In-Situ Biological Treatment

Introduction to In-Situ Biological Treatment Principles

In-situ bioremediation is a sustainable remediation method that leverages naturally occurring microorganisms (or their metabolic products) to degrade contaminants in soils and groundwater. This technique involves the encouragement of existing beneficial microbial populations to increase via the management (or sometimes introduction) of key growth factors typically oxygen, nutrients and moisture.

Depending upon the contaminant to be targeted the microbial population requires differing in ground conditions to develop:

With Oxygen (Aerobic or Oxidising Conditions): Oxygen is introduced to enhance microbial respiration and degrade contaminants into simpler compounds like carbon dioxide and water.
Without Oxygen (Anaerobic or Anoxic Conditions): Oxygen is excluded, and electron donors are added to create reducing conditions that support the degradation of specific contaminants.

The choice between aerobic and anaerobic systems depends on the nature of the contaminants present. This adaptability makes in-situ biological treatment effective for a wide variety of contamination challenges.

Contaminants Treated with In-Situ Biological Treatment

In-situ bioremediation is a versatile remediation process that can address a wide range of organic contaminants. By utilising aerobic or anaerobic conditions, most organic pollutants in soils and groundwater can be effectively degraded, making this technique suitable for various site-specific contamination scenarios.

Common Contaminants Remediated Through In-Situ Bioremediation

Contaminants that can be successfully treated with in-situ bioremediation include, but are not limited to:

Chlorinated solvents and hydrocarbons: Often found in industrial sites and requiring specialised anaerobic conditions for effective breakdown.
Diesel range hydrocarbons: Common in fuel spills and easily degraded under aerobic systems.
Mineral and lubricating oils: Found in heavy machinery areas, these contaminants respond well to microbial activity.
Some herbicides and pesticides: Agricultural pollutants that can be treated effectively with proper microbial conditions.
Phenols: Industrial chemicals known for their toxicity, which are broken down into less harmful compounds.
Polycyclic aromatic hydrocarbons (PAHs): Typically present in tar and coal-based sites, treatable through carefully managed bioremediation methods.

Aerobic vs. Anaerobic Treatment for Specific Contaminants

The type of contaminants present dictates whether the in-situ bioremediation process will be aerobic or anaerobic:

Aerobic Systems: Most hydrocarbons and organic contaminants respond better to aerobic conditions where oxygen is actively introduced.
Anaerobic Systems: Chlorinated hydrocarbons, such as trichloroethene (TCE), often require oxygen-free environments and the addition of electron donors to facilitate their degradation.

Aerobic Bioremediation

Aerobic bioremediation introduces air or oxygen into the contaminated area to increase oxygen levels, facilitating microbial respiration and growth. This approach enables the microbial population to degrade organic contaminants into simpler, non-toxic substances like carbon dioxide and water.

Key Features of Aerobic Bioremediation:

Oxygen Introduction: Air or oxygen is injected into the ground to enhance microbial activity.
Nutrient Addition: Nutrients may be supplemented if the plume is nutrient-deficient to support microbial growth.
Degradation Outcome: Organic contaminants are broken down into harmless byproducts.

Methods for Introducing Oxygen:

Air Sparging: Injecting air directly into groundwater to enhance oxygen levels.
Bioventing: Promoting the flow of oxygen into unsaturated soils to stimulate microbial activity.
Diffusion: Allowing oxygen to permeate naturally into the treatment zone.
Oxygen Release Compounds (ORCs): Slow-release products designed to provide oxygen over time.
Experimental Techniques: Other methods may be employed depending on site-specific conditions, though some may be less commercially viable.

These techniques are chosen based on the site’s characteristics and contaminant type, ensuring efficient and effective remediation.

Anaerobic Bioremediation

Anaerobic bioremediation involves creating oxygen-free (anoxic) conditions to support the development of anaerobic microbial cultures. This method is particularly effective for contaminants like chlorinated hydrocarbons, which require reducing conditions for degradation.

Key Features of Anaerobic Bioremediation:

Anoxic Conditions: Oxygen is excluded to allow anaerobic microorganisms to thrive.
Nutrient Addition: Nutrients and inhibitors may be added to prevent natural re-oxygenation of the groundwater.
Electron Donor Usage: Electron donors are introduced to maintain reducing conditions (negative REDOX), accelerating contaminant breakdown.

Common Electron Donors Used:

Molasses
Hydrogen Release Compounds
Emulsified Vegetable Oils (EVO)
Sodium Lactate
Cheese Whey

Process Overview:

Groundwater is abstracted, dosed with nutrients and electron donors, and reinjected into the contaminated plume.
This creates and sustains reducing conditions, allowing the reductive dehalogenation of pollutants like chlorinated hydrocarbons (e.g., trichloroethene or TCE).
Special care is taken to avoid aerating the groundwater during abstraction, dosing, or reinjection.

Bio-Augmentation for Enhanced Degradation

In both aerobic and anaerobic bioremediation, supplementary microorganisms (bio-augmentation) may be injected into the plume to enhance the natural microbial population and improve degradation rates. However, based on VertaseFLI’s extensive experience, this step is rarely necessary as indigenous microorganisms are typically sufficient to achieve remediation goals.

Examples of Successful In-Situ Biological Treatment Projects

Anaerobic Remediation Example: Chlorinated Solvent Plume in Newcastle

VertaseFLI was contracted to remediate a chlorinated solvent plume at an operational site in Newcastle upon Tyne. The plume, consisting of halogenated hydrocarbons, was confined within the water table. To address this, VertaseFLI designed and implemented an in-situ anaerobic abstraction and recirculation system. This system delivered emulsified vegetable oil (EVO) and KB-1 bacteria into the plume, creating optimal anaerobic conditions for contaminant degradation.

Key Project Highlights:

Drilling and Preparation: 33 dedicated abstraction and injection wells were drilled to facilitate the process.
Hydraulic Testing: Pump trials were conducted to assess site-specific hydraulic conditions.
System Design and Construction: A bespoke abstraction and injection system was designed and constructed.
Groundwater Management: Over 240,000 litres of groundwater were abstracted and reinjected into the site.
Electron Donor Delivery: Over 11,000 litres of emulsified vegetable oil (EVO) were injected to maintain reducing conditions.

The plume was monitored throughout the process to ensure complete degradation of contaminants, demonstrating the efficacy of anaerobic bioremediation for chlorinated hydrocarbons.

Aerobic Remediation Example: Hydrocarbon and Phenol Plume at Former Chemical Works

At a former tar and chemical works, VertaseFLI successfully remediated a hydrocarbon and phenol plume within a sandstone aquifer using an aerobic in-situ biological treatment system. The system was designed to sparge air into the aquifer, enhancing oxygen levels and supporting microbial degradation. Respiration products and volatile compounds, including xylene and toluene, were removed using a soil vapour extraction system.

Key Project Highlights:

Well Installation: 50 dedicated sparging and vapour recovery wells were drilled.
System Design and Construction: An air sparging and vapour recovery system was designed, built, and installed.
Pipework Installation: Treatment pipework was installed and buried to maintain system integrity.
Temperature Management: Warm air blowers increased the aquifer temperature by approximately 10 degrees Celsius, enhancing degradation rates.
Contaminant Degradation: Over 1 tonne of contaminants was successfully degraded, reducing phenols to detection limits and hydrocarbons to below the remedial target of 1 mg/l.

The treatment was completed within 12 months, achieving significant reductions in contaminant concentrations and meeting all risk-assessed targets.

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Proven Results in In-Situ Bioremediation

Both the anaerobic and aerobic projects showcase VertaseFLI’s expertise in designing tailored in-situ biological treatment systems. These approaches ensured effective remediation, reduced environmental impact, and maintained compliance with regulatory standards, setting a benchmark for sustainable site remediation solutions.

Advantages and Limitations of In-Situ Biological Treatment

In-situ biological treatment offers a sustainable, efficient solution for soil and groundwater remediation. However, like any remediation method, it has specific strengths and limitations depending on site conditions and contamination types.

Advantages of In-Situ Biological Treatment

In-situ bioremediation provides numerous benefits that make it a preferred choice for many remediation projects. Key advantages include:

No Excavation Required: This method is entirely in-situ, meaning there is no need for excavation, significantly reducing site disruption. Only a small area is required to set up equipment.
Effective Contaminant Removal: The process leads to the destruction or removal of harmful contaminants, restoring environmental quality.
Non-Intrusive to Structures: It causes minimal disruption to above-ground structures, making it ideal for operational sites or areas with limited access.
Treatment of Halogenated Hydrocarbons: Anaerobic systems are particularly effective for biodegrading halogenated hydrocarbons, such as trichloroethene (TCE).
Simultaneous Contaminant Treatment: This method can address multiple contaminants, including chlorinated solvents, hydrocarbons, and phenols, in one system.
Small Footprint: In-situ systems are compact and suitable for small or operational sites where space is a constraint.

These advantages make in-situ biological treatment a versatile and environmentally friendly option for remediating a variety of contaminants.

Limitations of In-Situ Biological Treatment

Despite its benefits, in-situ bioremediation has some limitations that must be considered during project planning:

Not Suitable for Free-Phase Contaminants Alone: This method cannot effectively treat plumes with free-phase contaminants unless combined with other forms of treatment, such as free product recovery.
Limited to Saturated Zones: The technique is only effective in the saturated zone, below the capillary fringe, where sufficient groundwater is present to support microbial activity.
Low-Permeability Soils: In-situ bioremediation is less suitable for low-permeability soils, such as clays, which may restrict the movement of oxygen or electron donors.

Understanding these limitations is critical to determining whether in-situ biological treatment is the most appropriate solution for a specific site.

From tender through to project completion, the team at Vertase FLI provided a very collaborative and thoroughly professional service with a positive problem solving approach.

Daniel Baker
Contracts Director, Brown and Mason Group Limited

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