| 1.
INTRODUCTION |
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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
utilizes 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 pH6.
This ballast water treatment system has undergone preliminary
laboratory tests at the Scripps Institution of Oceanography.
As expected, combination of hypoxia, pH decrease, and
raised CO2 levels severely affected the specimens in the
laboratory.
The paper is organized as follows.
First, we give background information including practical
requirements, current investigative efforts, and general
theoretical considerations. Then we describe survivability
experiments conducted at the Scripps Institution of Oceanography.
This is followed by discussion of analysis and design
considerations. In the final sections of the paper we
describe the ballast water treatment system and provide
a preliminary cost analysis. |
| 2.
BACKGROUND |
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| 2.1
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 Organization (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 favorable and
defensible, the Coast Guard will approve the treatment.
….." |
| 2.2
Current Investigative Efforts Of Alternative Technologies |
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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". |
| 2.3
Corrosion Considerations Of Various Treatment Systems |
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| 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. |
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