Page 200 - Environmental Control in Petroleum Engineering

186 Environmental Control in Petroleum Engineering

In most cases, no special attempt has been made to limit leaching
of metals or hydrocarbons from reserves pits or evaporation ponds. If
leaching is a problem, the pit can be constructed with an impermeable
liner and a leachate collection system with monitoring wells and
enhanced evaporation features (sprinkler recirculation to increase the
surface-to-volume ratio of the water). Lined pits are now required in
some areas for oil-based or salty drilling mud systems. Any sus-
pended or dissolved solids in the water will be concentrated as the
water evaporates. If the pond has completely dried, these materials
will be converted into a sludge, which may require further treatment
before disposal.
Before dewatering and closure of reserves pits, the pit contents can
segregate into layers. These layers can include a layer of free oil
floating on a layer of water. The water normally contains a high
concentration of dissolved solids. At the bottom is a layer of sludge
that contains most of the settled solids. As the oil layer is weathered,
a surface crust can also form. These top layers inhibit the evaporation
of water, delaying the natural dewatering of the pits.

Percolation

In some arid areas where the water table is very deep, aqueous
wastes can be placed in percolation ponds. These ponds have perme-
able sides and bottoms, allowing the water to percolate into the
surrounding soil, leaving the solids at the bottom of the pond. The
use of these ponds is highly restricted, however, because they allow
dissolved solids in the water to spread into the surrounding soil.

Mechanical Methods

In many cases, evaporation is too slow to remove water from solid
wastes. A number of mechanical methods are available to dewater
solids. Preliminary separation of free liquids from the solids should
be made with shale shakers, settling ponds, or hydrocyclones.
To further reduce the free water content of sludges, more advanced
(and expensive) technologies can be used. These technologies include
high-pressure filter presses, centrifuges, and vacuum filtering. Polymer
conditioning of sludges can also be used to enhance dewatering. The
low water content of the high-pressure filter presses can significantly