Niagara Falls carries 19,000 million US gallons (71,900 million litres) 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: 3,20,000 million gallons (more than one million litres). 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 (5400 million litres), including the water used by its factories and offices. That comes to about 200 gallons (750 litres) for every person in the city.
In Britain, the daily requirement – for domestic use only- of the Thames area, which includes London and Oxford, is 710 million imperial gallons (3200 million litres). The region’s 12 million inhabitants each use an average of 35 gallons (160 litres) of fresh water a day. More than a third is flushed in lavatories and a further and 12.5 gallons (56 litres) is used for washing, showers and parts. The remaining 10 gallons (45 litres) are used for washing clothes, dishes and cars, drinking, cooking and – depending on the season – gardening.
City water supply is generally come from rivers – New York, for example, gets most from the Hudson and Delaware Basins. The New York West and 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 miles (80 km) underground ring main consisting of pipes 8 feet (2.5 m) wide.
The water is channelled into screening and pumping stations, where coarse screens filter out the heavier debris is 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 the 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 maybe used immediately or diverted for temporary storage into service reservoirs or water towers. There are usually on high ground, although some service reservoirs are underground, beneath hills in public areas such as parks.
Picture Credit : Google