When volatile constituents are present, biosparging is often combined with soil vapor extraction (SVE) or bioventing and can also be used with other remedial technologies. When biosparging is combined with vapor extraction, the vapor extraction system creates a negative pressure in the vadose zone through a series of extraction wells that control the vapor plume migration. A biosparging pilot test or respirometery test is recommended.

Application
When used appropriately, biosparging is effective in reducing petroleum products at underground storage tank (UST) sites. Biosparging is most often used at sites with mid-weight petroleum products (e.g., diesel fuel, jet fuel); lighter petroleum products (e.g., gasoline) tend to volatilize readily and to be removed more rapidly using air sparging. Heavier products (e.g., lubricating oils) generally take longer to biodegrade than the lighter products, but biosparging can still be used at these sites.

Biosparging should NOT be used if the following site conditions exist:

The existing literature contains case histories describing both the success and failure of biosparging; however, since the technology is relatively new, there are few cases with substantial documentation of performance.

Operation Principles (Back to Top)
The biosparging process is similar to air sparging. However, while air sparging removes constituents primarily through volatilization, biosparging promotes biodegradation of constituents rather than volatilization (generally by using lower flow rates than are used in air sparging). In practice, some degree of volatilization and biodegradation occurs when either air sparging or biosparging is used.

The effectiveness of biosparging depends primarily on two factors:

1. The permeability of the soil which determines the rate at which oxygen can be supplied to the hydrocarbon-degrading microorganisms in the subsurface.
2. The biodegradability of the petroleum constituents which determines both the rate at which and the degree to which the constituents will be degraded by microorganisms.

In general, the type of soil will determine its permeability. Fine-grained soils (e.g., clays and silts) have lower permeabilities than coarse-grained soils (e.g., sands and gravels). The more permeable the soil, the more air that can be moved through it to reach the microorganisms.

Bacteria require a carbon source for cell growth and an energy source to sustain metabolic functions required for growth. The biodegradability of a petroleum constituent is a measure of its ability to be metabolized by hydrocarbon-degrading bacteria or other microorganisms. Petroleum constituents are generally biodegradable, regardless of their molecular weight, as long as indigenous microorganisms have an adequate supply of oxygen and nutrients. For heavier constituents (which are generally less volatile and less soluble than lighter constituents), biodegradation will exceed volatilization as the primary removal mechanism, even though biodegradation is generally slower for heavier constituents than for lighter constituents. The presence of very high concentrations of petroleum organics or heavy metals in site soils can be toxic or inhibit the growth and reproduction of bacteria responsible for biodegradation in biopiles. Conversely, very low concentrations of organic material will result in diminished levels of microbial activity.

Other factors that affect the efficacy of biosparging are those that affect the growth and viability of the microorganisms that degrade petroleum hydrocarbons. These factors include:

Bacterial growth rate is a function of temperature. Microbial activity has been shown to significantly decrease at temperatures below 10 degrees C. The microbial activity of most bacteria important to petroleum hydrocarbon biodegradation also diminishes at temperatures greater than 45 degrees C. Within the range of 10 degrees C to 45 degrees C, the rate of microbial activity typically doubles for every 10 degrees C rise in temperature.

To support bacterial growth, the pH should be within the 6 to 8 range, with a value of about 7 (neutral) being optimal. If the groundwater pH is outside of this range, it is possible to adjust the pH prior to and during biosparging operations. However, pH adjustment is often not cost-effective because natural buffering capacity of the groundwater system generally necessitates continuous adjustment and monitoring throughout the biosparging operation. In addition, efforts to adjust pH may lead to rapid changes in pH, which are also detrimental to bacterial activity.

For biosparging applications directed at petroleum products, bacteria that use oxygen as an electron acceptor (that is they metabolize organic contaminants aerobically) are most important in the degradation process. The rate of biodegradation will depend, in part, on the supply of oxygen to the contaminated area, because aerobic metabolism is much faster than anaerobic metabolism. When there is an insufficient amount of dissolved oxygen available, organisms that can use other electron acceptors may degrade the contaminants but at slower rates.

System Design (Back to Top)
The essential goals in designing and air sparging system are to configure the wells and monitoring points in such a way to:

1. optimize the influence on the plume, thereby maximizing the removal efficiency of the system, and
2. provide optimum monitoring and vapor extraction points to ensure minimal migration of the vapor plume and no undetected migration of either the dissolved phase or vapor phase plumes. In shallow applications, in large plume areas, or in locations under buildings or pavements, horizontal vapor extraction wells are very cost effective and efficient for controlling vapor migration.

The placement and number of air sparge points required to aerate the dissolved phase plume is determined primarily by the permeability and structure of the soil as these affect the sparging pressure and distribution of air in the saturated zone. The bubble radius (analogous to the radius of influence for air sparging systems) is defined as the greatest distance from a sparging well at which sufficient sparge pressure and airflow can be induced to enhance the biodegradation of contaminants. The bubble radius will determine the number and spacing of the sparging wells. The bubble radius should be determined based on the results of pilot tests. The bubble radius depends primarily on the hydraulic conductivity of the aquifer material in which sparging takes place. Other factors that affect the bubble radius include soil heterogeneities and differences between lateral and vertical permeability of the soils. Generally, the design bubble radius can range from 5 feet for fine-grained soils to 100 feet for coarse-grained soils.

Laboratory biodegradation studies can be used to estimate the rate of oxygen delivery and to determine if the addition of inorganic nutrients is necessary. However, laboratory studies cannot duplicate field conditions, and field tests are more reliable. A common biodegradation study for biosparging is the slurry study. Slurry studies involve the preparation of numerous "soil microcosms" consisting of small samples of site soils from the aquifer mixed into a slurry with the site groundwater.

Advantages and Disadvantages (Back to Top)