There are a wealth of options for enhancing bioremediation but still some challenges with relying on this method for large scale projects. This article discusses the challenges of enhanced bioremediation for soil and groundwater remediation, such as low permeable soils, and reviews some of the options, including electrokinetics, for overcoming these challenges.

Introduction
“Bioremediation Market Projected To Be Worth USD 334.70 Billion By 2027 Growing at a CAGR of 15.5%” states a recent report by Emergen Research. A prime example of the trend is KERP (Kuwait Environmental Remediation Program) which is moving to the remediation phase for the contaminated soil which remains from the Gulf War. Bioremediation has been chosen as one of the main remediation methods and the first remediation contracts are being awarded in 2021.

Due to the vast scale of global contaminated sites and increased focus from the authorities, more sustainable remediation solutions are being sort by a range of countries and projects. Simply excavating contaminated soil and replacing it with clean soil is not viewed as a long-term sustainable option. There have been concerns raised, and some misconceptions, about in situ solutions such as bioremediation.
This article addresses some of the concerns when applying enhanced bioremediation to sites with contaminated land and groundwater.

What is Enhanced Bioremediation?

Bioremediation relies on soil microbes and appropriate environmental conditions, to degrade organic contaminants. In aerobic conditions, in the presence of oxygen, the end products are carbon dioxide and water. And in anaerobic conditions contaminants metabolize to methane, carbon dioxide and hydrogen gas.

The microbes need to be compatible with the contaminants in the soil and available in sufficient quantities. Suitable microbes are often naturally present in the soil however they can be cultivated and then introduced to the contaminated area. Additionally, microbes need favourable soil conditions such as availability of oxygen, moisture, appropriate temperature range and a variety of macro- and micro-nutrients. All these factors need to be to be considered for the microbes to effectively function in their degradation role. The organic contaminants present in the soil will then be metabolized by the microbes, the contaminants effectively acting as the microbes energy source.

Enhanced bioremediation is optimizing the conditions for these biological processes by use of additives and other mechanisms such as electrokinetics.

In-Situ large scale remediation

The most sustainable, and often most economical, way to remediate contaminated soil is at the site, In-Situ, without excavating and transporting the contaminated soil to other locations for treatment. At very large scales In-Situ treatment is practically the only option. Large sites can however be complex, having a variety of soil conditions with contamination at different depths and ground water present. Low permeable and clay type soils have also historically been challenging to remediate in-situ.

Limitations of In-Situ bioremediation

Bioremediation by relying solely on natural attenuation has a poor record of success often simply consisting of long-term monitoring programs and very slow levels of natural degradation. Even in situations where bioremediation has been aided by interventions, inefficiency have been caused by challenges such as incomplete site surveys and poor background information and challenging geophysical environments such as low permeable soils.

Long timeframes were a major limiting factor with these types of bioremediation projects, especially when redeveloping sites, as the investors and developers typically need to meet strict schedules. In-Situ remediation was seen as more uncertain from a scheduling perspective than simple excavations and mass transfers.

Enhanced Bioremediation is not immune to the above challenges but several engineered solutions are overcoming the obstacles.

Limits for injection bioremediation agents

When injecting bioremediation enhancing agents the first questions are often where, how much and how to monitor the success in doing this. Often multiple injection points are required with little space between the points and even then differences in soil types and layers and pockets of soils may cause uneven results for spreading the enhancing agents to the soil. High hydraulic conductivity makes the injection task easier if pressure or gravitational gradient can be used.

This injection approach can be challenging when dealing with sites that have large infrastructure or buildings on top of the contaminations. Such sites are also very challenging to excavate and remove contaminants soil, so other in-situ methods are required.

Geophysical conditions limit the effectiveness of injecting additives

Tight soil matrices such as silt and clay limit the effective application of injected bioremediation agents. Fluids will follow the preferential flow paths and will not be evenly spread through the soil.The limitations in spreading bioremediation agents through the soil matrix in tight soils, deep locations and hard to reach contamination zones limits the effectiveness of injecting additives when used as the sole means of enhancing bioremediation.

Gas Exchange and availability of Oxidants and Reductants

When there are microbes present, nutrients available for keeping them growing and the contaminant has been made soluble enough to reach the microbes there can still be a lack of oxidants (or reductants in the case of halogenated hydrocarbons). This is especially the case with deep contaminations and in fine-textured soils. Possible solutions include air sparging, chemical oxidation, pump & treat, multi-phase extraction, supersaturated water injections and electrochemical approaches.

Electrokinetically enhanced bioremediation

In fine-textured soils, injection and diffusion can be extremely limited however counter to this, electrokinetic processes are more effective the finer the soils. Electrokinetic approaches thus offer a means to overcome the obstacles and limitations usually associated with injecting additives to fine textured and challenging soils.

Electro-osmosis can be used to transfer bioremediation supporting agents to targeted areas over a distance of 5 – 7 meters from the injection point and especially well through a low permeable soils as low hydraulic conductivity is often coupled with high electrical conductivity. Such applications are achieved by applying an external direct electric current to the soil which results in movement of water-soluble remediation agents via electrokinetic mechanisms.

Besides aiding the injection of bioremediation enhancing agents, electrokinetic phenomena offer methods for developing and sustaining optimum conditions for bioremediation. Pulsed Direct Current (DC) can be used to induce oxidative and reductive reactions in soil and groundwater.

The pulsed DC approach will increase the rate of degradation and biochemical reactions via electrokinetic mechanisms which increase the physical mixing at the soil particle level. The capacitive electric load on the soil particles from the pulsed electrokinetic approach will breakdown water molecules into oxygen and hydrogen, with the oxygen then being consumed as part of biochemical degradation reactions.

EKOGRID™ is an advanced oxidation and enhanced bioremediation technology which supports natural processes and can work with complementary remediation methods. EKOGRID™ utilizes electrochemical phenomena to generate oxygen and chemical radicals on soil particle surfaces and electrokinetic and electro-osmotic phenomena to increase availability of organic contaminants for bioremediation and chemical degradation.