Ballast water is placed in the hull of a ship to help stabilise it and is usually loaded in the port of departure and then discharged in the destination port while cargo is offloaded. This system means that seawater containing organisms from one region can be moved to a new location. Each year about 10 billion tons of ballast water are transported and exchanged around the world during maritime shipping.
Many non-native animals and plants have invaded new environments and ballast water is one means through which they spread. These invasive aliens threaten the survival of local species, sometimes fundamentally altering the ecosystem. The zebra mussel, which has invaded North America, is one example of an IAS that kills native species and causes infrastructure damage costing billions of dollars each year. The comb jelly in the Black Sea is another with significant ecological and economic impact. Little is currently known about smaller invading microbes such as bacteria and viruses. Many of these are harmless but there is potential for a risk to public health from disease carrying microbes being shifted from one location to another in ballast tanks.
The new system uses microwaves, similar to those found in a kitchen microwave oven, to heat and kill zooplankton, algae and oyster larvae in salt water. The continuous microwave system features high heating rates, low operating costs and effectiveness in hazy water. This offers advantages over other methods, which include using UV irradiation, conventional heating or chemicals.
The environmental advantages of this technology cannot be overlooked, the researchers suggest, and costs should be competitive if the system were manufactured in sufficient amounts. The technology might also be combined with other treatment methods.