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Waste Treatment Methods 179

to bacterial colonies that degrade the hydrocarbons. (American Petrol-
eum Institute, 1986a). The application of biological processes to other
waste streams will be discussed below.

6.1.2 Removal of Dissolved Hydrocarbons

In addition to suspended hydrocarbons, most produced water also
contains varying amounts of dissolved hydrocarbons. A variety of methods
are available to remove these dissolved hydrocarbons from the water.

Adsorption

An effective way to remove low levels of dissolved hydrocarbons
is to adsorb it onto a solid medium. The most widely used medium is
activated carbon. The pH and temperature of the system impacts the
effectiveness of activated carbon on removing different hydrocarbon
compounds. All free oil must be removed prior to the use of activated
carbon to prevent the oil from clogging the carbon. In some cases,
coal may also be used as an adsorption media. Natural and synthetic
resins have also been developed that have proven effective in removing
dissolved hydrocarbons from water.

Volatilization

Volatile organic carbon compounds (VOCs) can be removed from
water by lowering the partial pressure of the compound in the vapor
in contact with the water. When the partial pressure of the dissolved
VOCs in the water exceeds that of its vapor pressure, the compounds
will come out of solution and enter the vapor phase.
A variety of methods can be used to volatilize VOCs. Perhaps the
most common is air stripping. In this process, air and water are passed
through a containment vessel in countercurrent flow where VOCs
evaporate into the air. The removal of VOCs can be enhanced by
heating the air or by using steam, because higher temperatures increase
their vapor pressure. Volatilization can also be enhanced by pulling a
vacuum on the water, lowering the total system pressure.
One limitation to volatilization is that it transfers the VOCs from
water to a vapor phase, yielding a contaminated vapor stream that must
then be handled. If air is used, the oxygen will dissolve into the water,