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 more novel solutions on offer and some more outlandish proposed in 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.
The discharge standards of the IMO convention and the USCG rule both contain limits on the number of viable organisms of 10 micrometres and above that may be present in the discharge water. Particularly for the larger organisms of 50 micrometers and above, filtration is a proven and reliable technique for removal so it is not surprising that three out of every four systems make use of filtration as the first in a series of treatment. The types of filters will vary by system but all have the added advantage of also removing much of the inert sediment and so removing the issues of the ship carrying extra weight and tank damage referred to above.
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 three 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 filtration 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 situation where space may be limited.
At least one system – Hitachi’s Clear Ballast – employs coagulation treatment before filtration. Coagulation or flocculation makes use of a solid substance around which organisms congregate causing large flocs that can be removed by extremely coarse filters. The Hitachi system uses a magnetic floculant introduced into a tank and then removes the flocs with a magnetic separator before the water moves to a second filtration stage.
The main treatments
After any initial filtration, the next stage in any system is usually targeted at making any living organisms unviable. There is an important distinction to be made here between unviable and killing, because the convention wording does not use the latter term although US rules do. When updating the G8 guidelines there was much discussion over this difference with the eventual result that unviable was determined to mean either dead or incapable of reproduction. In practice it would require lengthy laboratory tests on any living organisms discharged in treated ballast to determine if they are viable or not, so most systems are designed to kill. Most system makers term this stage as disinfection which seems an appropriate choice. It is at this point that the variety of technologies proliferates. Even so there are many systems employing similar techniques or combinations of treatments.
As a first step, system makers, or more correctly the administration under which they plan to obtain type approval, need to decide if their product requires approval under the G8 or G9 (systems making use of an Active Substance) processes. The IMO convention states that the decision on whether a ballast water system makes use of Active Substances or not remains the prerogative of the administration. If applying for US approval there is no choice to be made.
What constitutes an active substance is not always immediately clear. Most would recognise that adding a chemical biocide would fall into this category but so do some of the neutralising chemicals used to remove chlorine produced by electrolysis. A fruit acid used in the Alfa Laval Pure-Ballast system for cleaning UV tubes was also considered to require IMO approval under G9.
Ultra Violet (UV)
Many systems employ UV radiation that can produce a short lived chemical change in water composition and while some administrations have determined this should fall under the G9 process, 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 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.
Maintenance is generally restricted to replacement of 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.
Some UV systems have been considered to have problems in meeting US rules because of the unviable v 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.
In 2015, the USCG decided that although UV systems may be permissible, the testing method that most makers had employed when gaining type-approval under the IMO convention was not. Some flag states have type-approved UV systems on the basis of the most probable number, or MPN, method, which is used to determine the viability of organisms rather than making the live/dead judgment. For all the reasons discussed above regarding the difficultly of determining viability and because the US discharge standard is expressed in terms of living organisms, the MPN method is not incorporated into the US type-approval procedures which recognises an alternative known as the CMFDA/FDA method that is sometimes referred to as the vital stain method.
Some of the systems currently undergoing US type-approval testing have previously been type approved elsewhere using the MPN method and in most cases the makers have requested – as they are permitted to do under US rules – that the MPN method be treated as being equivalent to the US rules. Unfortunately for those makers the USCG decided to refuse their requests. The USCG has said that as with all decisions made under the authority of Title 46 of the Code of Federal Regulations, the ruling is subject to appeal by the manufacturers. However, the USCG has repeated its ruling a number of times since.
Some system makers have decided that rather than challenging the USCG ruling, it would be better to retest their systems using the method approved. Two of the first three systems type-approved by the USCG are UV systems and were tested using the vital stain method.
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.
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. Chlorination can also be achieved through 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 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.
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 owners 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 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.
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.
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.
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.
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.
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, two of the newest systems to join the fray makes 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 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.
Another 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.
Testing ballast water systems on-board
Seafarers are not scientists and because the organisms that ballast treatment is supposed to protect against are microscopic, it makes knowing that a treatment has been effective mostly impossible.
For all practical purposes effectively treated ballast is indistinguishable from the water that passed through the initial filter when taken on board. Sampling by PSC inspectors will usually involve laboratory testing with the results known only after what may be a considerable delay. All of which leaves shipowners open to penalties if the treatment system is defective for any reason.
A number of specialist companies have developed products which are claimed to allow testing for some organisms present in ballast water. Although these devices do not test for every organism or bacteria mentioned in the IMO convention or US regulations, the presence of any living organisms in the range that can be tested for will be an indication that the system is not working effectively.
Systems such as the Ballast-Check 2 from California-based Turner Designs and UK-based Chelsea Technologies’ FastBallast are fluorometers that detect viable algal organisms in the 10-50μ size class. The first is a small handheld device while the FastBallast can be used as a stand-alone device or incorporated into the treatment system because it is capable of rapid measurements even flows.
The Speedy Breedy developed by Bactest of Cambridge, UK is a portable precision respirometer which detects and monitors microbial activity. Detection of microbial activity is determined as a consequence of pressure transients relating to gaseous exchanges within a closed culture vessel of 50 ml working volume, as a result of microbial respiration. Its maker says the system can be used by non-experts wanting to carry out microbiological tests and is also relatively inexpensive, which may make it a useful piece of equipment for measuring bacteria in fresh water supplies as well as for testing ballast. Although no treatment system maker has yet incorporated testing apparatus in its products, many believe that they may be obliged to do so in the future either because of changes in the regulations or because customers will demand it as an option.