Filling procedures: boats' water tanks

Using a storage cistern with fluid category 5 backflow protection

When filling a boat's water-holding tank, it is essential that the quality of water entering the tank is wholesome and that backflow of contaminants into the supply mains pipework is prevented. It must be considered that water already stored on board the boat may not be wholesome.

The recommended method is to supply the hoses used for filling water tanks from a storage cistern incorporating an air gap. (See Figures 1a and b).Without this protection in place, if the mains pressure dropped whilst the hose outlets were submerged in marina water, drains or puddles, backflow could cause contamination of mains water.

Figure 1a. Water supply pumped from a storage cistern incorporating a fluid category 5 air gap backflow protection.

Figure 1b. Water supply taken by gravity from a storage cistern in an arrangement which provides fluid category 5 backflow protection (an interposed cistern)

Requirements for a storage 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 cistern storing water for wholesome purposes must be designed manufactured and installed to ensure:

• all 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 five 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 no less effective warning device)
• a securely fitted lid and/or access cover to enable internal inspection and cleaning of the cistern
• thermal insulation to minimise freezing or undue warming

Marina owners have the responsibility to see that regular cistern maintenance is undertaken to ensure the water quality is wholesome.

Alternative using a mains-fed hose union tap

Subject to the terms and conditions required by us, and with our consent, an alternative arrangement for filling boats' water tanks is a mains fed hose union tap. The hose tap must be installed with an in-line double check valve (type EC or ED device) and be protected against damage by freezing by insulation or other means (see Figure 2). A method statement for the inspection, maintenance or replacement of the backflow device must be agreed with us and implemented by the marina operator. This arrangement is subject to our terms and conditions.

Figure 2. Water supply taken from mains-fed hose union via a double check valve

• hoses for use with mains-fed hose union taps must not exceed 22mm internal diameter
• all hoses left unattended on bank sides and pontoons must be removed by marina staff
• hoses used by the marina operator should be of the retractable lay flat type, removed and securely stored after use
• berth holders and visiting craft must provide their own hoses, preferably of the layflat type, to encourage removal after use
• the use of hose union taps and hoses should be restricted to authorised users by means of locked cabinets or other restrictions on access to them. Additional measures should be considered where vandalism is likely to result in interference with the equipment
• at sewage tank pump-out points, the outlets of mains fed hoses must never be placed in the sewage tanks

Other contamination risks

These include:

• water supply pipes must not be installed where they could be immersed in seawater or river water or exposed to sunlight
• power washers must incorporate an air gap and cistern for backflow protection
• unless fed by gravity from a storage cistern, hose union taps used with chemical toilet disposal points must be fitted with a permanent DC type pipe interrupter (see figure 3), an air-gap to drain or a tundish
• below ground hydrants are accepted only for fire-fighting purposes and must be capped when not in use

Figure 3. Hose union tap with DC device