A quick guide on providing clean safe drinking water on Marine vessels

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A quick guide on providing clean safe drinking

Potable Water On-board

Yachts and Marine Vessels

A quick guide on providing clean safe drinking water



This booklet is designed to help the captain, chief engineer and the rest of the crew to ensure that the quality of drinking water is of an acceptable quality and complies with the recent applicable requirements and regulations.

Clean safe drinking water is a particular challenge on board yachts, merchant ships and other sea going vessels. It either has to be produced from seawater via systems such as reverse osmosis and flash evaporation or bunkered from a shore or water barge supply. Therefore it is essential that the water bunkered or produced is of an acceptable biological, chemical and mineral quality, and that the holding tanks, distribution and treatment systems are clean, sterile and in good working condition.

 Regulations and guidelines for the quality of potable water on board, are stipulated by MLC, WHO, IWA, IHR, ILO, IMO, International Health Regulations, USEPA, European Council Directive 98/83/EC and MCA.


1.         Health risks associated with potable water on yachts (20m+)


Bacterial Analysis carried out on over 1000 samples from yachts between 20m and 100m+, both commercial and private, between April 2016 and April 2018 showed that a high number of vessels had water of a non potable quality within their fresh water systems (actual results are withheld to online users for legal reasons)

2.         Bunkering From a Known and Acceptable Source.

            (Where Documentation is available)

A port will usually receive potable water from a municipal supply, although for some ports this may come from a private supply. Most ports should have special arrangements for managing this water after it has entered the port, and generally documentation of the quality of this water should be available for the port office.

 When bunkering water it is important that the correct procedure is followed.

2.1      Water should never be bunkered through garden hose or any hose that is not specifically designed for this purpose, as the leeching of chemicals such as Cadmium, Phthalates, Lead, Bromine, Polybrominated Diphenyl ethers and Polyvinyl Chloride into the water may occur, hoses not designed for purpose are generally ideal breeding grounds for bacteria.

2.2      Bunkering hose should be blue in colour, and should have an inner core that prevents the growth of bacteria; it should not be used for any other purpose. All hoses used for bunkering potable water, including fittings and tools, should be stored in designated lockers that are closed and clean. Operators need to have a working  knowledge of water hygiene and good sanitary practice.

2.3      Facilities for disinfection, when and where necessary on board, need to be available. Regular cleaning and disinfection of hoses and fittings should be performed.

2.4      The water should pass through a filtration system to remove particles and trace chemicals where possible, prior to filling the holding tanks.

2.5      The filters prior to the holding tanks should be changed on a minimum of a monthly basis. During this change, the filter bowls should be cleaned and sterilized. This procedure should be noted in a fresh water logbook.

2.6      If any water conditioning units, such as water softeners are used prior to the water entering the holding tanks, ensure that these are sterilized with the appropriate products on a minimum of a monthly basis. This procedure should be noted in a fresh water logbook.

2.7      Always fill in the fresh water logbook, stating date, time and location of bunkering, stating any observations or abnormalities (if applicable).

 3.         Bunkering From a Questionable Source.

It is never recommended to bunker fresh water if the supply is in any way questionable. Try if possible, to obtain documentation and a recent analysis report from the source in question.

Only bunker if absolutely necessary. Do not risk contaminating the vessels water system or possible infection of the crew and guests.

When bunkering water from a questionable source it is important that the following procedure is implemented.

3.1      Water should never be bunkered through garden hose or any hose that is not specifically designed for this purpose, as the leeching of chemicals such as Cadmium, Phthalates, Lead, Bromine, Polybrominated Diphenyl ethers and Polyvinyl Chloride into the water may occur, hoses not designed for purpose are generally ideal breeding grounds for bacteria.

3.2      Clean and sterilize the shore water and bunkering hose connection. Flush the connection for at least 10 minutes before connecting the fresh water bunkering hose.

3.3      Test the shore water for disinfection residual (total and free chlorine, chlorine dioxide etc.)

3.4      Take samples of the shore water for analysis (after initial cleaning and flushing).

3.5      If you have the appropriate analysis equipment on board, begin the tests as soon as possible, if not send the samples to a laboratory (samples for bacterial analysis should be kept chilled and tested within 24 hours of the sample being taken)

3.6      Only use the yachts dedicated blue fresh water hose, do not accept hoses from the port or use any other type of hose.

3.7      If possible ensure the potable water holding tank to be filled is isolated from any other tanks.

3.8      When bunkering is complete, do not use the holding tanks concerned, before receiving the analysis results. (It will take a minimum of 24 hours to get results from the laboratory.)

3.9      Sterilize the bunkering hose and connections before draining and storing.

* New equipment is now available for testing bunkering water that will provide accurate results of TVC in a matter of minutes.

4.         Water Produced On-board.

Seawater contains bacteria, viruses, organic micro-contaminants and chemicals that are potentially damaging to health.

It is therefore necessary to be aware of the area in which you run the water maker, only to use the Reverse osmosis units in open sea – i.e. more than 20 nautical miles offshore. When there is a perceived risk of estuarial pollution you may need to increase this distance from land.

If however you are required to operate the water makers closer to land then the procedure detailed below becomes vitally important.

The water produced on board, must be analysed to ensure that the production quality does not surpass the 500ppm limit, and is free from bacteria.

4.1      Ensure the equipment is well maintained and serviced at regular intervals. Make sure that all work that is carried out is logged.

4.2      Always log the position of the vessel when using the water maker in the fresh water logbook.

4.3      Ensure the water being produced is sterilized before it enters the vessels fresh water tanks. (NB. UV sterilization may be used as a secondary barrier only)

4.4      Regularly check the calibration of the automatic diversion valve to ensure that product water of higher than 500ppm cannot enter the vessels fresh water tanks.

4.5      Ensure all filters on the production side of the water maker are changed on a monthly basis and logged in the fresh water logbook.

4.6      Always flush the water maker with fresh water after use. (Do not operate the high-pressure pump during this process)

4.7      If the unit is not going to be used for a period of 2 weeks or more, perform a chemical storage application, to ensure that there is no growth of bacteria within the system. (All procedures should be logged)

5. General Distribution Systems and Fresh Water Tank Maintenance.

The majority of water quality problems associated within the yachting community are to a greater extent, avoidable, providing the correct procedures are followed.

The procedures outlined below should form part of the regular maintenance schedule.

5.0.1   For bunkering from a known and acceptable source. Please refer to section 2.

5.0.2   For bunkering from a questionable source. Please refer to section 3.

5.0.3   For water produced on-board. Please refer to section 4.

5.0.4   Ensure weekly testing of the fresh water (Galley Tap recommended) for sterilization residual, pH, temperature and TDS.

Log all tests in the fresh water logbook.

5.0.5   Confirm that all filters within the fresh water system are replaced at least on a monthly basis and filter bowls are cleaned and sterilized before replacement.

Log all procedures in the fresh water logbook.

5.0.6   Ensure that all media filters are sterilized monthly where possible, and checked on a regular basis, the media should be replaced every two years.

Log cleaning, testing and replacement in the fresh water log book.

5.0.7   Ensure regular analysis is performed for bacterial content.

Log all results in the fresh water logbook.

5.0.8   Seldom-used taps, showers and outlets should be regularly flushed (weekly) to mitigate the chance of bacterial growth, formation of biofilms and legionella proliferation, due to water stagnation.

5.0.9   All tap filters and showerheads should be removed, cleaned and disinfected on a quarterly basis. Log all procedures in the fresh water logbook.

5.1.0   Any fresh water fire suppression system should be tested for legionella on a

bi-annual basis. Log all procedures in the fresh water logbook.

5.1.1   A visual Fresh water tank inspection should take place, where possible on a six monthly basis, and during every yard period, looking for signs of corrosion, bubbling, flaked or damaged paintwork, deposits and biological growth.

5.1.2   A Hypo-Chlorination of the tanks and distribution system, including all outlets at a free chlorine concentration of 50ppm, for a minimum period of four hours is required on an annual basis, After repairs or work on the holding tanks or distribution system and upon delivery of new builds *

* Please see section 10 for full details on performing a Hypo-chlorination.

6.         Water Analysis and Taking Samples.

When it comes to periodic water analysis involving complex chemical, mineral and microbiological tests, such as the MLC2006 analysis, samples should be taken and analyzed by a specialist on a minimum of a biannual basis.

( Basic bacterial testing can be performed with the correct equipment and basic knowledge of sampling and analysis procedures, by a competent member of crew on a regular basis.)

All samples should be taken in accordance with ISO 19458, using sealed sterile sample vessels containing a carefully measured amount Sodium thiosulfate, this is especially important when taking samples from bunkered water or from systems that are using chlorine as the means of sterilization. If the tap from which a sample is to be taken has a filter this should be removed, the tap should then be sterilized and run until a constant temperature is observed. Sterile gloves should be worn during the taking of the sample to avoid any cross contamination.

7. Controls and Water Analysis

The following table outlines the minimum amount of water tests you should undertake in a number of situations.

Table 1. Potable Water - Situations and Minimum Frequency of Analysis on-Board Yachts.


Water Produced On-Board

Bunkering from a known & acceptable source.

(With documentation available)

Bunkering from a questionable source

After work has been performed or New Builds

Disinfection residual, pH, TDS and water temperature


(TDS must be less than 500ppm)















From source at time of bunkering and after from system

After repairs have been completed and on delivery of new builds

Mineral & corrosion indicator analysis





From source at time of bunkering

After repairs have been completed and on delivery of new builds

Chemical analysis





From source at time of bunkering

After repairs have been completed and on delivery of new builds

Physicochemical Analysis





From source at time of bunkering

After repairs have been completed and on delivery of new builds

Organic contaminants

(Pesticides, and VOC’s)





From source at time of bunkering

New builds

And after holding tanks have been painted

Legionella Analysis




After repairs have been completed and on delivery of new builds

*HPC/TVC, Coliforms, E-Coli, Enterococci, and Pseudomonas Aeruginosa.

Table 2. Microbiological criteria for potable water on board Yachts.


Acceptable Level

Borderline Level

Inspection of system required

Action Level



0 in 100ml


≥1 in 100ml

E. Coli

0 in 100ml


≥ 1 in 100ml


0 in 100ml


≥ 1 in 100ml

Ps. Aeruginosa

0 in 100ml


≥ 1 in 100ml

Legionella spp

<102 / litre


≥102 / litre


<100 / ml


≥ 700 / ml

8.         Regular and Periodic Analysis

When it comes to periodic testing which involves complex chemical, mineral and microbiological analysis, samples should be taken and analysed by a specialist.

It is also possible to buy test kits so that crew can take samples and analyse water samples on board for simple parameters such as disinfection residual, simple bacterial testing and individual mineral testing such as aluminium.

It is important to be aware of the advantages and disadvantages of doing your own tests kits and conducting your own analyses.

Ensure the equipment to be purchased will achieve reliable results and that full back up is available from a water specialist.

It is also a requirement for crew to have been instructed in the correct way to carry out bacterial analysis and have a good working knowledge of the correct procedures for taking the samples.

If the Samples are to be sent to a laboratory they must be kept at 2-10°C for transport and examined within 24 hours.

9.         Pools and Spa Pools.

Spa pools are becoming increasingly popular. The agitated water is usually maintained at a relatively high temperature and this places a high demand on the spa pool’s disinfection and filtration systems. This can lead to an increase in microbial growth and a consequent risk of infection.

Even if the spa pool is emptied daily, the water within the treatment and circulation system, may begin to proliferate bacterial growth, infecting the water on refilling.

The spa pool must, therefore, be managed carefully to ensure water quality does not deteriorate. Experience has shown that inadequate management has led to illness for users, people in the vicinity or people passing nearby the spa pool.

Before spa pool use.

9.0.1   Check water clarity before first use.

9.0.2   Check automatic dosing or sterilization systems are operating correctly.

9.0.3   Check that the amounts of dosing chemicals in the reservoirs are adequate.

9.0.4   Determine pH value and residual disinfectant concentration.

Throughout the day.

9.0.5   Determine pH value and residual disinfectant concentration every 2 hours.

9.0.6   Determine the TDS, where appropriate.

9.0.7   Maintain pH at a level between 7.0 – 7.6.

After Use.

9.0.8   Clean water line, overflow channels and grills.

9.0.9   Clean spa pool surround.


9.1.0   Microbiological tests for bacteria.

9.1.1   Ensure filters are changed.

9.1.2   Disinfectant/pH controller - clean electrode and check calibration (where applicable)


9.1.3   Where possible clean and disinfect airlines.

9.1.4   Performa a test for Legionella

Table 3. Microbiological criteria for pool and spa pool water on board Yachts.


Satisfactory Level

Acceptable Level

Action Level


0 in 100ml


≥ 1 in 100ml

E. Coli

0 in 100ml


≥ 1 in 100ml


≤100 in 1ml


≥ 100 in 1ml

Ps. Aeruginosa Swimming pools

Spa pools

0 in 100ml

0 in 100ml

1-<10 in 100ml

0 in 100ml

≥ 10 in 100ml

≥ 1 in 100ml

Legionella spp

(Spa pools only)

Not Detected

Present <102 / litre

≥102 / litre

10.      Hypo-Chlorination of a ships Fresh water system

This section is designed to ensure that the correct procedure is followed during sterilization of the tanks and distribution systems, in line with current regulations and requirements.

The Hypochlorination procedure can be broken down into 6 main parts;

  • Pre-treatment bacterial analysis.
  • System survey.
  • Pre-Hypochlorination system preparation.
  • Hypochlorination and testing procedure.
  • Flushing and post procedure preparation 
  • Post-treatment bacterial analysis

Each section details the necessary protocols and actions required.

Many factors affect the ability of a hypo-chlorination procedure to successfully sterilize the on-board systems.

The presence of biofilms and specifically the bacterium Pseudomonas aeruginosa Legionella pneumophila are of particular concern, as these are unlikely to be removed by chlorine alone if present in numbers.

High turbidity can also decrease the effectiveness, whilst temperature can both increase or decrease it, a pH value of greater than 7.5 or less than 6 may require additional chlorine to be added to achieve the correct chemical level.

Saturation time is also an important aspect, as pipes have a much higher retention of free chlorine than tanks, (due to the exposure to oxygen), hence a minimum contact time of four hours and a recommended contact time of twelve hours should be observed.

10.1. Pre-treatment bacterial analysis

The presence of a biofilm within the vessels distribution system or holding tanks is unlikely to by removed by the Hypochlorination procedure. It is therefore important that an analysis is performed to confirm that such a biofilm is not present.

The manifestation of the bacterium Pseudomonas aeruginosa, along with high levels HPC bacteria is the most common indicator that a biofilm may be present within the system. To verify this, a multi point bacterial analysis is recommended prior to the sterilization procedure.

If the analysis results suggest a biofilm may be present, a separate procedure using a chemical capable of biofilm removal should be performed, prior to Hypochlorination.

10.2. System Survey

A system survey should be carried out prior to the application of the Hypochlorination procedure.

This requires a study of the vessels fresh water system drawings to ascertain the exact number and nature of water outlets. This should then be followed by a visual inspection to confirm any additional outlets or changes made since construction. 

The data should then be compiled into a list ensuring that during the Hypochlorination procedure, results of the free chlorine at each outlet can be logged.

Although the completion of system survey and subsequent list is labour intensive and time consuming, it is an important course of action, and provides proof that the system as a whole will be treated correctly. 

10.3. Pre-Hypochlorination system preparation

Preparation is the key to success for most procedures and Hypochlorination is no different.

Distribution filters and housings

Remove all filters from the distribution system, including if applicable;

  • Post Tank filters
  • Ice machine filters
  • Fridge filters
  • Drinking water system filters

Physically clean all filter housings and replace. DO NOT PUT FILTERS IN PLACE.

UV sterilization systems

Before the procedure any UV sterilizers should be serviced and the quartz tubes removed and physically cleaned, and then returned to service.

Water Makers

Disconnect the fresh water flush lines from the water makers to allow flushing with the sterilization chemical.

Fresh Water Fire systems

Ensure flushing of the fresh water fire systems is practical and possible.

Calorifiers / Water heaters

Ensure all calorifiers / water heaters are switched off before starting the Hypochlorination procedure.

Ice Makers

Check that your ice maker is compatible with chlorine, if not remove the supply lines in preparation for flushing, and clean you ice maker separately with a suitable sterilization solution.

10.4. Hypo-chlorination and testing procedure.

Determination of chemical volume

It is very important to determine the correct strength of the solution to be added, this will depend on a number of factors:

  • Total tank volume
  • Strength of chemical (NB. This must be fresh and unopened)
  • Size of calorifiers
  • Approximate distribution network volume.

With this in mind, prepare the correct volume of Sodium Hypo-Chlorite solution.

Please Note:

Before being diluted by the water in the potable water system Sodium Hypo-Chlorite:

  • Is dangerous to eyes and skin – see IMO Medical First Aid Guide;
  • Deteriorates on exposure to air – it should, therefore, be purchased, just before use and should be sealed tightly after use;
  • Should be kept in a cool, dry and dark place, but where it can be easily seen and removed when needed or if there is a likelihood of fire;
  • Be accompanied by a Material Safety Data Sheet. (MSDS)

Informing personal on board

All personal on board should be informed that this process will be performed and that the use of the water system for any purpose is prohibited.

Appropriate signs should be posted in areas of frequent water use specifying the water system is out of action and must not be used.

 Taking samples at each outlet

Samples should be taken at every outlet to determine if the correct saturation of chemical has been achieved. This requires specialist equipment and test kits capable of measuring free chlorine at high levels (50+ ppm).

Once saturation at an out let has been reached the test results should be logged in the sheets prepared during the system survey.

Ensuring all equipment is treated

Once all the available out lets have been verified to contain the minimum levels of free chlorine you should run the machines within the system that cannot be tested such as, but not exclusively;

  • Washing machines (cold wash recommended)
  • Dishwashers (rinse cycle)
  • Ice machines
  • Windscreen washers
  • Anchor chain wash
  • Fresh water fire systems (flushed from the end of the line)

These procedures should also be logged.

Topping up the tanks

In flushing the system a certain amount of the chlorinated water will have been drawn from the storage tanks. They should be refilled to overflowing, and chlorine solution should be added to make up the concentration in the tanks to 50 ppm.

Duration of contact

The chlorinated water should be allowed to remain in the storage tanks and distribution system for no less than four hours and where possible for twelve hours.

10.5. Flushing and post procedure preparation.

Flushing the system and holding tanks with HCE17

The use of HCE17 will drastically cut down the time required for the flushing procedure, and provides an enviromentaly responsible proceedure by removing the chlorine thus respecting the enviroment, this should be mixed and added when the contact time has been achieved.

The mixed HCE17 can be added directly to tanks and will remove the chlorine almost instantly, the water should then be passed through all the outlets as you did with the chlorine.

When the system has been emptied, you can re-fill and use immediately, the HCE17 is safe for use with potable water.

Flushing the system and holding tanks without HCE17

Once the saturation time has been achieved the remaining chemical should be flushed from the system, and the holding tanks should be emptied to as low as possible without losing prime on the hydrophores.

The tanks should then be refilled with filtered fresh water and flushed through the system.

The tanks should then be refilled a second time with filtered fresh water and flushed through the system once again.

Testing can begin on after the second refill process and

Fitting new filters

When the levels of chlorine are at an acceptable level, new filters should be fitted in the empty filter housings (Never replace used filters)

10.6. Post-treatment bacterial analysis

After approximately one week, a full bacterial analysis should be performed for confirmation of the sterilization procedure.


Association of Port Health Authorities/Health Protection Agency - THE MICROBIOLOGICAL QUALITY OF WATER ON BOARD SHIPS.  


World Health Organization – GUIDE TO SHIP SANITATION.  

Maritime & Coastguard Agency - MERCHANT SHIPPING NOTICE MSN1845 (M)   

Maritime & Coastguard Agency - MARINE GUIDANCE NOTE MGN 525 (M+F)   


Maritme and Coastgaurd Agency - Guidelines for the Provision of Food and Fresh Water on Merchant Ships and Fishing Vessels – MARINE GUIDANCE NOTE MGN 397 (M+F) 

Health Protection Agency and Health & Safety Executive - MANAGEMENT OF SPA POOLS - CONTROLLING THE RISKS OF INFECTION   

Guidance from - 

EU SHIPSAN ACT - ship inspections in compliance with European Union legislation and national Health Regulations.


Copyright© Dustin Wilson 2018 










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Octo Marine Code Potable Water On-board Yachts and Marine Vessels
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Manufacturer Octo Marine
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