What Is Ballast Water? Why Is It a Problem?

Ballast water is freshwater or ocean water stored in a ship's hull to provide stability and improve maneuverability during a voyage. When the ship reaches its destination, the ballast is emptied into the water at the new port, sometimes filled with a flurry of uninvited guests in the form of bacteria, microbes, small invertebrates, eggs, or larvae of various species that have hitched a ride from the original destination and may become invasive species.

When a ship receives or delivers cargo to a number of different ports, it will take or release ballast water at each one, creating a mixture of organisms from several different ecosystems. Some ships aren’t designed to carry ballast water, while others are able to carry permanent ballast water in sealed tanks to bypass the process altogether. In general, however, almost all sea-going vessels will take on some kind of ballast water.

Ballast Water Definition

Ballast is water brought on board to manage the ship's weight. It’s a practice that’s as old as steel-hulled ships themselves, and it helps reduce stress on the vessel, compensate for weight shifts as cargo loads change, and improve performance while navigating rough seas. Ballast water may also be used to increase the load so that a ship can sink low enough to pass under bridges and other structures.

A ship may carry anywhere from 30% to 50% of its total cargo in ballast, ranging from a hundred gallons to more than 2.5 million gallons depending on the ship’s size. According to the World Health Organization's Guide to Ship Sanitation, about 10 billion metric tonnes (approximately 11 billion U.S. tons) of ballast water are transported by ship around the world every year.

Why is this a problem? If an organism transferred through ballast water survives long enough to establish a reproductive population in its new environment, it can become an invasive species. This can cause irreparable damage to biodiversity as the new species outcompetes native ones or multiplies into uncontrollable numbers. Invasive species don’t just affect the animals who live there, but they can also devastate the economies and health of the local communities who rely on that balance for food and water.

Environmental Impact

Many of these foreign aquatic species have been responsible for some of the most profound damage to water bodies in recorded history. Invasions of zebra mussels in freshwater lakes, for example, can cause native fish species to grow slower in their first year of life. The round goby, another notorious invasive species, changes the food chain in its new habitat so quickly that it can increase the bioaccumulation of toxic substances in larger predatory fishes, putting the humans who eat them at risk.

And, according to the International Maritime Organization (IMO), the rate of bio-invasions is increasing at an "alarming" rate:

"The problem of invasive species in ships’ ballast water is largely due to the expanded trade and traffic volume over the last few decades and, since the volumes of seaborne trade continue to increase, the problem may not have reached its peak yet. The effects in many areas of the world have been devastating.”

It's not just sea environments under threat from ballast water—ships that travel through the open ocean to lakes are just as dangerous. According to the United States Environmental Protection Agency (EPA), at least 30% of the 25 invasive species introduced to the Great Lakes since the 1800s entered the ecosystems through ship ballast water.

The IMO set out guidelines for ballast water in 1991 under the Marine Environment Protection Committee, and after years of international negotiations, adopted the International Convention for the Control and Management of Ships' Ballast Water and Sediments (also known as the BWM Convention) in 2004. That same year, the U.S. Coast Guard established rules for controlling the discharge of organisms from ship ballast water in the United States.

The Coast Guard rules prohibiting ships from discharging untreated ballast water in U.S. waters went into effect in 2012, while the 2004 BWM Convention program for the development of ballast water guidelines and procedures came into effect in 2017. In 2019, the EPA proposed a new rule under the Vessel Incidental Discharge Act, though it has been criticized by conservation groups since it contains an exemption for large ships that operate in the Great Lakes.

Ballast Water Management Systems

Following the 2004 BWM Convention, different ballast water management strategies have been implemented around the world, using both physical (mechanical) and chemical methods. In many situations, different combinations of treatment systems are necessary to address varied species of organisms living inside a single ballast tank.

Some chemicals, while they have the power to inactivate 100% of organisms in ballast water, create high concentrations of toxic byproducts that can be harmful to the very native organisms they are trying to protect. Reducing these biocides can add another step to the treatment process, making the use of chemicals alone a costly and inefficient method. Even chemical treatments known to act faster than mechanical ones will likely cause more harm to the environment from toxic byproducts in the long run.

Environmentally speaking, using a primary mechanical treatment, such as removing particles with disk and screen filters during loading or using UV radiation to kill or sterilize the organisms outright, is considered the best option—at least for now.

Mechanical treatment methods can include filtration, magnetic separation, gravity separation, ultrasound technology, and heat, all of which have been found to inactivate organisms (especially zooplankton and bacteria). Studies have shown that filtration followed by the chemical compound hydroxyl radical is the most energy-efficient and cost-effective treatment method, plus it can inactivate 100% of organisms in ballast water and produces a low amount of toxic byproducts.

Ballast Water Exchange Methods

Beginning in 1993, international ships were required to exchange their freshwater ballast water with saltwater while still at sea, which was effective at killing any organisms that may have entered into the hull at its original port. By 2004, even smaller cargo ships containing no ballast water were required to take on a limited amount of seawater and eject it before entering port to prevent the unintentional transportation of invasive species.

To perform a ballast water exchange, the ship must be at least 200 nautical miles from the nearest landmass and operating in water at least 200 meters deep (656 feet). In some cases with boats that make shorter journeys or work in enclosed waters, the ship must exchange ballast water at least 50 nautical miles from the nearest land, but still in water that is 200 meters deep.

Ballast water exchange methods are most effective if the initial water originated from a freshwater or brackish source, since the abrupt salinity change is lethal to most freshwater species. Given the fact that efficient exchange is dependent on specific environments, such as changes in salinity or temperature, ships traveling from freshwater to freshwater, or from ocean to ocean, won’t benefit as much from ballast water exchange. There are, however, studies that show a combination or exchange plus treatment as more effective than treatment alone when destination ports are freshwater. Exchange followed by treatment also serves as an important backup strategy should onboard treatment systems fail.