Fresh Water Sterilisers

Silver ion sterilisers

The use of silver ion sterilised is recognised as a method of water sterilisation, the following are the requirements for its usage;

  1. A set of manufacturers instructions is to be located with the set.
  2. The system is designed to cope with the maximum flow of the production plant and is not to be bypassed and indeed no facility for by passing should be fitted.
  3. It should be readily accessible for maintenance, and be as close to the production equipment as practicable
  4. The system is to be fail safe in that it should prevent the passage of water by closing a fail-closed solenoid valve. An audible, independently supplied failure alarm should be fitted
  5. The set should be capable of maintaining the minimum concentration of 0.1 ppm silver ion concentration at maximum flow rate.
  6. The fresh water storage system should be so set up that a four hour period be allowed for contact with the silver ions. This should allow a maximum level of 0.08 ppm concentration of silver ions in the system, which should be checked regularly by a competent laboratory.
  7. Water conditioning should be installed after disinfecting unit.
  8. Spares of not less than the manufacturers recommendations should be carried.

A D.C. voltage driven silver coated anode releases silver ions into the water. Silver is toxic to many organisms. In addition the silver ions do not evaporate as in the case of chlorinating. The rate of ion release, and thus anode wastage is governed by the current flowing. This is adjustable on the controller.

Recommendations to prevent contamination of ships freshwater storage and distribution systems

With recent research it is required that more close attention is paid to the quality of fresh water storage and disinfection due to the potential dangers of bacteria such as legionella or to the prescience of toxic chemicals.

Freshwater loading and supply arrangements

Fresh water loaded from shore mains or water barge- should be done so via dedicated hoses. Every effort should be made to avoid possible contamination of the loading line and manifold

Treatment of freshwater- All fresh water loaded from shore or barge should be tested and treated to ensure a residual chloride reading of 0.2ppm

Fresh water from low pressure evaporator or reverse osmosis- Should only be made when outside 25 miles from shore or 50 miles from an estuary. The suctions should be separate to other suctions and forward of effluent outlets. Antifouling equipment used should be of an approved type suitable for use with potable water production.

New ships fitted with an ultra-violet steriliser must in addition be fitted with an automatic chlorinator system. As an alternative full flow silver ion systems may be used

Alternative means for sterilisation may be submitted to the classification society for approval.

Storage tank arrangements

Storage tanks and delivery systems intended for drinking or washing water-It is preferable that systems used for drinking and washing are kept isolated from the system supplying such circuits as jacket water and oil purifier seal water. Where this is impractical then effort must be made, say by the fitting of efficient non-return valves or an air break in the pipe work to prevent back contamination.

Sitting of tanks-Tanks designed for drinking water should be sited to allow easy inspection and maintenance. Fore peaks which are susceptible to damage should not be used or aft peaks which are too difficult to clean. No freshwater tank should be sited to an oil tank.

Construction-The internal structure should allow easy drainage and cleaning. The tank should be pressure tested every 10 years to ensure no seepage in or out, particularly where the tank is adjacent to the ships side. Manholes should be of an adequate size and raised above deck level. Only pipework containing the same quality of water in the tanks should be allowed to pass through the tank. Goose necks should be suitably designed to prevent the ingress of sea water

Coatings-Coatings should be applied and allowed to cure strictly with adherence of the manufacturers instructions.

Distribution systems

Water treatment, filters, mineralises, softeners etc.-Water supplied to evaporators and reverse osmosis plant should pass through a suitable filtration system. A form of sterilisation must be applied to fresh water. Neutralisers and mineralises may be optionally fitted to make the water more palatable.

Pumps-must be dedicated for freshwater only with no cross connections.

Calorifiers, hydrophore tanks etc.-Should be designed to allow proper cleaning and prevent sludge and scale build up

Design of fw systems-Dead legs are to be avoided, with temperatures between 15-50oC these can promote bacterial growth. Hot water line should be suitably insulated especially where they run adjacent to the cold water line. Special attention to the toxicity of material used in jointing and mechanical of the pump should be made at the design stage.


Freshwater storage tanks-These to be inspected every 12mths an refilled with sterilised water following cleaning with a 50ppm chlorine solution

Distribution system-Including sterilisation plant, should come under the planned maintenance system and regularly inspected. Chlorinators should be raised to 70oC before opening for inspection to kill of any bacteria colonies.
Opportunity should be taken at refits to flush the system with 50ppm chlorine solution.
Shower heads and taps should be cleaned in a similar solution every 3 months.

Where water production systems utilise treated water for cooling (Condensate cooled evap on steam ship) or heating (jacket water evap on motor ship) only approved treatment chemicals should be used.