Ballast water is water that is held in the ballast tanks and cargo holds of ships to provide stability and maneuverability during a voyage when ships are not carrying cargo, not carrying heavy enough cargo, or when more stability is required due to rough seas. Ballast water is pumped into ballast tanks when a ship has delivered cargo to a port and is departing with less cargo or no cargo. Ballast water is then transported and released at the next port-of-call where the ship picks up more cargo. If a ship is receiving or delivering cargo to a number of ports, it may release or take on a portion of ballast water at each port. In such cases, the ships ballast water contains a mix of waters from multiple ports.

While ballast water is essential for the safety of the ship, it can be harmful to the marine environment as its discharge can release potentially invasive species into a new marine environment. Ballast water discharge typically contains a variety of biological materials, including plants, animals, viruses, and bacteria. These materials often include non-native, nuisance, and exotic species that can cause ecological and economic damage. Therefore, proper management and treatment of ballast water significantly reduces this risk.