Emulsions are the result of the stable dispersion of droplets
of one immiscible liquid (not soluble in each other) in another. There
are many types and classifications of emulsions. The emulsion of concern
for the marine engineer is the oil in water (O/W) emulsion. For the
purposes of this Guide, oil will mean any water-insoluble hydrocarbon,
(i.e. diesel, benzene, etc.). It takes energy to disperse oil stably
in water. This energy can be mechanical (shearing due to pumping,
pressure, non-laminar flow in pipes and transfer operations) or chemical
(detergents, solvents). Both factors usually contribute to the formation
of stable emulsions on ships. Emulsions are problematic because oil
droplets of mean particle size of approximately less than (<) 15
microns are neutrally buoyant in water and will not separate under
the force of gravity alone, though some small OWS use centrifuges
or coalescers. These droplets will be detected by the OCM. In order
to destabilize an emulsion, one must cause the neutrally buoyant droplets
to coalesce into larger droplets, which will then have positive buoyancy
and separate from the effluent (approximately 50-100 microns) and
float to the surface. This can be accomplished through the use of
heat destabilization, pH adjustment or chemical addition. Polishing
technologies utilizing chemical affinity, ultra-filtration or flocculation
are also appropriate. The best approach is to minimize factors that
will contribute to and enhance emulsion formation. Resolution MEPC.107(49) OWS separators
were developed specifically to deal with bilge water in the form of
free oil and oily emulsions. Some of the less expensive 107(49) and older 60(33) OWS are not designed
to handle any or all of the contaminants, and may require retrofitting
with polishing units, and/or particulate filters and/or non-shearing
pumps. Poorly operated and maintained 107(49) separators
may not perform as expected.