System science – The mechanical G8 methods
Updated 11 Oct 2019
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 into 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.
Systems employing cavitation do not generally rely on it as the sole treatment method but as a means of making subsequent treatments more effective. Cavitation can be induced by injection of gases or liquids or by altering the shape of the ballast piping over an area of the flow. The forces caused by cavitation act on organisms, damaging or killing them depending upon their robustness. Ultrasound may be used as another means of inflicting shock damage to organisms and can be independently generated or induced by the piping profile.
Systems that make use of the waste heat of the ship’s engines and a heat exchanger to raise the temperature of the ballast water to levels sufficient to kill organisms have been proposed. In fact, this method was used in one of the earliest ballast treatment systems installed in a vessel.
While heat has tended to have been overlooked in favour of other technologies and considered by some impractical in operation – not least because the main engine may not be running if ballasting/de-ballasting takes place alongside the quay – some of the newest systems to come to the market do make use of heat.
In one, the ballast is continuously treated and used as the source of cooling water for the engine. The heat extracted from the engine treats the ballast after which the water passes through a second heat exchanger to produce hot water or steam to be used either directly in other ship systems or as part of a waste heat recovery system.
An early objection to heat was that high temperatures in ballast tanks may have a detrimental effect on some cargoes. However, heating ballast to lower temperatures than would cause cargo problems may improve the effectiveness of some chemical treatments and the heat can then be removed with a second heat exchanger.
These systems function by removing oxygen from the ballast water by venturi stripping or adding inert gases in sufficient quantities to bring the oxygen content below that needed to support life.
Deoxygenation can be combined with another means of disinfection or used on a stand-alone basis. On tankers where generation of inert gases is already carried out, the same equipment may be able to be used for treating the ballast flow. Deoxygenation is claimed to have a secondary benefit in that it will limit corrosion in the ballast system.
Ultra violet (UV)
This method is one that is very commonly used in systems but has not been without obstacles to overcome. They related to both the method of UV treatment and the testing process for type-approval.
Many systems employ UV radiation that can produce a short-lived chemical change in water composition and some administrations have determined this should fall under the G9 process, but others have not.
Some UV systems have been considered to have problems in meeting US rules because of the ‘unviable’ vs ‘killed’ wordings of the IMO and US standards. In contrast to IMO legislation, the USCG Ballast Water Discharge Standard defines treatment as effective when no organisms survive the treatment process. This has been considered a problem for UV-based systems, which kill many organisms outright but render others non-viable by making them unable to reproduce.
Among several technical challenges to proving viability, according to the USCG the most important is that it may not be possible to culture all of the types of organisms found in ballast water. Simply put, it is not yet known how to consistently induce them to reproduce in the laboratory. In addition, many organisms cannot be induced to reproduce under laboratory conditions but may be able to reproduce in natural environments. Finally, it is not clear that organisms rendered nonviable will remain so over time. It has been shown that some organisms have repair mechanisms that can undo damage caused by ultra-violet radiation and thus restore the ability to reproduce.
There have been issues for some system makers because the US authorities have in the past insisted that the testing method used during type-approval is restricted to the CMFDA/FDA method (sometimes referred to as the vital stain method) as opposed to the most probable number, or MPN, method, which has been used by most other regulatory bodies. The decision by the USCG does not make any approval granted to the systems under the IMO convention invalid but it may impact on the potential operation in US waters for vessels fitted with them.
Some system makers decided that, rather than challenge the USCG ruling, it would be better to retest their systems using the approved method.
In December 2018, a new law was passed in the US which will allow both methods of testing but as of the end of January 2019, no official notices about this have been issued.
UV is regularly used in shore-based water treatment and is considered effective.
At certain wavelengths, especially 254nm, it works by destroying cell walls and inducing changes in the DNA of micro-organisms thus destroying them or rendering them unviable. At different wavelengths, UV can cause production of ozone to take place. Ozone is a useful biocide in its own right.
A UV system employs several UV lamps in the water flow with the exact number being determined by the planned flow rate of the system. Pre-filtration is considered essential for most UV systems because otherwise the sediment in the flow would severely impede the efficiency of the irradiation process. Systems employing UV will usually have a feature aimed at keeping the lamp glasses clean and free from any scale or sediment build up for precisely the same reason.
The UV irradiation process requires organisms to be exposed sufficiently long enough for the breakdown of DNA to take place. If the flow is too fast the system may not function correctly. However, if the flow rate is restricted, lamps may overheat and fail. The layout and placement of lamps in systems employing UV treatment varies enormously but an owner should be able to expect that the problems mentioned here would have been considered at the design stage and found acceptable during the type-approval process.
Maintenance is generally restricted to replacing failed lamps and occasional cleaning. In shore systems where the flow may be continual day after day, lamps are generally considered to require annual replacement, even if they appear visually to be functioning properly, because their ability to produce UV of the requisite wavelength fades over time. In a ballast system that operates only for a few hours at a time and at irregular intervals, replacing the lamps will likely be a less regular operation.