2nd International Ballast Water
Treatment R&D Symposium
21-23 July 2003, IMO, London
Ballast water treatment by De-oxygenation with
elevated CO2 for a shipboard installation -
a potentially affordable solution
2. Paper author/s - M. Husain, MH Systems,
H. Felbeck, Scripps Institution of Oceanography, University
of California - San Diego
R. Apple, MH Systems, Inc,
D. Altshuller, MH Systems, Inc.
C. Quirmbach, MH Systems, Inc.
3. Contact details - MH Systems, Inc.
10951 Sorrento Valley Road, Suite 2F
San Diego, CA 92121 USA
T: 858-452-1280 F: 858-452-6035
4. Treatment options being researched
It is estimated that 21 billion gallons of ballast
taken on in foreign ports are discharged by commercial vessels annually
in the waters of the United States (Carlton et al. 1993). Specifically,
ballast water transport is a major vector for the introduction of
potentially invasive aquatic species.
The concept to combat Aquatic Nuisance Species
(ANS) invasion resulting from ballast water discharge, described in
this paper, is a technical extension of MH Systems' American Underpressure
System (AUPS). The AUPS utilises a slight negative pressure in the
tank's ullage space, in an inert environment, to prevent or minimize
oil spillage from tankers (Husain et al. 2001).
The ballast water treatment method consists of
bubbling the inert gas via a row of pipes (orifices at the bottom
of the pipes) located at the bottom of the tank, while maintaining
a negative pressures of -2 psi at the ullage space. The inert gas
from a standard shipboard inert gas generator is composed of 84% Nitrogen,
12-14% CO2 and 2% Oxygen. The ballast water will be equilibrated with
gas from an inert gas generator. As a result, the water will become
hypoxic, will contain CO2 levels much higher than normal, and the
pH will drop from the normal pH of seawater (pH8) to approximately
4.1 Ballast Water Treatment Standards
Standards for treatment of ballast water are still
in a state of flux. Efforts to define standards are ongoing in the
US Congress, International Maritime Organisation (IMO), and other
individual maritime nations. The US Congress (NAISA 2002) proposes
an Act that will, among other considerations, set the interim standards
for ballast water treatment (BWT). It states, "The interim standard
for BWT shall be a biological effectiveness of 95% reduction in aquatic
vertebrates, invertebrates, phytoplankton and macroalgae." There are
discussions about setting micron standards, i.e., x microns cut-off
for living organisms.
Currently, a fifty (50) micron standard is being
discussed in various circles, including IMO and US Coast Guard. The
default standard appears to be the Ballast Water Exchange (BWE), or
something close to it. Cangelosi (2002) states "… the Coast Guard
has set forth a "do-it-yourself" approach, directing interested ship
owners to conduct complex shipboard experiments (post-installation)
to undertake direct and real-time comparisons between BWE and treatment.
If the comparison is favourable and defensible, the Coast Guard will
approve the treatment. ….."
4.2 Current Investigative Efforts Of Alternative
Glosten (2002) provides a review of the numerous
treatment systems options being investigated. These include heat,
cyclonic separation, filtration, chemical biocides, ultraviolet light
radiation, ultrasound, and magnetic/electric field. The methods not
mentioned in this reference are hypoxia, carbonation, and their combination.
In studies of 18 months duration on a coal/ore vessel (Tamburri et
al. 2002), the ballast water dissolved O2 level was reduced and held
to concentrations at or below 0.8 mg/l by bubbling essentially pure
nitrogen. The experiments resulted in a treatment "that can dramatically
reduce the survivorship of most organisms found in the ballast water…"
In extensive experiments with gas of varying percent
CO2, N2 and O2 (McMahon 1995), the "…results indicate that CO2 injection
may be an easily applied, cost-effective, environmentally acceptable
molluscicide for mitigation and control a raw water system macrofouling
by Asian clams and zebra mussels".
4.3 Corrosion Considerations Of Various Treatment
Shipboard corrosion mitigation is always a priority
consideration. It requires the continual attention of the crew and,
if not carefully controlled, can actually compromise the strength
of the ship. Any installed ballast water treatment system must not
under any circumstances increase the potential for corrosion and,
if possible, should decrease the potential. The system discussed in
this proposal has considered the corrosion issue. As reported in literature
(Tamburri et al., 2002), corrosion might even be mitigated by deoxygenation.
Perry et al. (1984) states that unless pH level drops below 4, concerns
about corrosion are unfounded.
5. Timeframe of the project
We present initial proof of concept results, which
have been conducted during 2002-2003.
6. Aims and objectives of the project
Except for ballast water exchange, essentially
all treatment concepts involve the chemical change of the water to
cause an environment lethal for ANS. The chemical changes described
by Tamburri et al. (2002) and McMahon (1995) offer promising results,
i.e., reduce the dissolved O2 in the one case, and carbonate and reduce
the pH in the other case. In both cases the process involves the exchange
of gases, the extraction of the dissolved O2 and the introduction
of CO2. Surface contact area and partial pressure differentials permit
the gas exchanges to occur. The deoxygenation of the ballast water
is based on Henry's Law of gas solubility: The relative proportion
of any dissolved gas including oxygen in the ballast water is a function
of the concentration, equivalent to partial pressure of the gas (e.g.
oxygen), within the mixed gases over the ballast water. The depletion
of oxygen in the ballast water is primarily a function of the shared
surfaces and concentrations at the interfaces of the inert gases and
The pH of the ballast water is lowered by the chemical
All systems described thus far in the literature, including ballast
transfer, has left untreated the sediment buildup in the bottom of
the tanks. If the orifices in the lattice work of piping pointed down,
then the sediment could be stirred up facilitating the kill of the
The purpose of the preliminary experiments described
here was to obtain initial data on the effects of "inert gas" on marine
organisms. "Inert gas", hereinafter called trimix, a commercially
available gas mixture of 2% oxygen, 12% CO2 and 84% nitrogen resembles
the gas generated by commercially used marine "inert gas generators".
Adult or young adult animals were chosen for two reasons a) to make
the size of specimens amenable for the experimental setup and b) to
raise the significance of possible effects since adults of a species
are typically more tolerant of environmental changes than juveniles
or larvae. All animals were collected fresh from the coastal waters
off La Jolla, CA and used immediately. The plankton sample was collected
with a plankton net from a small boat.
7. Research methods, test protocols and experimental design
The schematic of the experimental setup is shown
in Figure 1. Three parallel incubations were done for each experiment.
Several organisms were incubated in 1.5L of seawater at 22°C in large
Erlenmeyer flasks. Each incubation was equilibrated with the respective
gas using aquarium stones before any organisms were introduced. The
aerobic control was bubbled from an aquarium pump for approximately
15 min and left open to the atmosphere after addition of specimens.
An anaerobic incubation was bubbled with 99.998% nitrogen for 15 min.
After introduction of the organisms, the bubbling was continued for
another 10 min and then the container was closed with a rubber stopper
or the bubbling was continued. The incubation in trimix was treated
similarly except that the gas mix was used instead of nitrogen. The
oxygen concentrations were measured after the initial bubbling period
using a Strathkelvin oxygen electrode with a Cameron instruments OM-200
oxygen analyser. Ph values were determined using a combination electrode
and a Radiometer pH meter.
Figure 1. Schematic of the
Survival of the specimens was determined visually by checking for
motile responses to tactile stimulus (e.g. mussels do not close their
shells, barnacles to not withdraw their feet, shrimp do not move their
mouthparts, worms appear limp and motionless). After each testing
of the animals, the incubation flasks were bubbled for 10 min to reestablish
original conditions. To verify survival of the specimens, they were
relocated to aerobic conditions and checked again after 30 min. If
they still did not respond, they were considered dead. The survival
of the bacterium Vibrio cholerae strain N16961 was monitored by repeated
plating on Luria-Bertani Agar with Rifampicin (100 µg/mL).
This setup allowed us to compare responses to nitrogen
and "trimix" while making sure that test specimens were not gravely
affected by other experimental parameters. Incubation in pure nitrogen
allow for a comparison with published results by others.
8.1 Experimental results and discussion.
The oxygen concentrations were measured at "non-detectable"
for the nitrogen incubations and 10% air saturation (=16Torr partial
pressure) for the "trimix". The pH value of the water bubbled with
trimix reached 5.5 after the initial 10 min of vigorous bubbling.
The aerobic and nitrogen bubbled seawater maintained their pH at 8.
The incubations showed clearly that "trimix" kills organisms considerably
faster than incubations in pure nitrogen Table 1. All organisms except
of Vibrio cholerae showed no mortality in aerobic conditions. The
shrimp and crabs incubated in "trimix" were dead after 15 min and
75 min, respectively. Even a transfer into aerated water did not result
in any movement. The brittle stars incubated under nitrogen started
to move again after transferred into aerated water. All the mussels
incubated in nitrogen and "trimix" were open after 95 min but only
the ones in nitrogen still responded to tactile stimuli by closing
their shells. The barnacles were judged dead after incubation in "trimix"
when they did not withdraw their feet when disturbed, the ones incubated
in nitrogen still behaved normally. The plankton sample mainly contained
copepods. They stopped moving after 15 min and could not be revived
in nitrogen and "trimix" incubations. The results are summarised in
Table 1. Effects of Trimix
on Marine Species
Low oxygen concentrations in water are a common
natural phenomenon and their effects on live organisms have been widely
discussed in the past. Oxygen may not be available to an organism
because no water for respiratory purposes is present, e.g., during
low tide in the intertidal zone.
Oxygen may also be removed in stagnant waters due
to bacterial or other "life based" actions, e.g., in ocean basins,
fjords, tide pools, or in waters with high organic content and consequently
high bacterial counts, e.g., in sewage, mangrove swamps, paper mill
effluent. In addition, oxygen can also be removed by chemical reactions,
e.g., in hot springs, industrial effluents. The manuscript by Tamburri
et al. (2000) summarises survival of a variety of larvae and adults
of organisms including some which may be significant as "nuisance
species" under hypoxic conditions. The publication supports extensively
that most organisms only survive strongly hypoxic conditions for a
few hours and only a few adults for several days. The authors suggest
that 72 h of hypoxia will be sufficient to kill most eucaryotic organisms,
adults or larvae in ballast water.
The effects of high CO2 on organisms in natural
waters have become a research focus because of proposals to dispose
atmospheric CO2 in the deep ocean (Haugan 1997, Omori et al. 1998,
Seibel and Walsh 2001). Two effects have to be distinguished when
looking at "trimix" incubations in seawater: a) the lowering of the
pH from pH 8 to about 5.5 and b) the raised CO2 concentrations in
the water. While the pH change caused by the incubations in "trimix"
are in the range of published experiments, the CO2 concentration in
"trimix" (about 14%) is much higher than those investigated in the
published literature (0.1% to 1%). Therefore, the effects of "trimix"
incubations should be much stronger than those published previously.
Several publications have shown the detrimental
effect of lower pH values and high CO2 levels on aquatic life. In
a recent publication, Yamada and Ikeda (1999) tested ten oceanic zooplankton
species for their pH tolerance. They found that the LC50 (=pH causing
50% mortality) after incubations of 96 hours was between pH 5.8 and
6.6 and after 48h it was between pH 5.0 and 6.4. Therefore, the pH
value caused by incubations with "trimix" is well within the lethal
range for this zooplankton. Huesemann et al. (2002) demonstrate that
marine nitrification is completely inhibited at a pH of 6. Larger
organisms were also investigated, a drop in seawater pH by only 0.5
diminishes the effectiveness of oxygen uptake in the midwater shrimp
Gnathophausia ingens (Mickel and Childress 1978) and Deep sea fish
hemoglobin may even be more sensitive to pH changes (Noble et al.
1986). It appears that a common metabolic response to raised CO2 levels
and concomitant lowered pH is a metabolic suppression (Barnhart and
McMahon 1988, Rees and Hand 1990). Most recently, first papers were
published investigating the effects of environmental hypercapnia in
detail (Poertner et al. 1998, Langenbuch and Poertner 2002). The effects
of pH changes on phytoplankton growth has been reviewed by Hinga (2002).
The review summarises data from 22 studies. Many of the cited studies
use elevated levels of CO2 to adjust pH. In almost all cases, the
growth of unicellular phytoplankton and diatom species was severely
affected by low pH below pH 6.5, only the species Nitzchia closterium
showed significant growth at pH 5.5. Since all of the studies cited
were done at high light levels and in aerobic conditions, it can be
safely assumed that the conditions in an hypoxic dark environment
as is found inside of an inert gas treated ballast tank is even more
detrimental to phytoplankton growth.
The trimix combines both of these effects on organisms
- hypoxia and hypercapnia. Preliminary results demonstrate the effectiveness
of this combination in quickly killing a variety of sample organisms.
Contrary to methods using additions of biocides or any chemicals in
general, nothing is added to the ballast water and, therefore, nothing
will be released into the environment when it is released again. Methods
using radiation, heating, or filtering ballast water before or during
a ship's trip, are much more expensive. The equipment needed to establish
a rapid gassing of ballast water is available off the shelf and has
been used in the marine environment. The plumbing and gas release
equipment has been optimised and has been used in application such
as aquaculture, sewage treatment and industrial uses. Extensive supporting
literature and research about the design and optimisation of equipment
for the aeration of water is available from public resources. Inert
gas generators are available for fire prevention purposes on ships
and other structures and are already installed on many ships, mainly
tankers. They can use a variety of fuels including marine diesel to
generate the inert gas. Several topics have to be further investigated
before a conclusive recommendation about the treatment of ballast
water with "inert gas" can be made: a) how are larvae, eggs, and plankton
effected and b) what is the affect of trimix type inert gas in fresh
water? If ballast water is taken up through a screen, larger animals
will not be included. The initial tests were made with adults because
of easy access to them. However, if adults of a species are effected
by "inert gas" it is most likely that their larvae will also be effected
probably even more so.
Future tests will be conducted with specimens from
plankton and larval cultures and with incubations of mixed plankton
collected from the ocean. Determinations of viability will be made
by microscopic observations (e.g. movement of mouthparts, swimming
behaviour), ATP measurements (the ATP levels rapidly decreases after
death of an organism), and the ability to bioluminesce (many planktonic
organisms emit light, an ability which ceases after death). Fresh
water organisms will be of interest because the pH change is not as
much as in seawater. Freshwater in its natural environment can have
pH values around 5.5. It has to be proven that raised CO2 concentrations
in combination with hypoxia will also affect these species. Only then
can the method be used for both, fresh and salt water ballast.
8.2 Analysis and Design Equations
In this section, we present mathematical descriptions
of the deoxygenation process and of the transfer of carbon dioxide
into the ballast water, which, in turn, leads to lowering of the pH
to the levels lethal to most ANS. We obtain closed-form mathematical
models, usable in design of a shipboard system from any set of given
specifications. The list of symbols used in the equations is given
at the end of the paper.
The system being analysed places a mixture of nitrogen
and carbon dioxide with a relatively small fraction of oxygen in contact
with ballast water. The oxygen level in the ballast water is assumed
to have reached equilibrium with air as a result of prolonged contact,
and therefore would contain a concentration of oxygen sufficient to
support a wide spectrum of life forms. The objective is to reduce
the oxygen content to a low level by interchange with the gas mixture.
The gas is bubbled through the ballast water, which assures uniform
distribution of dissolved gas throughout the ballast tank. Thus, diffusion
within the tank can be neglected. Bubbles are assumed to be small
and variation of hydrostatic pressure over the vertical dimension
of a bubble is neglected.
We do not discuss here the size of bubbles and
the frequency of their generation. These two issues are addressed
in existing reference literature (see, for example, Perry et al. 1984).
We assume that deoxygenation process follows Henry's
Law with equilibrium achieved within the residence time of each bubble.
The composition of the mixture in the bubble changes primarily due
to transfer of carbon dioxide, a dynamic chemical process assumed
to obey the mass action kinetics.
B. Deoxygenation Process
As trimix gas is flushed through the system, the
total weight of oxygen in the ballast water will be reduced. For the
purpose of analysing the deoxygenation process we neglect the presence
of carbon dioxide in the trimix.
When a small quantity of gas, dQ, is admitted,
it contains an oxygen molar fraction y0. By the time this quantity
of gas leaves the system it contains, according to Henry's Law, the
molar fraction Y/kH.
Therefore, we obtain the following differential
Integration of this equation
From this equation it follows
that pumping 5,200 m3 of gas into a 32,200 m3 tank reduces oxygen
concentration to 0.83 ppm. This level of hypoxia is lethal to many
ANS. With the flow rate of 38.2 m3/min this can be achieved in 135
min. The relationship between the size of the tank and the time required
to deoxygenate it is linear. Therefore, these results can be scaled
to any tank size.
C. Underpressure in Ullage Space of Ballast
Deoxygenation is enhanced by the under-pressure, as can be seen from
the following simple argument. Let p be pressure of water at a given
depth in the absence of underpressure. Let pu be the absolute value
of the negative pressure at the top. Let Y be the weight fraction
of oxygen in the water without underpressure and Yu - the same weight
fraction with underpressure. Then by Henry's Law:
From this equation we conclude that solubility
of oxygen is reduced by underpressure. This factor becomes even
more significant as a bubble rises to the surface, and the pressure
For example, if p=14.7 psi (the usual value at
the surface of the tank) and the absolute value of the underpressure
is 2 psi, then the solubility of oxygen is reduced by approximately
The maintenance of underpressure is not mandatory.
The underpressure helps accelerate the de-oxygenation process because,
by reducing the oxygen solubility, it also reduces the amount of
inert gas needed. For example, 2 psig underpressure will speed up
the de-oxygenation by 14%; 0.5 psig underpressure will speed it
up by 3.5%. Slight underpressure is also helpful in eliminating
the contaminated gas from the ullage space.
D. Carbon Dioxide Transfer
Since we assumed that the pressure inside the
bubble depends only on the pressure of the liquid surrounding it,
we can write:
By definition we have nCO2
= xn. Differentiating this equation we obtain:
However, since the reaction of carbon dioxide
with water is the dominant cause of change in the chemical composition,
we can write:
Combining this with the Equation (2) yields
the following equation:
In addition, we can solve for n
= xn+nN to obtain
From the Law of Mass Action kinetics we have:
For the partial pressure of carbon dioxide we have,
according to Dalton's Law pCO2 =
Combining the equations (1), (3), (4), and (5)
This equation can be integrated to obtain:
This equation can be used to calculate parameters
of the systems, including the residence time of a bubble, required
to achieve the desired molar fraction of carbon dioxide in the bubble.
The latter quantity is related to the pH and the concentration of
carbon dioxide in the water, as we shall see in the next subsection.
E. Concentration of Carbon Dioxide in Water
and pH Calculation
Concentration of carbon dioxide in water can be
determined as the ratio of the number of moles transferred from the
bubble to the volume of the tank. The number of moles transferred
from each bubble can be determined from the value of x as follows.
By definition, we have:
Solving for nCO2
which gives the following answer for the concentration
of carbon dioxide in water:
The concentration of the hydrogen ions in the water
can be calculated from c by solving
the following equation for h:
The pH can be then found by taking the -log h.
We can also solve the Equation (8) for c and substitute
the result into the Equation (7). This yields after some tedious,
but straightforward algebra the following relationship between the
desired molar fraction of carbon dioxide in the bubble and the desired
concentration of hydrogen ions in the water:
The equations (6) and (9) constitute a closed-form
mathematical model of carbon dioxide transfer usable for design of
the treatment system.
8.3 The MH Systems' Ballast Water Treatment
(Note: The Authors are cognizant that a large
tanker of the size as 300,000 DWT may not be an ideal candidate for
ballast water treatment features. However, this hypothetical design
study can be easily modified for smaller tankers.)
The MH Systems Ballast Water Treatment System is a combination of
two other effective treatment systems, i.e. deoxygenation and carbonation.
It also is an extension of the MH Systems American Underpressure System
- AUPS (Husain et al. 2001). The inert gas, supplied by the standard
marine gas generator, is 84% nitrogen, 12-14% carbon dioxide and about
2% oxygen. This inert gas has all the ingredients necessary to combine
the two very effective treatments of hypoxia and carbonation at a
very reasonable cost. The laboratory tests at Scripps, described previously,
show that this gas needs very little contact time to be effective.
The analyses described earlier established the flow rates and control
time for hypoxia carbonated conditions.
Each ballast tank has rows of pipe at the tank
floor with downward pointing nozzles. The pressurized inert gas is
jetted downward out of the piping. The jets stir up the sediment for
contact with the inert gas bubbles. The bubbles then rise through
the ballast water to the space above the water surface, which has
previously been underpressurized to -2 psi. For the purposes of this
paper, a 300,000 DWT single hull tanker was used for design studies
of this system to test practicality and affordability. Applicability
to a 300,000 DWT double hull tanker was also examined. Figure 2 shows
inboard profile, deck plan view, piping layout, nozzle detail and
section through ballast tank. Figure 3 shows schematic of the system
and Figure 4 shows isometric of one tank. A 300,000 DWT double hull
tanker has somewhat less installation costs since the tank bottom
is smooth as shown in Figure 4.
For the 300,000 DWT tanker, there are 8 ballast
tanks as follows in Table 2:
Table 2. Ballast Water Tank
From analyses and experience (Tamburri et al. 2002),
it is estimated the hypoxia and pH conditions can be set in at least
8 hours, even in the largest tanks, B3 Port and Starboard. The flow
rate is 1350 cfm for each of these tanks. With one 1500 cfm marine
gas generator, and treating each tank sequentially, it is estimated
that all 8 tanks can be in a hypoxia, low-pH (5.5 - 6) condition in
less than 48 hours. Contact time for essentially total lethality may
not require more than another 24 hours although the remainder of the
2 to 3 week voyage is available.
The space above the liquid in each tank is underpressurized
to about -2 psi and maintained throughout the voyage. As the gas bubbles
rise up to the surface, they are evacuated by a blower to maintain
the underpressure of the inert gas blanket at the surface. The underpressure
further facilitates the solubility of the oxygen (see analysis) and
tends to compensate for the oxygen captured in the bubbles as they
Since the ballast tanks are treated sequentially,
only two 700 cfm compressors are required to compress the gas. The
gas is compressed enough to offset the hydrostatic head plus an additional
25% psi to provide a jet force for stirring the sediment. Two compressors
are provided for redundancy. If there are some concerns with the dumping
of hypoxia and carbonated treated water, it is easily countered with
the system discussed in this paper. The compressors will shift over
from the gas generator to atmospheric and the ballast water will be
oxygenated within just a few hours. In this same period of time the
CO2 is readily washed out since the air contains no CO2 component.
Sensors are needed to monitor the pH to ensure
that it never goes below about 5.5. Sensors will measure dissolved
oxygen content to ensure that adequate deoxygenation is established.
Sensors will also monitor the underpressure. The control system will
remotely start and stop the gas generator, the compressor and the
blower. The control system also remotely controls the valves off of
the inert gas manifold to each ballast tank and the valving for the
It is expected that system will be controlled by
a suitably designed arrangement of programmable logic controllers
(PLCs). These devices are commercially available. They are also easy
to program and maintain.
A control console with displays will integrate
the functions of the inert gas generator and the AUPS ballast water
treatment system as well as provide for monitoring, status displays
and manual override, if required.
Tests were conducted with the AUPS System installed
on a naval reserve fleet tanker. They verified the structural capability
of tanks to withstand the pressure of -3 psi and the controls needed
to maintain the required underpressure. These findings are applicable
to the equipment and controls that will be used for the ballast water
The following are the design features of the shipboard
||Dry docking is not required for the
installation of the system. The system can be retrofitted at
||The system includes mainly off-the-shelf
||The system is fully automated. Data
can be transmitted in real time to a shore-side facility, if
||Sensors are installed at different
locations inside the tank to determine pH and oxygen levels.
||The system requires low maintenance.
8.4 Economic Evaluation of MH Systems' Ballast
Water Treatment System for a 300,000 DWT Tanker
In making an economic evaluation, the analysis
methodology described in Mackey et al. (2000) was used. This method
states, "a logical basis for economic comparisons would be a change
in Required Freight Rate (RFR)." Since there would be no change in
cargo capacity, then:
Mackey et al. (2000) stated that the economic payback
period for conversions is typically 5 years.
selected a 300,000 DWT tanker for analysis. As stated earlier, a
ballast water treatment system applicable for ships must have the
capacity for treating huge quantities of ballast water. If a system
is practical and economical for treating a ship with 8 ballast tanks
of 110,823 cubic meters, then it is practical for all ship types.
The economics would have to be assessed for ships of other, smaller
ballast capacity, as the economics might not scale. But obviously,
the effectiveness as well as the practicality of the system would
Table 3 lists the principal parts and materials
in the ballast water treatment system together with estimated prices
and labour costs.
The total cost is approximately $3,057,100. All
tankers already have some type of inert gas generating capability.
The newer tankers have generators with a gas mixture discharge similar
to the mix used in the experiments at Scripps. Nevertheless, for
conservatism, the generator has been included in the cost. Similarly
tankers probably have sufficient excess electrical capacity to supply
the load of this equipment - the compressors and blower. This is
especially true since this is on the return trip in ballast and
the machinery will only run about 48 hours each trip. Nevertheless,
again for extreme conservation, a 300 KW generator has been included.
To make a usefully indicative estimate of operating
costs, the following assumptions were made:
||The tanker will operate to 360 days
||Six (6) voyages per year between
Persian Gulf and USA.
||Half of the voyages are return trips
in ballast, or 6 trips a year.
||Assume the 2 compressors and blower
must operate 48 hours to obtain hypoxia and carbonation in all
8 tanks (note that actually the cfm of both compressors is only
required for tanks B3 port and starboard and B6 port and starboard.
||Operating costs are primarily the
fuel costs for the inert gas generator and the 300 KW generator.
||n is 5 years (economic payback period)
and i (interest rate) is 8%.
Table 3. Ballast Water
Treatment by De-oxygenation with Elevated CO2 for a Shipboard
An Affordable Solution
If the gas and electric generators operate 48 hours
for each of 6 voyages, then the total operating time is 288 hours
per year for each generator. About 6,000 gallons of diesel fuel would
be consumed by the electric generator and for the gas generator about
16,500 gallons. This is a total of 22,500 gallons. At a cost $1.25
per gallon, the yearly operating cost will be about $28,125. Considering
the few hours per year that the machinery operates and the fact that
the ship has no cargo and therefore less requirements of the crew,
minimal cost has been allocated for maintenance.
In estimating the cost of treatment per ton of
ballast water, the estimated annual operating costs of $28,125 is
used. The approximate 4 million cubic feet of ballast is 128,000 tons.
Six trips are made in ballast, which is a total of 768,000 tons treated.
Therefore, cost of ballast water treatment is 3.7 cents per ton.
This ballast water treatment system is focused
on treating the huge amounts of ballast water discharged into US harbours.
It has the capacity to readily treat these huge quantities using standard
marine components. For tankers that already have the major components
on board, it would be very affordable. And for tankers with the AUPS
spill containment, the added cost would be even less expensive.
Also, it appears (although not tested) that this
system may be adequately effective in treating sediments. Ballast
Water Exchange leaves sediment and other residue untreated. In fact,
only the filtration concept treats sediment, by eliminating it.
9. Conclusions and recommendations
Based on the preliminary study, we conclude that
a combination of hypoxia and elevated CO2 levels are expected to kill
in excess of 95% of marine phytoplankton, zooplankton, macroalgae,
and invertebrates as required by the interim standard proposed by
the US Congress. The treatment system proposed requires only off-the-shelf
components which can be installed at pier side, without dry-docking.
The system can be fully automated. Installing pH and oxygen sensors
at multiple locations inside the tank can assure continuous remote
monitoring of the ballast water.
It will be necessary to continue the laboratory
tests, especially to include experiments on the effects of the system
on phytoplankton, cysts and spores. In addition, the practical application
of the system should be verified in a large scale effort using land
based tanks or ballast water tanks in ships.
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||concentration of carbon dioxide in
the water, including ions produced by electrolytic dissociation.
||concentration of hydrogen ions in
||dissociation constant of carbonic
( = 4.3 x 10-7 mol/liter).
||reaction rate constant.
|| Henry's Law constant
(= 39.79 x 10-6).
||total number of bubbles generated.
||total number of gas moles in the
||number of moles of carbon dioxide
in the bubble.
||number of moles of nitrogen in the
||total pressure inside the bubble.
||partial pressure of carbon dioxide
in the bubble.
||gas weight flow rate.
||volume of the tank.
||molar fraction of carbon dioxide
in the bubble.
||weight fraction of oxygen in the
||molar fraction of oxygen in the bubble.
||density of the ballast water.
|Superscript 0 refers to quantities
in the gas bubble when it is first introduced into the tank.
Subscript 0 refers to quantities in the water at the time t=0.