There are several different methods of ballast treatment offered by system makers. Some are more suited to different ship types than others and some more suited to the operating profile of the ship. Both these considerations along with type-approval status and of course cost and availability should be factors taken account of when choosing.
Before looking at the system technology itself it is worth considering the question of filtration. Most of the systems on the market have a filter included as part of the system but some do not. There will be a degree of filtration provided by the strainers at the ballast intake but that is to protect the pumps and prevent piping blockages and not considered as part of the treatment.
The reason for filters being used is to assist in meeting the IMO’s D2 discharge standard which requires that treated ballast must contain:
- fewer than ten viable organisms greater than or equal to 50 micrometres in minimum dimension per cubic metre;
- fewer than ten viable organisms less than 50 micrometres in minimum dimension and greater than or equal to 10 micrometres in minimum dimension per millilitre.
It stands to reason that if such organisms could be removed by filtration at the outset then meeting the discharge standard is only problematic due to smaller organisms growing whilst held within the ballast tanks.
Filtration also removes sediments that could both diminish the treatment efficacy (especially in UV systems) and cause build up in the ballast tanks which adds needless extra weight to the ship and which can also provide ideal conditions for organisms to live and grow.
On the negative side, filters add to the expense of the system and need regular backflushing to prevent blockages and thus slow the intake of ballast. Another argument that is made against filters is that the very small mesh size of 50 micrometres or less can easily be enlarged over time by the action of sediment and will thus eventually become ineffective. The microscopic mesh size would make any such degradation almost impossible to detect by sight.
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.
The topic of filters has become something of a battleground with makers of filtered systems and makers of full-flow systems each arguing in favour of their chosen options. In almost all cases where filtration is used, its presence will have been part of the type-approval process, therefore specifying such a system without the filter would not be acceptable and would place the ship in danger of falling foul of a PSC inspection. If an owner decided that filtration would be desirable but the system does not rely on a filter, it would be possible to add one prior to the treatment stage but this could negatively affect the performance of the system and may invalidate any warranties.
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 including electrochlorination and most other methods must also undergo approval for substances under the 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 the 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.
These systems 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. 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 with the water then passing through a second heat exchanger to produce hot water or steam to either be used 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 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 some 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. There is some question as to whether lack of oxygen is injurious to some life stages of specific organisms
Ultra Violet (UV)
Many systems employ UV radiation that can produce a short-lived chemical change in water composition. 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 other 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 mention would have been considered at the design stage and found acceptable during the type approval process.
There are several systems that employ oxidising substances including chlorine, chlorine dioxide, ozone, peracetic acid, hydrogen peroxide or sodium hypochlorite. The oxidation mechanism consists of electron transfer with organisms that destroys the cell wall structure.
When a stronger oxidant is used, the electrons are transferred to the micro-organism much faster, causing the micro-organism to be deactivated rapidly. Long in use as a sterilisation method for land-based water supplies and with a proven kill rate although considered ineffective against some cyst forming organisms except at high dosages. Systems making use of this method require dosing using liquid or powder chemicals.
Chlorine dioxide is used in some systems and is considered by many to be better for treating water of high turbidity. There are several methods available to produce chlorine dioxide some of which require the use of hazardous chemical reagents and others which do not. In practice seafarers should not experience any more problems in dealing with the reagents than they do with other chemicals in use on board vessels although they do need to be made aware of the problems during initial training on the equipment and procedures may need to be added to the owner’s safety management system.
Peracetic acid reacts with water to form hydrogen peroxide which can also be used as an additive itself. These chemicals are freely available but price can vary widely and of course the required quantity will depend on the ballast capacity of the ship and sufficient storage space will be required on board.
Chlorination can also be achieved through electrochlorination by passing a DC electric current through the ballast water with chlorine being produced by the electrolytic reaction.
This method is more effective in waters with a high salt content and in cases where ballast is taken from a fresh or brackish source may not be effective. In such cases the addition of brine into the ballast flow will be required. There is therefore a need to carry supplies for operation in areas where different degrees of water salinity may be encountered.
Typically, a system that makes use of any chemical biocide or disinfectant will need to ensure that at discharge the ballast water does not retain any active substances that would have a detrimental effect on local species. This will usually require the addition of a neutralising additive that would also require approval under the G9 guidelines.
Has some similarity with Electrochlorination in that a DC electric current is passed through the ballast water. However, these systems do not rely on chlorine salts in the water or added to it to produce chlorine but rely instead on the production of very short-lived hydroxyl radicals which also have the ability to destroy cellular structures. In some systems a catalyst that speeds the reaction and makes it more efficient may also be present. The catalyst may either be attached to the surface of the electrode or even the electrode itself. In all systems where an electric current is passed through the water certain gases – notably hydrogen and perhaps chlorine – will be formed as by-products of the disinfectant or treatment process. The quantity of such gases may be small but since they are considered hazardous there will need to be some form of venting system in place so that they can be removed from the vessel.
Ozone is another oxidising biocide that is highly effective against micro-organisms and used in many water treatment processes. On board ship, it can be generated as a gas using an ozone generator and bubbled through the ballast flow and as already mentioned, UV light at some wavelengths can be used to produce ozone directly in the ballast water itself. Ozone reacts with the ballast water producing bromates which are highly effective at destroying organisms unaffected by the ozone itself.
Ph values and temperature of the ballast water intake can affect the efficiency and speed of the chemical reactions that take place and system makers should be able to give guidance on this. Higher temperatures usually mean more efficient treatment is possible. As an example, at a temperature of 15°C and a pH value of 7, five times more peracetic acid is required to effectively deactivate pathogens than at a pH value of 7 and a temperature of 35°C. Seawater has a pH value of around 8 – 8.5 which also slows the reaction but again system makers will have taken this into account when determining dosing quantities.