It is only in recent times that treating ballast to prevent species transfer has become regulated. Because of this there were no obvious methods or systems that had been tested on ships to treat the quantities and highly variable qualities of ballast water. Shore-based treatment of water to destroy unwanted organisms has been practiced for decades and so it was to this source of experience that the industry and system makers have been obliged to turn.
There are of course numerous difficulties in adapting shore systems for use on ships. Aside from initial filtration, which was already a feature on some ships to limit the build-up of sediment in ballast tanks, all of the other elements must be slotted into much smaller spaces than are available at water treatment plants ashore. In addition, the power requirements must be reduced and the dynamic movement of ships at sea factored in to the design of systems.
None of the regulations drawn up around ballast have mandated any one treatment type so system developers have followed a variety of routes and chosen a range of different technologies in designing systems. Many have drawn on shore-based water treatment technology but there are also some novel solutions on offer and some more outlandish proposals from the infancy of system design have been abandoned over time. A very small number of the systems rely on a single means of achieving the required standards. Most though make use of two or more methods and although this would seem to indicate a larger and more complex system, that is not always the case.
Where filters are used as part of the treatment process, most makers have opted for simple filter types. A small number of systems employ hydrocyclone technology as the method of removing larger solids. In these systems, the water is pumped to a specially shaped chamber where a vortex is induced by the flow. Sediment and some organisms will be channelled away from the water which continues on its way to the next treatment stage. In both instances there will be a large amount of solids to be returned to the water. In a filtration system this will be done by back-flushing, which is also essential to prevent filter clogging and maintain the flow in the system.
Where no filter or hydrocyclone is included in the system design, owners may opt for installing one upstream of the system to reduce sediment and enhance the treatment process. The decision may be more difficult in a retrofit, where space may be limited.
After filtration any of several methods can be employed. Mechanical processes are tested only to the G8 rules as are most UV systems but oxidation, electrochlorination and most other methods must also undergo approval for active substances under the IMO Convention’s G9 process.