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Domestic Water System Merchant Ships

Domestic water systems usually comprise a fresh water system for washing and drinking and a salt water system for sanitary purposes . Both use a basically similar arrangement of an automatic pump supplying the liquid to a tank which is pressurised by compressed air.

The compressed air provides the head or pressure to supply the water when required. The pump is started automatically by a pressure switch which operates as the water level falls to a predetermined level. The fresh water system has, in addition, a calorifier or heater which is heated, usually with steam.

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Fresh water supplied for drinking and culinary purposes must meet purity stndards specified by the Department of Transport. Water produced from most evaporator/distillers will not meet these standards and must be treated to ensure it is biologically pure and neutral or slightly alkaline.

automatic domestic water supply system
Fig: Automatic domestic water supply system

The fresh water pumps take suction from the tank that is in use, pump the water via a neutraliser, which makes the water slightly alkaline and improves taste, and a hypochlorinator to sterilise the water, to the pneupress tank, which comprises of water and an air space at the top and is pressurised to around 4 bar with compressed air.

As the water level drops, the pressure drops and when the pressure drops to around 3 bar, a pressure switch is activated which starts the fresh water pump. The water level in the pneupress tank will then rise, causing the pressure to build up, and once it reaches 4 bar another pressure switch is activated that shuts down the fresh water pump.

From the pneupress tank, the water passes through a sand bed and carbon bed filter to absorb any excess chlorine in the water. It then branches off to the cold supply or through a calorifier for the hot water supply. Note that all these filters can be backflushed by arranging the cocks on them for cleaning.

In domestic fresh water systems, carbonates of calcium and magnesium are used to produce water that is slightly alkaline and to improve its taste.

Chlorine is used in fresh water systems to sterilise the water, the amount used is a solution of 100000 : 1.

Domestic water systems

Fig: Domestic water systems on board

. The water is sterilised by an excess dose of chlorine provided as hypochlorite tablets. It is then dechlorinated in a bed of activated carbon to remove the excess chlorine. Any colour, taste and odour which was present in the water will also be removed by the carbon. Excess chlorine is originally applied to ensure that complete sterilisation occurs,



A treatment plant suitable for a general cargo ship is shown in Figure

Domestic water treatment

Fig: Domestic water treatment on board


Fresh water supply

Domestic water supply either produced on board or bunkered must be sampled frequently and treated in accordance with statutory regulations. Records are to be maintained of chemical treatment dosage. In case of any doubt on the quality of drinking and cooking water, the office must be consulted. Equipment for the sterilisation and or demineralisation of drinking water is to be maintained as specified by the Manufacturers.

Every vessel shall carry designated hoses for taking on potable water. They are to be carefully stowed and labelled “Potable Water Use Only.” Care must be taken to ensure that these hoses are kept clean and are kept free from any contamination.

Potable water tanks are to be emptied for inspection and maintenance as specified in the vessel’s planned maintenance system. They are to be thoroughly scrubbed, flushed out and the entire system disinfected with chlorine as described in the International Medical Guide for Ships or the UK Ship Captain’s Medical Guide. Any personnel entering a potable water tank must wear absolutely clean clothing and footwear. No person suffering from skin infections or communicable diseases are to be allowed to enter the tank.



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