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Electrolytic Sodium Hypochlorite System for Treatment of Ballast Water
ArticleinJournal of Ship Production · August 2006
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Electrolytic Sodium Hypochlorite System for Treatment of
Ballast Water
Rudolf C. Matousek,*David W. Hill,* Russell P. Herwig,
†
Jeffery R. Cordell,
†
Bryan C. Nielsen,
†
Nissa C. Ferm,
†
David J. Lawrence,
†
and Jake C. Perrins
†
*Severn Trent DeNora, Sugar Land, Texas, USA
†
University of Washington, Seattle, Washington, USA
The potential problems of organisms introduced by ballast water are well docu-
mented. In other settings, electrolytic generation of sodium hypochlorite from sea-
water has proven to be a simple and safe method of handling and injecting a biocide
into water. After the hypochlorite oxidizes organisms, it reverts back to the chloride
ion. Mesocosm-scale testing of this technology combined with filtration, using organ-
isms from Puget Sound, Washington, demonstrated that hypochlorite generation and
use may be a viable method to eliminate aquatic nuisance species from ballast water
while minimizing disinfection byproducts and residual toxicity. These experiments
were conducted at the U.S. Geological Survey Marine Field Station on Marrowstone
Island, Washington. Results from the first set of studies of the system showed that
hypochlorite levels greater than 3.0 ppm hypochlorite with or without filtration reduced
bacteria by more than 99.999%, reduced phytoplankton by more than 99%, and
reduced mesozooplankton by more than 99%. Filtration improved efficacy only when
hypochlorite concentration was initially less than 1.5 ppm.
1. Introduction
1.1 Regulations
THE MARINE ENVIRONMENT PROTECTION COMMITTEE (MEPC) of
the International Maritime Organization met in February 2004.
The committee adopted a new world Ballast Water Convention
that will enter into force 12 months after ratification by 30 member
states representing 35% of the world’s gross tonnage. The Con-
vention is divided into articles and, more importantly for the de-
velopment of ballast water treatment technologies, an annex that
includes technical standards and requirements in the regulations
for the control and management of ship ballast. All ships, includ-
ing submersibles, floating craft, floating storage units (FSUs), and
floating production, storage, and offloading units (FPSOs), are to
manage their ballast water in accordance with an approved ballast
water management plan and record such management in a ballast
water record book. All ships greater than or equal to 400 gt are to
be surveyed (initial, annual intermediate, and renewal) and
certificated (not exceeding 5 years). Table 1 summarizes the
implementation schedule of the type of treatment required accord-
ing to the age of ship and its ballast capacity as per the provi-
sions of the Convention (International Maritime Organization
2004).
Indicator microbe concentrations shall not exceed:
• Toxicogenic Vibrio cholerae: 1 colony forming unit (cfu)
per 100 ml
•Escherichia coli: 250 cfu per 100 ml
• Intestinal Enterococci: 100 cfu per 100 ml.
Ballast water exchange is to take place as follows:
• At least 200 nautical miles from the nearest land and 200
m water depth OR
• In the event throughout the intended route the sea area
does not afford the above characteristics, in a sea area desig-
nated by the port state. There may be a need to alter the ship’s
intended route to exchange ballast in the designated area.
States may establish additional ballast water management mea-
sures for ships to meet based on guidelines, which remain to be
Presented at the 2005 Ship Production Symposium, Society of Naval Ar-
chitects and Marine Engineers, October 19 to 21, Houston, Texas.
Journal of Ship Production, Vol. 22, No. 3, August 2006, pp. 160–171
160 AUGUST 2006 8756/1417/06/2203-0160$00.49/0 JOURNAL OF SHIP PRODUCTION

developed. The MEPC shall undertake a review of the Ballast
Water Standards no later than 2006 and is to include an assessment
of the technologies available that achieve the standard. As part of
the assessment, the MEPC requires significant documentation of
the system, with efficacy data for both land-based and shipboard
tests. Criteria for such testing have been established and outlined
in MEPC document MEPC 52/WP.7 titled “Guidelines for Ap-
proval of Ballast Water Management Systems.”
As a result of these regulations, there is a need for a proven,
viable, cost-effective ballast water treatment system by 2007. This
will allow ship designers to specify and incorporate such devices
into ships that begin construction after January 1, 2009. This paper
is the summary of land-based work on a ballast water treatment
(BWT) system utilizing on-site hypochlorite generation.
1.2. Ballast water management strategies
1.2.1. Open-ocean ballast water exchange. Most environmental
scientists agree that ballast water that is exchanged in the open
seas presents less of an ecological threat or risk to the receiving
waters. The question is whether the threat has been reduced to an
acceptable level. As presently engineered and practiced, reballast-
ing does not exchange 100% of the ballast water or remove all of
the sediments that are found in ballast water tanks. The amount of
the exchange varies from ship to ship. Older ships tend to ex-
change less efficiently than newer ships. Dickman and Zhang
(1999) examined four containerships that took on ballast in
Mexico and discharged 21 days later in Hong Kong. After this
period, few of the dinoflagellate and diatom species taken on in
Mexico were alive in the ballast water in Hong Kong. Five ships
that reballasted in the open ocean reduced the diatom and dino-
flagellate populations by 48%. They concluded from results of this
and previous study that older vessels were less effective in remov-
ing diatom and dinoflagellate species than newer ships. The reason
could be that the reballasting design of older ships was less effi-
cient in removing water and sediments located near the bottom of
the ballast tanks and that the bottom water is associated with a
large number of resting cysts and cells. Ships sampled in Puget
Sound by the University of Washington’s ballast water team
showed that even in ships that reported open ocean exchange, up
to 50% of the organisms were nonnative to the northeastern Pa-
cific.
1.2.2. Ballast water treatment technologies. A variety of treat-
ment technologies are suggested for the removal or reduction of
organisms that are found in ballast water. The challenge is to
develop a technology that is effective against a variety of biologi-
cal taxa and that is capable of quickly treating the very large
volumes of ballast water that are associated with large ships.
Treatment technologies may be developed for on-board or at-port
applications. Technologies that have received most of the recent
consideration include (1) cyclonic separation, (2) heat treatment,
(3) electric pulse, (4) ultraviolet light, (5) filtration, and (6) bio-
cides. These technologies were listed and reviewed in the National
Research Council publication “Stemming the Tide: Controlling
Introductions of Nonindigenous Species by Ships’Ballast Water”
(National Research Council 1996). We briefly review the pro-
posed technologies and provide some comment.
Cyclonic separation. A more sophisticated and more recently
developed technology involves cyclonic separation. This is nor-
mally accomplished using hydrocyclones. If properly designed
and applied, hydrocyclones will require less pump pressure than
screen filters and will allow separation of sediments and other
suspended solids to approximately 20 µm. Hydrocyclones are lim-
ited, however, to separating solids with a specific gravity greater
than, or less than, water. Many types of organisms (e.g., bacteria
and other microorganisms) will not be separated from the water
since their specific gravity is extremely close to seawater.
Ultraviolet light. In close contact, ultraviolet (UV) light treat-
ment is effective in killing non–spore-forming microorganisms. It
does not appear effective in inactivating higher organisms or the
cyst or resting stages of protozoa. Ultraviolet light was not effec-
tive in totally eliminating dinoflagellates. Montani et al. (1995)
found that after exposure to ultraviolet light for 2 hours, the ger-
mination of Chattonella sp. cysts decreased to 6% of the control,
whereas germination of cysts of other species of dinoflagellates
(Alexandrium sp. and Gymnodinium sp.) was more than 40% of
the controls. The University of Washington research team has
performed controlled mesocosm tests at the Marrowstone Marine
Field Station in which Puget Sound seawater was amended with
additional zooplankton. The amended seawater was exposed to
moderate levels of UV light. They found that zooplankton mor-
tality was delayed and did not immediately occur. The microbial
populations were quickly reduced in number following exposure
to the UV light but rebounded to their original levels within a few
days.
Heat treatment. The use of waste heat from the ship’s propul-
sion and service cooling is an attractive option for the inactivation
of organisms in ballast water. No chemical byproducts or residuals
Table 1 Ballast water treatment implementation schedule
Ballast Capacity (m
3
) Construction Date 2009 2010 2011 2012 2013 2014 2015 2016 2017
<1,500 <2009 D1 or D2 D2
>2009 D2
>1,500 <2009 D1 or D2 D2
<5,000 >2009 D2
>5,000 <2012 D1 or D2
>2012 D2
D1 ⳱ballast water exchange (95% volumetric exchange) or pumping through three times the volume of each tank.
D2 ⳱ballast water treatment systems approved by the Administration with treatment efficacy of:
+ Not more than 10 viable organisms per m
3
>50 ␮m.
+ Not more than 10 viable organisms per milliliter >10 and <50 ␮m.
AUGUST 2006 JOURNAL OF SHIP PRODUCTION 161

would be associated with the discharge of heat-treated ballast
water. A number of factors will limit the practicality of thermal
treatment primarily related to the volume of water that is associ-
ated with large vessels and the amount of energy required to heat
the volume. Thermal treatment may be more applicable to ballast
water originating from warmer environments. Heat required for
thermal treatment could be reduced where water temperatures are
at tropical or summer levels (30 deg C or higher). The heat loss to
the ambient waters outside of the hull must be considered. Dif-
ferent types of organisms or organisms from different parts of the
world may have different sensitivities to heat. A few recent studies
suggest that heating ballast water is the best method for killing a
variety of higher organisms and microorganisms that are found in
ballast water. Rigby et al. (1999) showed how a cost-effective
heating technique using waste heat from a ship engine could be
used to kill many unwanted organisms. In an ocean trial, heated
water flushed through one of the ballast tanks resulted in the
destruction of all zooplankton with very limited survival of the
original phytoplankton.
Electric pulse. Small-scale experiments have been performed
by applying electrical voltages in the 15 to 45 kV range with pulse
duration of 1 µs. Large energy sources would be required for
systems capable of treating large volumes of ballast water (Na-
tional Research Council 1996).
Filtration. The physical separation and removal of organisms
can be accomplished during ballasting operations using a ship-
board filtration system. Some would argue that this technology is
the most promising choice. Filtering ballast water as it is loaded is
an attractive option since it would minimize the introduction of
unwanted organisms. The options for onboard filtration systems
are either mesh strainers or deep media filtration. Many problems
associated with a strainer technology have been solved with the
development of commercially available continuously cleaned
screening systems. Media filters are attractive in principal because
small-size particles can be removed, but these filters are most
likely unrealistic for on-board treatment of ballast water because
of the large footprint that would be required. The primary disad-
vantage of the strainer filters is that many organisms are smaller
than strainers and would pass through the treatment system. A
study for the Canadian Coast Guard (1992) concluded that the
physical removal of organisms by filtering may be an effective
stand-alone treatment process or may be used in conjunction with
other technologies, such as chemical treatment or UV sterilization.
Flow-through centrifugation systems can separate particles prior
to filtering to reduce filter clogging.
Biocides. The addition of chemicals that would kill or inactivate
a variety of organisms found in ballast water is an attractive treat-
ment technology because of the ease of application. A biocide
could simply be added to the ballast tank and allowed to react for
a specified period. Biocides are among the most widely used in-
dustrial chemicals, and there is a large body of knowledge about
their use in wastewater treatment. If similar concentrations were
required to inactivate organisms found in ballast water, then a
large ship would need to carry only a few cubic meters of biocide
per voyage. The use of biocides for ballast water treatment has
been rejected by some for several reasons, including the reluctance
to add toxic chemicals to water that may be discharged back into
the ocean, the unknown effectiveness of biocides against target
organisms, and compliance with discharge regulations around the
world. Oxidizing biocides such as chlorine, chlorine salts, and
ozone have been used for decades in a variety of sanitizing appli-
cations. Ozone is a strong oxidizing agent used for treatment of
potable and industrial waters. With the increasing environmental
concern associated with the use of chlorine, ozone has received
greater attention in recent years. Ozone is an unstable gas that
must be generated as needed, and some reviewers have concluded
that ozone may not be practical for shipboard use (National Re-
search Council 1996). Along with generation, ozone efficacy is a
problem due to the gas–liquid contact requiring elaborate diffu-
sion equipment. In salt water, ozone produces many of the similar
residual compounds as chlorination. Nonoxidizing biocides such
as glutaraldehyde or vitamin K have also have been suggested.
2. Electrochlorination
2.1. Background
Seawater (normally between 15 and 35 grams/liter) or other
water containing NaCl may be used to generate a disinfecting
solution containing chlorine by passing a direct electrical current
through the solution. On-site generation of hypochlorite from sea-
water has been used for over 25 years. These systems can be
purchased as completely skid-mounted systems that generate so-
dium hypochlorite from seawater. These systems are used in re-
fining, petrochemical power plants, offshore drilling production,
and marine applications around the world. Systems can be scaled
to the appropriate size depending on the quantity of hypochlorite
required.
The type of electrolytic cell commonly used in these marine and
offshore applications is a “tube within a tube.”A cell consists of
one anode, one cathode, and one bipolar tube with the necessary
ancillary hardware to facilitate assembly. The outer anode and
cathode are manufactured from seamless titanium pipe. The anode
surface is coated with proprietary precious metal oxides, primarily
ruthenium and iridium. Seawater enters one end of the cell and
passes between the cathode, the anode, and bipolar tube annular
spaces. When direct current is applied to the cell, sodium hypo-
chlorite results. One cell can produce up to 5.5 kg/day, and a
maximum of 12 cells can be connected in series for a capacity of
65 kg/day per train.
In some applications, such as ballast water treatment, a dechlo-
rination step can be added to the process. This requires adding a
reducing agent, such as sodium sulfite, to the end of the system to
neutralize any residual chlorine at the point of discharge. The end
result is a nontoxic stream with no free chlorine.
2.2. Chemistry
The process is based on the partial electrolysis of NaCl present
in seawater as it flows through an unseparated electrolytic cell.
The resulting solution exiting the cell is a mixture of seawater,
sodium hypochlorite (hypo), hydrogen gas, and hypochlorous
acid. Electrolysis of sodium chloride solution (seawater in this
study) is the passage of direct current between an anode (positive
pole) and a cathode (negative pole) to separate salt and water into
their basic elements. Chlorine generated at the anode immediately
goes through chemical reactions to form sodium hypochlorite and
hypochlorous acid. Reactions are shown below:
Cl
−
→Cl
2
(aq) + 2e
−
Eo ⳱1.396 V (1)
162 AUGUST 2006 JOURNAL OF SHIP PRODUCTION

which is hydrolyzed in solution to form hypochlorous acid:
Cl
2
+2H
2
O→2HOCl + 2H
+
(2)
Hypochlorous acid dissociates to hypochlorite at alkaline pH levels:
HOCl →OCl
−
+H
+
pKa =7.5 (3)
In seawater bromide ions are present, together with a range of
inorganic cations as well as possibly ammonia and a variety of
organic compounds. The reaction of molecular chlorine or hypo-
chlorite ions with ammonia or amino compounds leads to the
disinfectants, and they react to destroy bacteria and microorgan-
isms in the water just as do chlorine, hypochlorous acid, and
hypochlorite ions. The rapid oxidation of bromide ions will also
occur and (as with chlorine) in the aqueous environment form
hypobromous acid (HOBr) and hypobromite ions (OBr
−
). These
reactions will also be equilibrium processes, dependent on tem-
perature and pH. Note also that brominated species will react with
ammonia and/or amino compounds if present in the water, just like
the chlorine analogues.
Hydrogen and hydroxides are formed at the cathode, the hy-
drogen forms a gas and is vented, and the hydroxide aids in the
formation of sodium hypobromite and increases the exit stream pH
to approximately 8.5. This reaction is shown as follows:
2H
2
O+2e
−
→H
2
(g) +2OH
−
Eo =−0.828 V (4)
Because the electrolytic cell used for this application is unsepa-
rated, the reactants at both anode and cathode can further react to
form the respective end products shown in the overall electro-
chemical and chemical reaction as follows:
NaCl +H
+
+Br
−
+2e →NaOBr +H
2
+Cl
−
Salt +Water +Energy →Sodium Hypochlorite +Hydrogen
(5)
2.3. Disinfection by-products
Disinfecting agents, such as chlorine, ozone, chlorine dioxide,
and chloramines, react with natural organic material present in
water to produce disinfection by-products (DBPs). Most of the
research and interest in DBPs has been with drinking water. DBPs
have been known since 1974, when chloroform was identified as
DBP resulting from the chlorination of tap water. Since then,
hundreds of DBPs have been identified in drinking water. The
benefit of disinfecting drinking water is obvious, as thousands of
people died from waterborne disease before municipalities began
to disinfect drinking water, but it is also generally recognized that
it is important to minimize the formation of DBPs in drinking
water. Several DBPs have been linked to cancer in laboratory
animals, and as a result, the US Environmental Protection Agency
(EPA) has regulated some DBPs.
While we anticipate that most people will not be drinking bal-
last water and other treated seawater, we have evaluated the for-
mation of selected DBPs that may be formed following the gen-
eration of hypochlorite in seawater. Seawater is significantly
different from freshwater, not just because of the relatively high
concentration of Na
+
and Cl
-
and oftentimes higher levels of natu-
ral organic material, but also because of the presence of Br
−
(bro-
mide). The presence of this ion may lead to the formation of
bromate (BrO
3
−
), a compound that is considered a possible human
carcinogen. In the United States, bromate is regulated at 10 µg/L
(10 parts per billion) in drinking water. Seawater contains a typical
bromide concentration of 65 mg/L, so the concentration in sea-
water is significant. Therefore, bromate was one of the DBPs
measured in our research. In addition, the presence of haloacetic
acids (HAAs) and trihalomethanes (THMs) are a concern and
were measured as part of the overall study.
One other component of the ballast water treatment system is
the neutralization of the free halogen (hypochlorite and hypobro-
mite) prior to discharge from the ballast tanks. Sodium sulfite is
used, and the simplified reaction is shown below to form sodium
sulfate. As shown in the equation, neutralization occurs at one to
one molar ratio but two to one (sulfite to halogen) as a weight
ratio.
Na
2
SO
3
+Cl
2
+H
2
O–>Na
2
SO
4
+2HCl
Sodium Sulfite +Chlorine +Water –> Sodium Sulfate
+Hydrochloric Acid (6)
Typically, there are 4 g/L sulfate in seawater, and based on the
concentrations of halogen required, only 10 mg of sulfate will be
added to the discharge ballast water. Also, the amount of HCl
generated is negligible and will not change the pH of the discharge
ballast water.
3. Pilot electrochlorination ballast water treatment
3.1. Background
The University of Washington School of Aquatic and Fishery
Sciences performed third-party verification tests on the Severn
Trent DeNora’s Electrochlorination Ballast Water Treatment
(BWT) System (BalPure; Severn Trent DeNora, Sugar Land, TX).
The pilot plant system consisted of two 5.7 m
3
(1,500 gallon) raw
seawater holding tanks. The water was pumped through a 50-␮m
self-cleaning filter. When the pressure drop reached a preset value,
the filter was automatically back flushed while continuing to op-
erate. A stream volume of roughly 10% of the inlet flow to the
filter was generated, containing the removed organisms and solid
contaminants. During a ballast water operation, this stream would
be discharged overboard. For purposes of this test, the stream was
collected.
After the water was filtered, a side stream was fed to the elec-
trolytic cell, where oxidizers were generated. This oxidized stream
was then injected and mixed with the main stream. After the
hypochlorite stream was mixed with the mainstream, the water
was sampled and the free halogens were measured and recorded.
This on-line real-time free halogen value was used to automati-
cally adjust the amount of hypochlorite generated for the ballast
water treatment. Based on previous lab tests, the expected required
dosage after filtration ranged from 1 ppm up to 5 ppm.
Once the water was treated, it was placed in replicate dark
mesocosms. This step was analogous to ballast water in ballast
tank conditions. After 7 to 14 days of storage, the seawater was
monitored before it was “discharged.”A dehalogenation agent,
such as sodium sulfite, was injected into the water to react or
neutralize any free halogens. A schematic of the pilot process is
shown in Fig. 1.
AUGUST 2006 JOURNAL OF SHIP PRODUCTION 163