Remediation techniques

Bioremediation is a treatment process that uses naturally occurring micro-organisms (yeast, fungi, or bacteria) to break down or degrade hazardous substances into less toxic or non-toxic substances. Like humans, micro-organisms eat and digest organic substances for nutrients and energy. In chemical terms, 'organic' compounds are those that contain carbon and hydrogen atoms. Certain micro-organisms can digest organic substances such as fuels or solvents that are hazardous to humans. The micro-organisms break down the organic contaminants into harmless products, mainly carbon dioxide and water.

Once the contaminants are degraded, the micro-organism population is reduced because they have used their entire food source. Dead micro-organisms or small populations in the absence of food pose no contamination risk.
Bioremediation can be used as a clean-up method for contaminated soil and water.

Bioremediation applications fall into 2 broad categories, in situ or ex situ.

• In situ bioremediation treats the contaminated soil or groundwater in the location in which it was found.
• Ex situ bioremediation processes require excavation of contaminated soil or pumping of groundwater before they can be treated.

In situ bioremediation of soil

In situ techniques do not require excavation of the contaminated soils so may be less expensive, create less dust, and cause less release of contaminants than ex situ techniques. Also, it is possible to treat a large volume of soil at once. However, in situ techniques may be slower than ex situ techniques, may be difficult to manage and are most effective at sites with permeable (sandy or uncompacted) soil.

The goal of aerobic in situ bioremediation is to supply oxygen and nutrients to the micro-organisms in the soil. For this reason in situ bioremediation may not work well in clays or in highly layered subsurface environments because oxygen cannot be evenly distributed throughout the treatment area. In situ remediation often requires years to reach clean-up goals, depending mainly on how biodegradable specific contaminants are. Less time may be required with easily degraded contaminants.

In situ bioremediation of groundwater

In situ bioremediation of groundwater speeds the natural biodegradation processes that take place in the water soaked underground region that lies below the water table. For sites at which both the soil and groundwater are contaminated, this single technology can be  effective at treating both.

Ex situ bioremediation of soil

Ex situ techniques can be faster, and easier to control. They are used to treat a wider range of contaminants and soil types than in situ techniques, but they require excavation and treatment of the contaminated soil before and, sometimes, after the actual bioremediation step. Ex situ techniques include slurry-phase bioremediation and solid phase bioremediation.

Slurry-phase bioremediation

Contaminated soil is combined with water and other additives in a large tank called a 'bioreactor' and mixed to keep the micro-organisms, which are already present in the soil, in contact with the contaminants in the soil. Nutrients and oxygen are added, and conditions in the bioreactor are controlled to create the optimum environment for the micro-organisms to degrade the contaminants. Upon completion of the treatment, the water is removed from the solids, which are disposed of or treated further if they still contain pollutants.

Solid-phase bioremediation

Solid-phase bioremediation is a process that treats soils in above-ground treatment areas equipped with collection systems to prevent any contaminant from escaping the treatment. Moisture, heat, nutrients or oxygen are controlled to enhance biodegradation for the application of this treatment. Solid-phase systems are relatively simple to operate and maintain, require a large amount of space, and clean-ups require more time to complete than with slurry-phase processes. Solid-phase soil treatment processes include landfarming, soil biopiles, and composting.

Landfarming

In this relatively simple treatment method, contaminated soils are excavated and spread on a pad with a built-in system to collect any 'leachate' or contaminated liquids that seep out of contaminant-soaked soil. The soils are periodically turned over to mix air into the waste. Moisture and nutrients are controlled to enhance bioremediation. The length of time for bioremediation to occur will be longer if nutrients, oxygen or temperature are not properly controlled. In some cases, reduction of contaminant concentrations actually may be attributed more to volatilisation than biodegradation. When the process is conducted in enclosures controlling escaping volatile contaminants, volatilisation losses are minimised.

Soil biopiles

Contaminated soil is piled in heaps several metres high over an air distribution system. Aeration is provided by pulling air through the heap with a vacuum pump. Moisture and nutrient levels are maintained at levels that maximise bioremediation. The soil heaps can be placed in enclosures. Volatile contaminants are easily controlled since they are usually part of the air stream being pulled through the pile.

Composting

Biodegradable waste is mixed with a bulking agent such as straw or hay to make it easier to deliver the optimum levels of air and water to the micro-organisms. Three common designs are:

• static pile composting (compost is formed into piles and aerated with blowers or vacuum pumps);
• mechanically agitated in-vessel composting (compost is placed in a treatment vessel where it is mixed and aerated); and
• windrow composting (compost is placed in long piles known as windrows and periodically mixed by tractors or similar equipment).

Will it work at every site?

Biodegradation is useful for many types of organic wastes and is a cost-effective, natural process. Many techniques can be conducted on site, eliminating the need to transport hazardous materials. The extent of biodegradation is highly dependent on the toxicity and initial concentrations of the contaminants, their biodegradability, the properties of the contaminated soil, and the particular treatment system selected. Contaminants targeted for biodegradation treatment are non-halogenated volatile and semi-volatile organics and fuels. The effectiveness of bioremediation is limited at sites with high concentrations of metals, highly chlorinated organics or inorganic salts, because these compounds are toxic to the micro-organisms.