FIELD OF THE INVENTION
The present invention relates to a method and apparatus for the control
of microbial contaminants in liquids and, more particularly, to the destruction
of such contaminants in shipboard fuels and ballast waters by the use of ultrasonic
vibration to cause cavitation within these liquids, that apparatus being designed
to prevent cavitation from damaging parts of the apparatus that create the ultrasonic
BACKGROUND OF THE INVENTION
Microbiological contamination of hydrocarbon fuels presents a variety
of problems to the operators of naval vessels. Some of the organisms responsible
for such contamination are fungi, yeast and bacteria.
In naval vessels, it is common for water to be found in on board fuel
tanks. This water originates from various sources such as condensation from the
fuel, water leakage into the fuel or from water taken on as ballast in the tanks.
The presence of water in the fuel tank results in the proliferation of yeasts and
fungi at the fuel/water interface where the microbial contaminants extract oxygen
from the water and nutrients from the fuel layer. Some forms of these microorganisms
produce water as a byproduct, thereby altering the environment of the fuel/water
interface and allowing other microbial forms to flourish.
Various problems arise from the microbiological contamination of fuel
- (a) Mat-like or slimy deposits at the fuel/water interface;
- (b) Blockages of valves, pumps, filters and coalescers;
- (c) Reduction in interfacial tension resulting in the malfunction of water separating
- (d) Accelerated corrosion of steel and aluminum;
- (e) Black stains on copper alloys or silver plated components;
- (f) Injector fouling; and
- (g) Probe fouling and incorrect volume measurement.
Some of these problems have previously been documented (R.D. Haggett
and R.M. Morchat, Intl. Biodeterioration & Biodegradation 29 (1992) 87-99).
These consequences can be tolerated at minor levels of infection.
However, as the microbial population flourishes, serious and costly failures are
inevitable. Generally, contamination problems are only investigated when the failure
or malfunction of equipment occurs. Fuel tanks, and associated systems, found to
contain such contaminants must be drained, cleaned, dried and inspected prior to
Completely sterile natural environments are rare and without strong
chemical additives toxic to microbes, some level of contamination can always be
expected. However, if the levels of this contamination can be kept below critical
levels, their proliferation can be prevented and the damaging consequences avoided.
The only means of controlling microbiological contamination in ship
board fuel systems at present is to prevent water from accumulating in fuel tanks,
which is extremely difficult and impractical, or to treat the contaminated fuel
with biocidal agents. However, the use of such biocides presents environmental and
health and safety concerns. Questions have arisen concerning the effect of biocide
containing fuel on personnel working daily with fuel system components as well as
personnel working in confined spaces where they may be exposed to vapours containing
the biocide. The environmental concern relates to the effect that such biocidal
agents may have if introduced into already sensitive marine ecosystems. The selective
nature of biocides presents a further problem in their usage. For example, while
some biocides are effective against fungi they have little or no effect on bacteria.
Further, while some biocides inhibit growth of pure microbial cultures, their effectiveness
is drastically reduced when applied to mixtures of fungi, yeasts and bacteria.
The use of ultrasound as a germicidal agent has been investigated
previously by G. Scherba et al (Applied and Environmental Microbiology 1991,
2079-2084) and H. Kinsloe et al (J. Bacteriology 68 (1954) 373-380). The
literature on the treatment of microorganisms using ultrasonics is sparse, but all
studies that have been carried out agree that it is an effective means of destroying
microorganisms. A shipboard application of this technology is waste water treatment.
This possibility was studied by the U.S. Navy Coastal Systems Station in 1976 (A.J.
Ciesluk, "Acoustic Sterilization for Shipboard Waste Management", U.S. Navy Coastal
Systems Station Technical Report, NCSC-TR-329-78). In this study, two commercial
ultrasonic cleaners were used at two different power levels. However, it was concluded
that the basin volumes of these cleaners were too large to lead to effective cell
disruption. That literature does not describe the use of ultrasound to control microbial
populations in fuel systems although the possibility has been proposed (E.C. Hill
(1986), "Microbial Problems In Offshore Oil Industry" Proceedings of the International
Conference, Inst. Petroleum Microbiology Committee, Aberdeen, U.K.).
Because of its inherent safety and relatively low power requirements
compared to other physical control measures, ultrasound may represent the ideal
solution to microbiological contamination of fuel systems. If the fuel and/or the
water in the vicinity of the fuel/water interface is treated on an ongoing basis,
the microbial populations can likely be kept below critical levels. This would represent
a more environmentally friendly and more effective control measure than the biocides
currently in use.
United States Patent 5,395,592 to Bolleman et al describes an innovative
transducer for use in an ultrasonic treatment apparatus. The apparatus includes
a treatment container (17) that is used to confine the contaminated liquid (14)
to be treated by cavitation. The treatment container (17) is surrounded by an outer
container (12) for containing a transmission fluid (16). The transducer (13) is
mounted on the outer surface of the treatment container (17) and is submerged in
the transmission fluid. Both the contaminated liquid and the transmission fluid
can be pressurised to increase their cavitation threshold. United States Patent
5,395,592 teaches that the pressure between the contaminated liquid and the transmission
fluid must be the same and the apparatus therefore includes a pressure equalising
means such as an elastic membrane, for example.
SUMMARY OF THE INVENTION
Accordingly, it is an object of the present invention to overcome
the limitations of known fuel decontamination methods and provide a safe and effective
process for the control of microbial populations in fuel systems. It is also an
object of the present invention to provide a system and a process for the effective
treatment of microbiologically contaminated ballast waters prior to disposal thereof.
Specifically, the present invention is directed to an apparatus for
neutralizing microbiological contamination of a liquid fuel comprising subjecting
the fuel to ultrasonic vibrations in order to cause cavitation within the liquid
and, thereby, to destroy the microbial contaminants and which is designed to avoid
any cavitation from damaging the parts of the apparatus that create the ultrasonic
The present invention provides an apparatus for the ultrasonic treatment
of a microbiologically contaminated liquid, comprising a module having a treatment
container, an outer container for containing a transmission fluid that contacts
an outer surface of the treatment container and an ultrasonic generating means for
subjecting the contaminated liquid in the treatment container to ultrasonic vibrations
to result in cavitation in the contaminated liquid and the destruction of microbial
contaminants contained therein, wherein the ultrasonic generating means is located
outside of the treatment container and submerged in the transmission fluid which
is pressurised to prevent cavitation occurring in the transmission fluid at areas
surrounding the ultrasonic generating means;
DESCRIPTION OF THE DRAWINGS
CHARACTERISED IN THAT the transmission fluid is pressurised by
a hydraulic cylinder connected to the interior of the outer container, the hydraulic
cylinder being provided with means to apply a predetermined pressure to a piston
in the hydraulic cylinder.
These and other features of the invention will become more apparent
in the following detailed description in which reference is made to the appended
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
- Figure 1 is a schematic view of an ultrasonic decontamination system according
to the present invention;
- Figure 2 is a cross-sectional view of the ultrasonic decontamination system
according to the present invention;
- Figure 3 is a schematic view of a flow-through test system according to one
embodiment of the present invention;
- Figure 4 is a perspective view of a bank of ultrasonic decontamination modules
connected in parallel according to a further embodiment of the present invention;
- Figure 5 is a front view of the embodiment shown in Figure 4.
A flowthrough ultrasonic system for destruction of microorganisms
in fuels and ballast waters was described by Randall Haggett et al in Canadian Patent
Application 2,151,874 that was filed on June 15, 1995. A series of potential configurations
for the design of Ultrasonic Destruction of Microorganisms (UDM) have been built
and evaluated. These configurations included:
- (1) a flowthrough bank of ultrasonic horns;
- (2) a submerged coil; and
- (3) piezoelectric rings.
The ring concept appears to be the most suitable at providing a fairly
even distribution of energy in a fluid such as diesel fuel. There were problems,
however, associated with this concept. The generation of cavitation to kill microorganisms
in the fluid was accompanied by serious erosion problems associated with that cavitation.
Initial tests with the piezoelectric ceramic ring concept showed that the erosion
would drastically limit the life of the electrodes on the ceramics and then the
ceramic rings themselves. In early tests, the electrodes were being eroded in minutes.
Various coatings for protection from cavitation erosion were tested on the ceramic
rings and evaluated for power transfer characteristic as well as erosion protection.
None were found to provide satisfactory protection. A solution to that problem,
according to the present invention, was the introduction of a second fluid (a transmission
medium) around the ceramic rings and their electrodes. That second fluid was selected
to minimize cavitation around the piezoelectric rings while transmitting ultrasonic
energy to an inner tube through which the fluid to be decontaminated flowed.
The basic concept of an ultrasonic decontamination system according
to the present invention is illustrated, schematically, in Figure 1. A number, three
being shown in Figure 1, of piezoelectric ceramic rings (or elements) 1 surround
an inner metal tube 2 that forms a treatment container through which a fluid 3 to
be decontaminated, such as diesel fuel, flows. The rings 1 are connected to and
electrically excited by driver electronics 6 to generate ultrasonic sound of an
intensity to create destruction of microorganisms in that diesel fuel. The generated
ultrasonic sound is sufficiently intense to induce cavitation in the liquid 3, illustrated
by bubbles 9 in Figure 1, and it is this cavitation 9 that is responsible for microorganism
destruction in the liquid 3. The piezoelectric rings 1 are immersed in a transmission
medium 8 in container 4, the transmission medium 8 in container 4 being pressurized
to a pressure sufficient to prevent cavitation occurring in that transmission medium.
If cavitation were allowed to occur in the medium in contact with the piezoelectric
rings, destruction of the thin metal coated electrodes on the rings would rapidly
occur because of surface erosion action caused by cavitation. Pressurization of
the medium prevents cavitation occurring and damaging the piezoelectric rings electrodes.
A prototype mechanical module (or cavitator) 40 for an ultrasonic
decontamination system according to the present invention was built and consisted
of 4 piezoelectric rings stacked axially around a 3.8 cm (1.5 inch) outside diameter
(OD) stainless steel tube 2 as illustrated in the cross-sectional view in Figure
2. The housing design is such that the transmission fluid surrounding the piezoelectric
rings 1 can be pressurized and the liquid to be decontaminated can be pumped continuously
through the tube.
The prototype module, according to one embodiment of the invention
and illustrated in Figure 2, has an outer housing 4 with a bottom flange 14 and
top flange 12 with a bottom 16 being attached to flange 14 and a cover 10 attached
to flange 12 to form a chamber to hold a transmission medium such as oil. These
components were formed of aluminium in this prototype but might be formed of stainless
steel in other systems so as to avoid possible corrosion problems.
The bottom 16 has a central opening formed by protrusion 18 with that
central opening being connected to a stainless steel inner tube 2 that extends through
the chamber and out through a central opening in the cover 10. The inner tube 2
is swaged onto a flared section on an extension of protrusion 18 that protrudes
into the chamber and an O-ring 24' between a support structure 20 attached to flange
16 and flange 16 effects sealing at that end. The tube is sealed at the other end
by an O-ring 29 between the cover 10 and tube 2. This allows the tube to expand
and contract freely (except for friction of the O-ring) at that end so as to accommodate
manufacturing tolerances and thermal expansion/contraction.
A support structure 20 is connected to bottom 16 and supports four
piezoelectric ceramic rings 1 around the inner tube 2. The rings each have a 5.0
cm (2 inch) OD, a 4.4 cm (1.75 inch) ID, a 1.4 cm (0.55 inch) length and are stacked
one above the other. The rings are mounted between nylon flanges with silicone rubber
O-ring separators 28, 28'. The separators avoid overconstraining the rings and prevent
any buildup of axial stress due to differential thermal expansion between the components
as temperature changes occur. The piezoelectric ring assembly is fixed in the chamber
and the electric connections to it are brought out to a connector (not shown) on
the cover 10. An O-ring 24' is located between the bottom of support 20 and bottom
16 and another 24 is located between bottom 16 and flange 14 to seal the chamber
at that end. An O-ring 26 between the top flange 12 and cover 10 seals the chamber
at the top end.
To allow filling the chamber formed by components 4, 10 and 16 with
a transmission medium without trapping air in that chamber, a fill hole (not shown)
is provided at the bottom and an air release vent (not shown) is provided at the
top with a circular groove being machined in top 10 to collect air from all points
around the circumference and lead it to the air release vent. That vent is sealed
with a screw after the chamber is filled and the air vented. Once the chamber is
filled with the transmission medium, it surrounds the piezoelectric rings 1 and
transmits ultrasonic energy generated by the rings to the inner tube 2 through which
a fluid to be decontaminated flows.
The transmission medium in the chamber must have the following characteristics:
Possible fluids that could be used as a transmission medium are natural and synthetic
lubricating oils, transformer oil, or oils used in some high power sonar transducers.
In this prototype system, ordinary SAE 10W30 motor oil was used as a transmission
- 1. be electrically insulating;
- 2. be compatible with all material that it contacts, such as the aluminium (or
stainless steel) housing, the inner tube, the nylon (or other plastic) support used
to mount the piezoceramic rings, the ceramic rings and their silver or nickel electrodes;
- 3. be resistant to cavitation when pressurized to low levels; and
- 4. have a low loss to acoustic energy.
Once the chamber is filled with the transmission medium, that medium
must be pressurized to a controlled level to prevent cavitation in it. In this prototype
system, that pressurization was accomplished by using a small hydraulic cylinder
37 (see Figure 3) connected to the chamber and which is loaded by a manually adjusted
screw 38. Another method would be to use an air cylinder rather than a screw to
load the hydraulic cylinder. Thermal expansion of the oil as the temperature rises
during operation causes the pressure to rise making the screw and hydraulic cylinder
difficult to control. To alleviate that problem, a small bellows or expansion bulb
could be added to the fill port.
The inner tube 2 through which the liquid to be decontaminated flows
is formed of stainless steel in order to ensure maximum compatibility with a ship's
diesel fuel. The tube 2 should be as thin as possible to avoid screening the acoustic
field from the medium in the pipe but thick enough to avoid collapse under the external
pressure caused by the pressurization of the transmission medium in the chamber.
The tube 2 used in this prototype system had a 0.03 cm (0.12 inch) wall thickness
and an outside diameter (OD) of 3.8 cm (1.5 inch).
The test assembly of the prototype flowthrough system is schematically
illustrated in Figure 3. In that system, a tank 30 contains contaminated water which
is pumped by pump 36 through pipes and valve assembly 34 to a decontamination module
40 mounted on a stand 42. A power supply 6 is connected to the top of module 40
and treated water exits from the top of module 40 and is then piped to tank 32.
The flow rate through the module is dictated by the duration which
the liquid in tube 2, such as diesel fuel, must be exposed to the ultrasonic field
in order to obtain an adequate microorganism kill rate. The flow rate is also dependent
on the type of application. It need not kill all microorganism in a single pass,
for instance, if the system is run continuously with fuel circulating from a tank
to the module, through the system, then back to the tank with the goal of keeping
microorganism growth down. If the goal is to attempt to sterilize fuel that has
been brought on board from a contaminated source, or which is being off-loaded after
contamination has been allowed to build up, then a high kill rate would be required.
This would, therefore, require a lower flow rate through the module resulting in
a longer residence time.
The prototype module was designed with the criterion that the fuel
should be exposed to the most intense sound field (assumed to be directly adjacent
to the piezoelectric rings) for 4 seconds. That time was based on experience with
an experimental unit where, at least for some organisms, the required kill rate
was achieved in 4 seconds. With the dimensions used in this module, a flow rate
through the module of 15 Imperial gallons per hour (GPH) (0.068m3 per
hour) would be sufficient. If shorter durations are found to be acceptable, greater
flow rates can be used. The pressure drop through the tube is very small at these
Decontamination modules, similar to those previously described can
be arranged into banks where a number of modules 40 are connected in parallel between
an input manifold 50 and an output manifold 52 assembled on support 48 as shown
in Figure 4. The input manifold 50 is divided into two input manifolds 44 and 46
located on each side of a central support 48 (see Figure 5) with 5 modules being
connected between each inlet manifold 50 and the output 52 on each side of support
48. If each module has a flow rate of 15 GPH (0.068m3 per hour) then
the total flow through the bank illustrated in Figures 4 and 5 would be 150 GPH
(0.68m3 per hour). The overall dimensions of this type of bank is expected
to be approximately 40 by 15 by 15 inches (100 by 38 by 38 cm) with a weight of
about 60 pounds (27 kg).
By arranging the modules into banks, a number of advantages would
be obtained such as that a bank could form a convenient Line Replacement Unit (LRU)
with only two couplings to the fuel system and with only a small number of external
electrical connections. All other mechanical and electrical connections within each
module could be made at a factory where the banks are assembled. A single pressurization
system connected to each module can be used. Another advantage is that systems of
different sizes and flow rates can be configured by assembling varying number of
banks. Furthermore, the overall shape or form factor of the system can be varied
to suit unique mounting consideration in different applications
The prototype's driver electronics unit 6 contained three sections,
a function generator, a power amplifier and a power supply. The function generator
had a frequency range from .01 Hz to 300 kHz and could provide four different wave
shapes. Those shapes (sinsoidal, triangular, half-square and full-square) have different
spectral compositions and excite the ceramic rings at different efficiencies. The
electronics unit allowed the excitation to be tuned to optimal frequencies. The
power amplifier section was a commercial unit modified to provide a signal response
up to 70 kHz. Its power output was 300 W at 15 kHz tapering to 32 W at 59 kHz. When
the piezoelectric elements are driven below 40 kHz, the power transferred is at
least 60 W. A high-voltage transformer takes the power amplifier output of 70 V
and steps it up to approximately 1000 V in order to provide the voltage required
by the piezoelectric elements. The ultrasonic frequencies can range from 22kHz to
40kHz or higher. In the prototype apparatus, that frequency was variable along with
the power supplied to it in order to optimize the performance.
Various modifications may be made to the preferred embodiments. For
example, the module described herein has a flow through pipe but the treatment container
could be, for instances, an open tank.