Environmental Engineering Reference
In-Depth Information
not preferred by ship operators owing to cost and unavailability of consumables at foreign
ports of call. Often, bilge water is heated to achieve an optimal temperature for centrifugal
separation, reducing viscosity of the continuous phase and facilitating formation of larger
oil droplets. A number of commercial systems are available on the market that use these
various physical separation processes. Performance at <5 ppm is typically only guaranteed
with use of optional inal polishing ilters that can be bypassed during routine operation.
After separation, sludge is often dewatered and stored. In certain cases, recovered oil can
be used as fuel; however, contamination may prevent usage in this manner. Oil-in-water
content monitors are often employed in the discharge stream to be in compliance with
IMO Resolution MEPC 107(49).
Alternative approaches to physical separation have been investigated. Electrocoagulation
has been studied as a means to improve separation, especially for emulsiied systems.
13
The process of electrocoagulation uses an electric current to dissolve a sacriicial iron or
aluminum anode creating cations that are attracted to the ine negatively charge droplets
of contaminants. Agglomeration of the droplets or particles destabilizes the suspension,
leading to phase separation. Hydrolysis at the electrodes creates gas bubbles of oxygen
or hydrogen that facilitate separation by lotation but also create a signiicant explosion
hazard owing to the combustibility in enclosed environments. Laboratory-scale measure-
ments indicated that a >95% decrease in oil and >99% decrease total suspended solids
were achievable using electrocoagulation. Biodegradation has also been examined for
bilge water treatment at the laboratory scale. Aerobic bioreactors were seeded with sedi-
ment from the mouth of the Cheng-Jenn River in Kaohsiung Harbor (Taiwan), which was
presumed to contain bacteria necessary to degrade hydrocarbons associated with bilge
water.
14
Experiments indicate that a >90% reduction in total organic carbon levels was
achieved with emulsiied diesel fuel being the contaminant. Practical disadvantages are
the time it takes to degrade the oil (rate = 0.4 kg oil/m
3
d) and the necessity to maintain
the microbial population over time. An evaluation of bilge water treatment by a combina-
tion of ultrailtration (UF) and membrane distillation (MD)
15
and reverse osmosis
16
has
also been conducted. Substantial removal of organics to <5 ppm was achieved by the UF/
MD method. Permeate low varied according to oil concentration and temperature, but
the system generally demonstrated ~200 kg/m
2
d of permeate lux.
15
Oil-water separation
methods adapted from the petroleum industry also serve as potential methods for bilge
water separation.
17
7.3 Membrane Separation
7.3.1 Nanotechnology-Based Membranes
Numerous nanotechnology-based membrane separation methods have been described for
separation of oil and water that have application to bilge water treatment.
18
Separation
of oily water emulsions is a relatively mature technology typically involving the use
of microiltration (MF) with pore sizes of 100-10,000 nm; however, UF, which employs
smaller pore sizes (100-500 nm), is sometimes required for colloidal waste streams.
19, 20
Application of nanotechnology to membrane design has allowed for improvements in the
functionality of traditional levels, including higher permeability, better selectivity, and
increased resistance to fouling. There are three general types of nanotechnology-based
