Mexico City’s wastewater system is undergoing major surgery, with work under way on a large tunnel and a wastewater treatment plant as well as several pumping stations.

The Valley of Mexico, which contains 21.4 million residents across 2,020 sq kilometers, is an enclosed valley with no natural outlet for water to flow. It contains the lakebeds of five extinct lakes.

Overextraction of groundwater has been causing the land on which the city rests to sink for decades. Currently, the rate of sinking is between five and 40 centimeters per year. The existing system of tunnels and pumps, which carry sewage and stormwater runoff out of the valley, has been weakened by the subsidence.

Mexico’s National Water Commission (Conagua) has embarked on a wide-ranging, $4.5-billion program to “increase capacity of the valley drainage to eliminate flooding vulnerability,” according to Jorge Luis Meza Reyna, adviser to the commission’s director general.

One major element of the program is the Eastern Outfall Tunnel. Six tunnel-boring machines are currently at work on the 62-km-long, 7-meter-dia tunnel, which will have a discharge capacity of 150 cu m per second.

Three Mexican contractors—Carso Infraestructura y Construcciones S.A. B de CV, Ingenieros Civiles Asociados (ICA), and Constructora Estrella y Lombardo Construcciones—have been awarded segments of the tunnel, with Constructora Mexicana de Infraestructura Subterranea (Comissa) serving as construction manager. Both Robbins and Herrenknecht TBMs are at work at depths ranging from 26 m to 150 m.

Outfall construction began in October 2010, with completion expected by August 2013. The estimated cost of building the outfall is $1.45 billion, according to Conagua. The eastern outfall will double the drainage capacity of the Mexico City basin. It will convey rainwater and wastewater to the Atotonilco de Tula wastewater treatment plant, which will be one of the largest wastewater treatment plants in North America upon its completion in 2014.

The plant is being built by Consorcio Aguas Tratadas del Valle de Mexico, a consortium made up of Atlaztec, Grupo Carso, ICA, Acciona, DYCUSA and Gas Pioneer. Englewood, Colo.-based CH2M Hill is providing project and construction management. The estimated cost is $750 million. “It will have the capacity to treat 23 cu m per second during the dry season and up to 12 cu m more in the rainy season,” Conagua says.

The plant’s maximum capacity of 35 cu m per day, or 800 million gallon per day, will be larger than most U.S. plants, with the exception of Chicago’s Stickney Water Reclamation Plant, with a 1.44-billion-gallon-per-day capacity; Boston's Deer Island sewage treatment plant, with a 1.27-bgpd capacity; and Detroit’s plant, with a 930-mgd capacity.

Currently, only 6% of Mexico City’s wastewater is treated, compared to the national average of 54%, an “acute lag,” according to Conagua. The completion of the Atotonilco plant will increase Mexico City’s treatment rate to 60%, a tenfold jump. Five additional treatment plants are planned, which, when completed, will bring the treatment rate to 100%. Only one of those, the El Caracol plant, is currently in the design stage.

The Atotonilco plant holds the promise of several dramatic improvements. For one, it will drastically reduce the volume of untreated sewage discharged, thus "facilitating better health conditions for 300,000 people living in the region," says Meza Reyna.

In addition, it will supply the treated wastewater to irrigation districts in the Tula Valley, encompassing 80,000 hectares; the districts currently draw their water from the aquifer. This tactic will help reduce the unbalanced extraction rate—that is, far more water is pumped out than replaced.

Because the Valley of Mexico has no natural outlets, it has long been vulnerable to flooding when it rains. Further, due to land subsidence, many sewers now lack the required gradient. So Conagua is building three massive pumping stations.

The first, La Caldera, which was completed in May 2011, is capable of pumping 40 cu m of wastewater or stormwater per second. A second pumping station, El Caracol, is expected to be completed by June 2012 and will empty into the first segment of the East Outfall Tunnel. A third pumping station, Casa Colorada Profunda, is also under construction.

“During the rainy season from May to August, whole parts of Mexico City had normally been flooded with a mixture of rain and wastewater,” says Christoph Pauly, media relations director for Germany-based KSB, which is supplying the pumps.