Page 206 - Environmental Control in Petroleum Engineering

192 Environmental Control in Petroleum Engineering

Naturally occurring bacteria can effectively degrade naturally occur-
ring hydrocarbons, such as crude oil. In most cases, the appropriate
bacteria are already present in the environment and their populations
can be increased just by adding nutrients. In some cases, naturally
occurring bacteria have been artificially cultured and then released in
greater numbers to accelerate biodegradation of the hydrocarbons, but
the effectiveness of this augmentation is uncertain. Genetically engi-
neered bacteria may be necessary to degrade some refined hydro-
carbons, such as chlorinated solvents.
The most significant limitation for many bioremediation applications
is a lack of nutrients for bacterial growth. These nutrients, e.g.,
nitrogen, phosphorus, and some trace elements, can be added by way
of fertilizer. The amount and composition of fertilizer needed for
optimum degradation depends on what hydrocarbon is being degraded
and the bacteria being enhanced.
Oxygen is also needed for bioremediation to convert the hydro-
carbons to carbon dioxide and water. Anaerobic biological degradation
(without oxygen) also occurs, but is much slower and less efficient
than aerobic degradation. Oxygen is normally provided by ensuring
that the pore system within the solids is sufficiently open for air to
flow through it. One way to enhance the pore system is by adding
inert bulking agents like wood chips, bark, sawdust, tires, and shredded
vegetation to increase the mixture porosity. The use of inert bulking
agents is called composting bioremediation.
In most cases, water is also needed because it is the medium in which
the bacteria live. Bacterial growth normally occurs at water/hydrocarbon
interfaces. For optimum degradation, the water content of the solids must
be balanced. If not enough water is present, bacterial growth will be
inhibited. If too much water is present, the access of oxygen and nutrients
to the bacteria will be limited, again inhibiting bacterial growth.
In some cases, surfactants have been added to the nutrient mixture
to solubilize and emulsify low-solubility hydrocarbons, including
heavy aromatics and PAHs. Surfactants can also mobilize sorbed
microbial cells and contaminants from the soil surface to provide
greater access to microbial attack.
The degradation rate of hydrocarbons depends on the structure of
the hydrocarbon molecule and the type of bacteria involved. Paraffins
are the most susceptible to microbial attack, followed by isoparaffins
and aromatics. The polycyclic aromatic hydrocarbons (PAHs) are the