Systems that use an active substance to kill organisms must undergo additional testing and pass a different approval process. Firstly, the substance used has to be investigated and if it is found to be effective the system will be given what is called Basic Approval at a meeting of MEPC. It must then be further tested and found not only to be effective but also capable of being neutralised or removed from the ballast before being discharged during de-ballasting. The substance and the system will then be given Final Approval by the MEPC. It will then remain for the system to be tested at sea and given type-approval in the same way that the mechanical systems are.
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 it to be deactivated rapidly. Oxidation has long been in use as a sterilisation method for land-based water supplies and has a proven kill rate, although it is 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. 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.
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. 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.
Peracetic acid reacts with water to form hydrogen peroxide which can also be used as an additive itself. These chemicals are freely available but their price can vary widely and,of course, the required quantity will depend on the ballast capacity of the ship; sufficient storage space will be required on board.
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 slows the reaction but system makers will have taken this into account when determining dosing quantities.
Chlorination can also be achieved not by adding the chemical but by electrochlorination and there are many systems available that use this method. Electrochlorination is achieved 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 may not be effective in cases where ballast is taken from a fresh or brackish source. 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. The brine can be normal seawater which is taken in and stored in a separate tank.
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.
This 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 to be 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 be 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 safely removed from the vessel.