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9.4 WATER TREATMENT EQUIPMENT

The various produced water treatment equipment mentioned are described
in this section. The main function of the treating equipment is to separate
the free oil droplets from the water. The fluid may contain some dissolved
gas, which will be liberated in the treating equipment and must be
removed. Therefore, the produced water treatment equipment are, in
essence, similar to the three-phase oil–water–gas separators. The main
difference is that for water treatment equipment, water is the main and
continuous phase and oil represents a small volume of the fluid mixture.

9.4.1 Filters
One of the very efficient ways of removing oil droplets from water is the
use of filters. In this method of water treatment, produced water is made
to flow through a bed of porous medium, normally sand, where the oil
droplets are trapped in the filtering medium. At least two filters arranged
in parallel are used. As the filter in use gets clogged, the flow is directed to
the other filter and the clogged filter is backwashed using water or solvent.
The backwash fluid must be treated or disposed of properly, which adds
more complications and cost to the water treatment process. Several
onshore successful operations have been reported in which sand filters
were used to yield treated produced water with oil content as little as
25 mg/L of water.

9.4.2 Precipitators
In this method of treatment, the produced water is directed through a bed
of porous material, such as excelsior, placed inside a horizontal vessel that
is similar in design to the three-phase separator to promote the coalescence
of oil droplets. The coalesced large oil droplets flow upward, counter-
current to the downward flow of the water where it can be skimmed out of
the vessel. Although this method has been effective in treating produced
water to desired quality, clogging of the coalescing medium represented a
serious problem, which limited the use of such precipitators.

9.4.3 Skim Tanks and Vessels
The skim tanks and vessels are the simplest equipment used for primary
treatment of produced water. Skim tanks and skim vessels are generally
similar in shape, components, and function. However, the designation of
skim tanks is associated with atmospheric treatment, whereas skim vessels

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