The main reason why ships pump water out from the sides is because of ballasting operations. Ballasting is a very important aspect of a ship’s ability to maintain stability and safe handling at sea. When a ship is being built, ballast tanks are strategically placed within the vessel’s hull. These tanks are filled with seawater or emptied depending on sea conditions and ship operations, to adjust the weight distribution, balance, and trim of the vessel. When filled, the ship sits lower in the water, when empty, it’s more buoyant, similar to how a submarine uses ballasting to dive or rise to the surface.

A ship’s center of gravity is important for seaworthiness, and there are several factors that affect it, namely loading and unloading cargo ships, cruise passenger embarkation, debarkation, and taking on fuel. Taking in water or pumping ballast water out from side ports counteracts gravity shifting changes, helping to reduce stress on the hull and improve propulsion and maneuverability. But while this process is necessary for stability and navigation, it can have serious environmental consequences.

According to the International Maritime Organization, ballast tanks often have microscopic life living in them, such as bacteria, plankton, small invertebrates, and larvae taken in at the port of origin, so when a ship discharges water from its ballast tanks, water is not the only thing being released. If these organisms survive the journey to the next stop and are discharged in a different region, they can become invasive species, disrupting local ecosystems and out-competing native marine life. Invasive species can wreak havoc, spreading very quickly, disrupting food chains, damaging habitats, and creating long-term ecological imbalances.

Aside from environmental damage, the economic impact can be huge. A review of global studies by SciSpace found that aquatic invasive species can cause billions of dollars in annual losses. Ballast water discharge can also contain pollutants like oil residue, chemicals, and heavy metals in the tanks, further degrading water quality and endangering marine organisms. Due to the risks involved, ballast water management has become a pivotal factor for environmental protection in global shipping.

In 2004, the International Maritime Organization (IMO) introduced the Ballast Water Management (BWM) Convention, which officially took effect in 2017, requiring ships to implement approved ballast water management plans, maintain detailed record books, and comply with strict standards limiting the number of viable organisms in discharged water. The usual way vessels can maintain compliance is through an onboard Ballast Water Management System (BWMS) that treats water before release or through a mid-ocean ballast water exchange. The system typically combines mechanical filtration with physical or chemical treatment methods such as ultraviolet light, electro-chlorination, ozone, or chemical injection to neutralize or remove harmful organisms.

Meeting these requirements can be difficult for some older ships that need to be retrofitted with expensive BWMS systems, especially if they have limited space. Conditions like temperature, salinity, and water clarity can have an effect on the system, too. To avoid non-compliance by port state control authorities, ship crew members must be trained to operate and maintain these systems to safeguard marine ecosystems while allowing global shipping to operate safely and efficiently throughout the world.