Backflow means the flow of water in a direction opposite to the intended normal direction and can occur either by backpressure – where the water in the plumbing system is subjected to a greater pressure at a 'downstream' point than that upstream, allowing it to flow backwards, or by backsiphonage – where the downstream end of a pipe or device is only at atmospheric pressure, but the pressure upstream is less, effectively sucking water back upstream.

Boats in marinas 

Consider the use and storage of hosepipes and access to them. Backflow could cause drinking water supplies on boats, elsewhere in the marina or in the surrounding neighbourhood to be contaminated with toxic chemicals or disease causing micro-organisms (bacteria or viruses) from other water, for example sewage, puddles, rivers or sea water. A common cause is hosepipes being left submerged in river or seawater.

Ships in ports 

There have been a number of significant incidents in the British Isles where the public water supply has been contaminated due to backflow at ports and harbours. The most common cause is the cross-connection of pressurised seawater pipework and water supply pipes, resulting in water from the dock being pumped into drinking water systems within the port area.

When initially filling or replenishing ships' wholesome water tanks, backflow could contaminate drinking water supplies on ships, elsewhere in the port premises or in the surrounding neighbourhood. Contaminants could include toxic chemicals or disease-causing micro-organisms (pathogenic bacteria or viruses) from the dockside, from the harbour itself or from unwholesome water already stored on-board.