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Different types of ballast water treatment system

Filtration

Filtration

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.

Where filtration is included as part of the treatment system it is not the coarse filters that are used to protect seachests and piping from damage by large objects but fine filters of 40-50 microns designed to remove most of the organisms that are regulated by the D2 discharge standard.

It is very possible that over time the filters could become damaged by small amounts of grit and debris increasing the mesh beyond the design limits. In turn that could impact on the effectiveness of further treatment.

UV treatment

UV treatment

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.

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.

Mechanical treatment

Mechanical treatment

Cavitation/Ultrasound

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.

Deoxygenation

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.

Heat Treatment

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.

Electro Chemical

Electro Chemical

Electrochlorination

Chlorination can 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.

Electrolysis/Electrocatalysis

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.

Active substances

Active substances

Oxidation

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 described in the mechanical treatment section, 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.

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