertical engineered barriers (VEBs) are a critical tool in this effort. These are walls built below the ground to control the flow of groundwater and prevent the migration of contaminant plumes. By using impermeable barriers, VEBs ensure that contaminated groundwater does not mix with clean water sources. This method is particularly effective in diverting contaminated water away from drinking water wells, wetlands, and streams, thus safeguarding human health and the environment.

VEBs are designed to be vertical, engineered, and subsurface. They differ from permeable reactive barriers, which allow water to pass through and aim to intercept and treat contaminated plumes. In contrast, VEBs are impermeable, focusing solely on containing and diverting the flow of contaminated groundwater. This distinction is crucial in understanding the role of VEBs in groundwater remediation.

Common types of VEBs include slurry walls and sheet pile walls. These walls are constructed using specialized techniques to ensure their effectiveness in blocking the flow of groundwater. The choice of VEB type depends on various factors, including the specific site characteristics and the nature of the contamination.

VEBs are often used in conjunction with other remediation methods, such as pump and treat systems, to comprehensively address groundwater contamination. By drawing contaminated water towards wells, pump and treat systems help keep the contaminant plume from spreading. This combination of containment and pumping prevents further pollution of water sources while also controlling the flow of groundwater.

The effectiveness of VEBs in preventing plume migration makes them a valuable tool in the overall effort to clean up polluted groundwater. By containing the spread, VEBs provide time and flexibility to implement other remediation techniques. This containment approach is particularly useful when restoring groundwater to beneficial uses is not immediately feasible.

In conclusion, vertical, impermeable barriers, known as VEBs, are a critical tool in preventing the migration of contaminated groundwater plumes. By diverting the flow away from sensitive areas and containing the pollution, these barriers protect human health and the environment. The use of VEBs, often in combination with other remediation methods, is a vital step in the process of cleaning up polluted groundwater and ensuring the safety of water sources.

Frequently asked questions