By David Lazar

With the ground sinking due to the depletion of the aquifer, Mexico City was faced with the prospect of building pump stations to lift wastewater up and out of the city, which was once served by a gravity system. The municipal government selected 20 large propeller pumps which will be used to pump wastewater and storm water out of the metropolis.

One of the world’s biggest cities
Mexico City is the cultural, economic and industrial center for the Mexican nation. With a population of approximately 22 million, roughly equivalent to the entire state of Texas or New York, it has become a magnet for growth. There is a continuous migration of people from rural areas to the city in search of work and the general economic benefits generated from the centers of political power. Many of these emigrants settle illegally in the urban fringe as squatters, which complicates water and sewer issues further.

The provision of water and wastewater service for this growing population presents a formidable challenge. Almost 72 percent of the city’s water supply is derived from the Mexico City aquifer, which underlies the metropolitan area and which is being substantially over-exploited.

A sinking feeling
In 1519, when colonizing Spaniards conquered the Aztec capital, Tenochtit-lan, there was plenty of water; the city straddled two lakes. Then those Spanish conquistadors built their own Mexico City adjacent to the Aztec’s and brought engineers to drain the lakes.

Early in this century the city exhausted its natural springs. Well digging began, and as the city pumped more and more water the soil began to give way. Scientists claim Mexico City is slowly sinking due to the depletion of the subterranean water level. Giant aquifers that serve as water reservoirs are being emptied faster than they can be refilled, causing the ground to sink. In the early decades of this century, ‘sinkage’ measured at the city’s center averaged about two inches lost annually, but when it peaked at mid-century the soil was collapsing away at the astonishing rate of 19 inches a year.

Many cities have experienced subsidence. The most famous in that regard, Venice, Italy, sank about nine inches during the 20th century as its water table dropped. Yet by comparison, Venice’s problems seem marginal. Mexico City has sunk an astounding 30 feet in the same time period.

The water difficulties have become a vicious circle: as the city grows, more water is pumped from the aquifer. As more is pumped, the city sinks further. The sinkage ruptures more underground water pipes, sending fresh water gushing into the sewers, aggravating the shortage, requiring more water to be pumped from the aquifer, and so on.

This subsidence has also caused serious sewage disposal problems. The Gran Canal de Desague is the sewage canal for the city, but as the city fell so did the Canal. Since the 1970’s, the city has sunk so much that the water authorities could no longer count on gravity to carry the sewage away as Mexico City has expanded the sewage treatment and water recycling facilities. Today it takes 11 pumping stations running 24 hours a day all year round to keep the sewage flowing and keep the summer rains from washing it back to the city.

Two new stations
With its huge population, Mexico City produces a lot of waste and wastewater: approximately 60,000 liters of sewage are created every second. In addition, there is storm water, 95 percent of which falls between April and October. Very powerful pumps are needed to move such quantities. In December 2001, Mexico’s largest water company, Direccion General de Construcccion y Operation Hydraulica, DGCOH, placed an order for 20 high voltage propeller pumps for use in two newly built pump stations.

These units are designed to pump large volumes of water at relatively low heads. Typical application areas for this series of pumps include storm water stations, sewage treatment plants, land drainage, irrigation, aquaculture and water attractions. In Mexico City, 14 of the 300 kW pumps will operate at Gran Canal pump station, located on the Gran Canal, which flows through the city to the northeast. In addition, six 480 kW pumps will be sited on the Rio Hondo River, which flows through the city to the northwest. At the end of July, 2002, the 20 heavy duty submersible propeller pumps went into operation in the Grand Canal and the Rio Hondo River as part of Mexico City’s improved wastewater pumping system.

Conclusion
Although the water is now extracted from deep wells instead of the aquifer, the clay is still affected. Throughout the metropolitan area of Mexico City, the ground level continues to sink by between 5 and 40 centimeters a year.

About the author
David Lazar is a freelance writer specializing in articles relating to fluid technologies. He can be reached at ITT Fluid Technology, Inc., c/o Bjorn von Euler, 10 Mountainview Road, Upper Saddle River, NJ 07458, (201) 750-9800, (201) 760-9692 (fax), email: [email protected] or the website www.ittind.com. For more information: www.flygt.com

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