Four ways to disinfect the drinking water system offshore
How to disinfect a drinking water system offshore?
Disinfection is a process where most or all microorganisms are wiped out. Some bacterial spores may still survive disinfection. The most common ways to disinfect an offshore drinking water system are by heat treatment, by using chemicals such as hypochlorite or hydrogen peroxide or by UV irradiation.
The distribution net and the potable water tanks must be disinfected once a year. The tanks should also be cleaned and disinfected if work has been done inside the tanks. Hyperchlorite or hydrogen peroxide are common disinfectants used in these processes. UV and temperature are used as barriers to microbiological pollution.
Ambio has assisted several boats and installations with annual disinfection processes. Bjørn Tore was once on a boat that was docked due to excessive detection of Legionella. Here it took two rounds of disinfection using chlorine to remove everything. In the first round, 99% of the bacteria were taken, while after round two the analyzes showed that no more legionella bacteria were present in the drinking water system anymore. Such a process can be time-consuming and challenging, but it is a safe way to remove microorganisms if done thoroughly.
Read more about the four most common ways to disinfect the drinking water system:
Sodium and calcium hypochlorite are different types of chlorine used for disinfection processes offshore. Sodium hypochlorite is in liquid form and therefore has limited shelf life, while calcium hypochlorite is in powder form and has virtually unlimited shelf life. However, once the calcium hypochlorite is dissolved in water, the shelf life is the same as sodium hypochlorite.
Common to these two is that they form the same active chlorine compounds in water, subchloric acid (HOCl) and hypochlorite ion (OCl-).
Free chlorine, a common term for these chlorine compounds, is unstable and reacts with organic matter. The more organic matter the water contains, the more chlorine one must add. Bunkered water contains some organic material, while the water one produces himself, via evaporator or reverse osmosis, contains little or no organic matter.
Chlorine is a disinfectant in that it acts as an oxidizer on microorganisms. Since chlorine spends a certain amount of time destroying microorganisms, the drinking water regulations require that a free chlorine residue be detected after 30 minutes. Due to the measurement uncertainty of instruments used offshore, it should be possible to detect 1.0 mg/l of free chlorine residue after bunkering.
The main challenge with chlorination is to ensure a good mix of chlorine. If chlorine is added to self-produced water, it is best to add it before the alkalization filter, since chlorine is more effective in an acidic environment. A circulation loop makes it possible to mix more chlorine after bunkering.
Chlorine is effective against microorganisms, but only works if it reaches all tubes. If there is stagnant water in some pipelines, the chlorine will not be able to reach here.
Hydrogen peroxide is a colorless, viscous liquid used as an oxidizing agent against microorganisms.
Oxyl-Pro uses hydrogen peroxide as an active disinfectant, but it is packaged in a gel that is activated and opened only upon contact with specific biological markers. This means that the hydrogen peroxide is only released when and where it is needed.
TIP: Remove all dead ends to get chlorine and hydrogen peroxide out of all piping lines.
Disinfection by UV irradiation
UV irradiation is an incredibly effective way of inactivating microorganisms. The UV-C rays work on microorganisms as it does on us humans. It destroys our genetic material and gives us skin cancer, while the microorganisms become sterile and can no longer multiply. In order to inactivate the microorganisms, they must be exposed to a sufficiently high UV-C dose.
Water quality is important in UV disinfection. Color and turbidity can reduce the intensity of the chamber and reduce the UV dose. Particles can also screen microbes through the plant.
Irradiation with UV should be the final treatment step before the water is sent online. UV facilities used offshore must be type-approved by the Norwegian Institute of Public Health.
TIP: Good maintenance and operation of the UV system is essential for optimal disinfection. Replace all the lights at the same time.
The majority of microbes that can make us ill thrives at body temperature. Keeping the cold water temperature below 20oC and hot water temperatures above 60oC will greatly reduce the growth of microbes. In an offshore potable water system it is important to keep the cold and hot water separate, as well as document that the temperature is sufficient throughout the system. By establishing a sampling system you will get a good overview of where the risk of growing up is greatest.
TIP: Take temperature measurements of the cold water where it is warmest and of the hot water where it is coldest.