The ‘living vs viable’ organism debate is a complex matter. According to Coldharbour Marine, ballast water treatment systems (BWTS) can be separated into three types, being broadly based on electrochemical, ultraviolet (UV) or ultrasonic/physical technologies. The electrochemical and UV systems treat ballast water as it is pumped onboard at a terminal. For LNG operators, this is an extremely busy time, and risks to the vessel are at a maximum.
All BWTSs suffer from some degree of microbial re-growth, and scientific data shows that organism re-growth during extended ballast voyages is a reality that cannot be ignored. Official testing for re-growth under the International Maritime Organization (IMO) certification is five days, whilst the US Coast Guard (USCG) certification requires only one day. LNG tankers can typically be in ballast for up to 20 days, by which time re-growth in the ballast tanks, following treatment at ballast uptake, could be severe. The longer the ballast voyage, the greater the risk of microbial counts approaching and even exceeding pretreatment levels. Since both the IMO and USCG regulations set ballast water discharge standards rather than simple treatment standards, operators of LNG carriers are faced with a problem.
The ballast water discharge standard in the IMO convention is phrased in terms of ‘viable’ organisms. Specifically, it states that discharged ballast water must contain no more than 10 ‘viable’ organisms per cubic metre or cubic millilitre, depending on the size class of the organism. The IMO’s G8 guidelines specify that, for the purposes of approving BWTSs, ‘viable’ is defined as ‘living’. The USCG interpretation has always insisted that ‘non-viable’ means dead.
One major problem surrounds the practical difficulty of determining ‘viability’. Some flag administrations use reproductive ability tests, which raise a number of technical challenges, the most important being that it may not be possible to culture all of the organisms found in any one ballast water sample. Many organisms cannot be induced to reproduce at all under laboratory conditions, but do reproduce when pumped out into natural environments.
So far, there have been limited peer-reviewed academic research studies into the effectiveness of BWTSs. Peter Stehouwer of the Royal Netherlands Institute for the Sea compared six different treatments and demonstrated that although microorganism populations are heavily depleted by BWTSs, no system discharges sterile water. He also showed that different BWTSs have differing impacts on different microbes.1
Most commercial BWM treatments start by physically removing – usually by filtration – solid particles and organisms larger than 50 µm, followed by either a physical or a chemical treatment. Furthermore, since BWTSs remove the organisms that prey on and control algae and bacteria, (copepods and phytoplankton), any residual post-treatment microbes can regrow unhindered, feeding on the organic remains of their dead predators. Even worse, regrowth results in far larger microbial populations in the ballast water than originally existed. Others have demonstrated that high concentrations of organisms permit the initiation of genetic exchange mechanisms, which result in the emergence of totally new strains of organisms.
UV-based BWTSs work by damaging the DNA of marine organisms, leaving them viable, but unable to reproduce. Crucially, some organisms have repair mechanisms that are able to undo this damage and thus, over time, restore their ability to grow, survive and reproduce. Bear in mind that a ship’s ballast tank is far from a sterile environment. Its warm, dark, enclosed environment is an ideal bacterial/algal incubator. This means that as soon as UV treated water is pumped into the ballast tanks, the water can become contaminated by resident organisms and, unless retreated, there will inevitably be millions of organisms discarded with the ballast water.
Stehouwer demonstrated that when UV treated ballast water is brought onboard through the UV treatment system, the microbial population reduces over time, reaching a minimum after five to eight days in the tanks. Then regrowth starts, reaching a steady state after 12 to 15 days carrying a higher level of microbes than were pumped onboard initially. This implies that there is an optimum period for UV-treated water to be held in the tanks and, should journey times take longer than eight days, the ballast water should be UV-treated again immediately before discharge. Only then will ship owners be sure that they are discharging the minimal concentration of organisms.
Stehouwer showed that the alternative chemical in-tank treatments kill microbes effectively, but any remaining active ingredients require careful neutralisation before tanks can be discharged. Chemical treatments need care, because they can damage tank coatings and the neutralisation of any residual active ingredients can be a challenge in the absence of technical staff able to calculate the precise quantities needed for each tank.
Operators of large tankers, carrying thousands of tonnes of ballast water, and travelling over extended periods, are discovering that in-voyage, in-tank physical treatment systems provide excellent value for the company and are environmentally friendly. These owners are opting for continuous in-tank, in-voyage BWTSs and take advantage of three different physical (hypoxia, hypercapnia and ultrasonic shock) in-tank treatment strategies. The processes take place in the ballast tanks and only begin once loading and ballasting is complete, after the ship has left the terminal.
Monitoring shows that these systems gradually reduce the bacterial load over time during a typical 10-day voyage and, being continuous, maintain low microbial counts throughout the voyage, delivering clean ballast water at the end of the voyage. These physical systems succeed in removing all risks of regrowth or damage to ballast tank coatings as they are able to operate in waters of any salinity, temperature or turbidity, and do not use chemically active ingredients.
So, should the few viable organisms that are accepted under both the IMO and the USCG standards be a cause for concern? Charles Darwin realised that for any invasive organism to successfully establish itself in a new environment, there needed to be the following:
- A sufficiently large number of organisms introduced – propagule size (PS).
- A number of discrete introductions – propagule number (PN).
The resulting (PS x PN) ‘propagule pressure’ explains why some introductions persist while others do not. Species regularly introduced in large quantities (high PS and high PN) have a high propagule pressure and are more likely to survive than species rarely introduced (low PN) or introduced in small numbers (low PS) at small propagule pressure. The IMO standard has, therefore, been set at levels designed to minimise propagule pressures. In summary, if the ambition of preventing the successful introduction of new species is to be achieved, small propagule numbers and small propagule sizes are vital.
- STEHOUWER, P. P., BUMA, A., and PEPERZAK, L., ‘A comparison of six different ballast water treatment systems based on UV radiation, electrochlorination and chlorine dioxide’, Environmental Technology, (2015).
Written by Mark Wells, Coldharbour Marine Ltd, UK. Edited by Callum O'Reilly
Read the article online at: https://www.lngindustry.com/special-reports/25052016/dead-or-alive-1478/