How a major city gets its water

Niagara Falls carries 19,000 million US gallons of water over its brink every day. It would take the thundering flow of Niagara 17 days to fill up the 21 main reservoirs that New York City relies upon: 320,000 million gallons. The largest city reservoir, Pepacton, alone holds enough water to flood the whole of Manhattan to a depth of 40ft (12m).

Each day New York consumes 1423 million gallons, including the water used by its factories and offices. That comes to about 200 gallons for every person in the city. New York’s distribution system takes it to the consumers’ taps via more than 5700 miles (9000km) of water pipes.

In Britain, the daily requirement  for domestic use only – of the Thames area, which includes London and Oxford, is 710 million Imperial gallons. The region’s 12 million inhabitants each use an average of 35 gallons of fresh water a day. More than a third is flushed in lavatories and a further 12.5 gallons is used for washing, showers and baths. The remaining 10 gallons are used for washing clothes, dishes and cars, drinking, cooking and – depending on the season – gardening.

City water supplies generally come from rivers – New York, for example, gets most from the Hudson and Delaware Basins. The New York-West Delaware supply tunnel, which runs for 106 miles (170km), is the world’s longest tunnel of any kind.

More than half the tap water supplied in the Thames area is extracted from the River Thames itself, while the rest is raised from underground lakes and streams through boreholes or chalk wells. By 1996, London’s water will be distributed via a 50 mile (80km) underground ring main consisting of pipes 8ft (2.5m) wide.

The water is channelled into screening and pumping stations, where coarse screens filter out the heavier debris and pumps raise the water to storage reservoirs.

Because water in reservoirs is still, solids sink to the bottom. At the same time, oxygen from the air neutralises other chemical or organic impurities.

A system of sluices takes water from the storage reservoirs to a treatment plant, where further purification takes place. The usual method involves filtering the water twice through sand beds which are cleaned daily. In the first bed, the water sinks through coarse sand, which traps larger impurities. The process is repeated through finer sand.

The water is chemically treated with chlorine in a closed tank to kill bacteria, and then de-chlorinated to remove the chemical taste. It is then pumped under pressure into trunk mains – large underground or overground pipes – which carry it to the users’ taps.

Treated water pumped into the mains may be used immediately – or diverted for temporary storage into service reservoirs or water towers. They are usually on high ground, although some service reservoirs are underground, beneath hills in public areas such as parks.