Small Ships Breed Mighty Microbes
Many operational microbiological problems occur on small ships which rarely affect large ones.
For example, ships operating in confined inshore waters around ports or estuaries are not permitted to discharge untreated bilge water into the sea. A cheap option is to hold the bilge on-board and discharge it when berthed – but this bilge becomes stagnant and oxygen deficient on board – and stagnation stimulates the growth of sulphate reducing Bacteria, SRB. These produce hydrogen sulphide (the bad egg gas), which causes health problems, sometimes fatal ones, and is the agent for dramatic accelerated pitting corrosion of steel hull bottom plate.
Ships working around river estuaries may have to bunker at facilities which also service river craft – and these must use low sulphur diesel fuel. Due to the lack of appropriate infra-structure or just for convenience, the fuel supplied may be automotive diesel fuel, which is mandated in most countries to contain bio-components such as fatty acid esters (7% in Europe). Highly nutritious stuff for microbes – so more slimy growth to block pipes, filters and injectors.
Many small ships, particularly super yachts are under-utilised and most fluid systems become stagnant and oxygen deficient (anaerobic) – good news for many microbes waiting to start their families in sludges and slimes. There are particular implications for the ships’ potable water systems, whether used in the galley, for hot and cold washing or for drinking. Any residual protective disinfectant, such as the standard 0.2% chlorine, is rapidly depleted and the few live microbes still in the water are free to grow. They will need to be killed by super-chlorination or thermal disinfection before the water systems can be used safely.
A specific effect of stagnation can occur in cooling water if nitrite is used as the anti-corrosion agent. In operational use, cooling water systems are well aerated and probably too hot to allow much growth. But when a system is shut down, the temperature falls and dissolved oxygen disappears. Some clever microbes called Nitrite Reducing Bacteria (NRB) use dissolved oxygen if it’s there but if it’s not, they can remove the oxygen molecules from nitrite, producing various reduced molecules such as ammonia and nitrogen. So the normal anti-corrosion protection is lost and ammonia, which can be aggressive to some copper alloys is produced.
An inevitable consequence of under-utilisation is that any microbes suspended in fuel, oil and water systems will slowly settle downwards as they are significantly heavier than fuel and oil and a little heavier than potable water. Hence monitoring regimes recognise this and sampling from system low points will usually give early warning of a developing problem.
So do small ships breed mighty microbes relative to larger ships? Probably not – they are just the usual clever ones taking advantage of better living conditions. Those of you at the sharp end of small ship operations may know of other examples. An understanding of all of these factors promotes the development of effective monitoring and treatment strategies – which is what we do at ECHA Microbiology.