Filling procedures: ships' water tanks

When filling a ship's water holding tank, it is essential that the quality of water entering the tank is wholesome and contaminants are prevented from entering the supply mains pipework by backflow.

The water being loaded onto the ship can become contaminated by mixing with unwholesome water already stored on board or by cross connection with pipelines carrying sewage or other contaminants within the vessel. Prevention of this includes good design and maintenance of the ship’s water and waste systems and good management by the vessel's operators of water supply, waste and other systems, such as fire-fighting systems.

Hoses used for filling operations can lead to contamination of mains water by the hose outlet being submerged in the dock water or in drains, puddles or other fluids on the dockside or by backflow of contaminated water from the ship's holding tanks. These are Fluid Category risks and the backflow prevention devices allowed by the Byelaws are all types of physical air gaps between the mains pipework and the ship's intake. A tundish arrangement can provide this air gap, but it is unlikely to be practicable. The recommended method is to supply the hoses used for filling the ship from a break cistern incorporating an air gap (see figure 1).

Figure 1: Break cistern and booster pump for backflow protection of mains water supply during filling of ships with water 

Requirements for a break cistern

The cistern, whether fixed or mobile, must be of a suitable size to encourage good turnover of stored water and the inlets and outlets must be designed and located to minimise stagnation. The cistern must be protected against the ingress of environmental contaminants.

Every break cistern storing water for wholesome purposes must be designed manufactured and installed to ensure:

• all construction materials in contact with the stored water are suitable and do not cause deterioration of water quality; non-metallic materials must comply with British Standard BS 6920
• a suitable air gap to provide fluid category 5 backflow protection. If using a type AB air gap, there must be a slot type weir conforming to the requirements for a type AB air gap, sized to suit the maximum inflow and screened to prevent ingress of insects or other contaminants. A spreadsheet to assist in designing a type AB air gap is given on the WRAS website
• a screened warning pipe (or other no less effective warning device) and a screened overflow to prevent the ingress of contaminants
• a securely fitting lid and/or access cover to enable internal inspection and cleaning of the cistern
• thermal insulation to minimise freezing or undue warming

The port authority has responsibility to see that regular cistern maintenance is undertaken to ensure the water quality is wholesome.