FRESH WATER STORAGE AND DISTRIBUTION SYSTEM

The following principal features should be carefully considered as per regulation to ensure the supply of wholesome fresh water on board ships which is to be bacteria free, clear and virtually colourless.

1.                   Fresh water obtained from shore mains supply or water barge - The transfer hose for fresh water is to be marked and kept exclusively for that purpose. The ends must be capped after use and hose must be stored clear of the deck to reduce the risk of contamination.

2.         All freshwater taken from,shore should be chlorinated on loading to a sufficient concentration to ensure a residual free chlorine content of 0.2 ppm.  This concentration may be achieved by traditional manual method or by using an automatic chlorination unit in the ship's deck filling line. The concentration may be checked by means of a Lovibond Comparator Kit.

3.         Fresh water from low pressure evaporator or reverse osmosis plant - Water from such plants should in general only be produced when the vessel is at least 20 miles from land. The sea water suction to evaporator or reverse osmosis should be separate from other machinery sea suction and sited forward and on the opposite side of the ship from sanitary or bilge discharges. All water making plants should be fitted with automatic chlorination unit, ultra - violet sterilization or Electro - Katadyn  silver ion system.

STORAGE TANK ARRANGEMENT :-

4.         Storage tanks and delivery system intended for drinking or washing water should be independent of any other services wherever possible. For a common service line, there should be a clear air break, efficient non - return valve, vacuum breaker or back - flow preventer.

5.         Siting of tanks - Tanks intended for drinking water should normally be sited above the inner bottom and independent of the hull, but tanks other than peak tanks not independent of hull may be utilised if they are of all welded construction and readily accessible to faseilitate inspection, cleaning and coating. In ships with only one fresh water storage tank sited in the double bottom, an alternative reserve drinking water tank should be provided for use in emergency.

6.         Construction - Internal structure should be designed to ensure efficient drainage through adequate limber holes to the suction and in general, continuous welding should be used. No freshwater should have a common boundary with a tank containing oil or any other liquid except clean water ballast.

At intervals of 5 years a pressure test of fresh water has to be carried out.

7.         Coatings - Freshwater tanks when new. should be thoroughly wire brushed, scrubbed and primed before coating with cement wash and thoroughly aired before filling.

8.         Distribution System - Water treatment, filters, mineralisers,        softeners etc. – All sea water drawn by an evaporator or reverse osmosis plant should be passed through separate sand filters, before being used in water - making. A chlorination plant and a neutraliser or mineraliser may be inserted between the water – maker and auto - chlorinator.

9.         Freshwater distribution pumps - These should be dedicated to domestic freshwater services only

10.         Calorifier, pressure tanks etc. - These should be designed to avoid stagnant zones forming to ensure complete draining of loose scales or sludge.

11.         Piping - Care should be taken not to run hot and cold water pipes adjacent to one another, unless the pipe, are adequately insulated to prevent transfer of heat from hot to cold lines.

12. Overall design - System should be designed to provide maximum circulation of the system and to avoid dead lags where temperature could rise which may promote bacterial growth (15°C to 50°C)

13. Fittings & accessories - All items used in the construction of ship's freshwater plumbing systems should not provide a habitat for bacteria, which can occur in the case of natural rubber, various plastics and fibre accessories or leach out toxic constituents.

MAINTENANCE :

Freshwater Storage Tanks - It is recommended that these should be opened up, emptied, ventilated and inspected at intervals not exceeding 12 months and thoroughly cleaned, recoated as necessary, aired and refilled with clean freshwater chlorinated to a concentration of 0.2 ppm. The cleaning process should include disinfection with a solution of 50 ppm chlorine. In addition tanks should be thoroughly pumped out and where necessary, hosed prior to refilling at approximately 6 month intervals.

It is further recommended that tanks should be super - chlorinated at a concentration of 50 ppm for a period of not less than 4 hours and then completely flushed out and refilled at 0.2 ppm concentration at every refit or dry docking period.

Persons inspecting or working in freshwater tanks should wear clean clothing and footwear which has not been used for any other work areas and should not be suffering from any skin infection or communicable disorder.

Distribution System :- The various elements of the freshwater production, treatment, and delivery system i.e. sand filters, evaporators, reverse osmosis plant, auto - chlorinator, neutraliser/mineraliser, softeners, pumps, pressure tank, calorifier, carbon filter, U-V sterilizer should be inspected, cleaned, flushed out, back washed, re-charged or items replaced where appropriate, in accordance with maker's instructions.

Some items e.g. filters should be cleaned on monthly basis or to be replaced to ensure it has not become contaminated by bacteria or other foreign matter. Calorifiers should be opened up and Inspected, scaled and cleaned periodically and before draining should be raised to a temperature of 70°C for at least 1 hour to ensure destruction of bacteria which may have colonised the lower and cooler zone of the unit. At every refit or dry docking period the whole delivery, tanks and distribution system should be charged with super - chlorinated freshwater at a concentration of 50 ppm residual free chlorine and left for a period of 12 hours.

After flushing through the storage tanks should be chlorinated to a concentration sufficient to maintain 0.2 ppm residual free chlorine.

Shower heads and their flexible pipes where fitted should be thoroughly cleaned in a 50 ppm chlorine solution routinely every 3 months.

Hoses :- Disinfection of hoses should be carried out as a routine measure every 6 months with a solution of 50 ppm residual free chlorine and allowed to stand for a period of at least 1 hour, before the hoses are emptied and restowed.

BACTERIA INFECTION :

The low operating temperature of the evaporators is not sufficient to sterilize the product. There is a likelihood that while in domestic water tank, water may become infested with bacteria.

Micro - organisms, of which the Legionelfa bacteria is one, can enter the human body by three routes■:-

(i) Via the digestive tract - contaminated water can enter the stomach and dilutes the resident acid there, thus reducing its anti - microbial properties.

(ii) Via cuts and abrasions - microbes that are not normally invasive can cross the normally impermeable barrier of the skin.

(iii) Via the respiratory system with very small water droplets - only very small droplets (5 micron and less) can penetrate the respiratory system and can cause a reaction of infection in the terminal air passages and air spaces. Legionaires disease is a form pneumonia., which is caused by aerolised water (droplets less than 5 micron) when inhaled in showers and air - conditioning system. This bacteria grows over a range of temperature 24° - 45°C, with particularly rapid growth at 36°C. It feeds on the by - products of slime, forming microbes. Hence any situation which is warm and contaminated with microbial slime can encourage the proliferation of Legionella. Legionella is an opportunist pathogen, a parasite, producing damage in its host.

DISINFECTION :

1.     Chlorine Sterilisation and Conditioning - Initial treatment involves passing the distillate through a neutralite unit containing magnesium and calcium carbonate.
Some absorption of Carbon dioxide from the water and the neutralizing effect of these compounds, removes acidity. The addition of hardness salts also gives the water better taste. The water then passes to chlorine sterilisation unit via a
control orifice. Chlorine being a gas, is carried into the water as a constituent of sodiumhypochlorite (a liquid) or in granules of calcium chloride dissolved in water.
The addition is set to bring chlorine content to 0.2 ppm. The water then passes through a sand filter and Carbon filter which removes the chlorine taste.

Chlorine is a blue/green gas and vaporisation takes places by sunlight or heat. In addition to its disinfection property, it is toxic to marine life. The lethal concentration for fish is 0.3 to 1.0 mg/litre of free chlorine in water. Up to 50 mg/litre can be tolerated by humans, as chlorine is destroyed by saliva and reaction with gastric juices.

2.    Electro - Katadyn method of Silver - ion sterilisation - Silver - ion sterilisation is accepted as an alternative to chlorination. There are three methods of adding silver ions to water -

i)    The contact principle has water flowing over a large silver area, the metal slowly dissolving.

ii)   Manually adding silver salts or solutions, this allows for precise dosing. Large volume flows being treated by a metering pump.

iii)   Electrolytic process, where positive silver ions are releasd from a silver anode.

in the electro - katadyn process, silver anode injects silver ions (Ag ⁺ ) into the distilled water product of low temperature evaporator. Silver is toxic to various risk organisms. Unlike the gas chlorine, it will not evaporate but remains suspended in water.

The steriliser is placed close to the production equipment with the conditioning unit being installed after the sterilizer and before the storage tank. The amount of metal released to water passing through the unit, is controlled by the current setting. If a large volume has to be treated, only part is by passed through and a high current setting is used to inject a large amount of silver. The bypassed water is then added to the rest in the pipeline with low water flow, all of the water is delivered through the device and the current setting is such as to give a concentration of 0.1 ppm of. silver. The silver content of water in the domestic system should be 0.08 ppm maximum.

Silver is non - toxic, non - corrosive, poses no health hazard and residual  silver in the water prevents reinfection and does not have the evaporation problem. The operating cost is relatively low. One electrode has the capacity to treat 4000 m³ of water during its life span.                                           

Ultra - violet irradiation - Disinfection can be achieved by UV - radiation, however, use of this method does not inhibit re -Infection.

The UV - unit is positioned after the hydrophore tank and as close as possible to tap supply points. The stainless steel irradiation chamber containing low pressure mercury vapour tubes, housed in a quartz jacket. Tubes are wired in series with a transformer for safety. A wiper is fitted within the chamber to clean the jackets and lamp observation windows.

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