Atotonilco tender gives boost to Mexican reuse

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As the tender process for Mexico’s giant Atotonilco WWTP finally gets underway, Mark Walsh speaks to the man in charge of delivering the project.

The tender for the two million m3/d Atotonilco wastewater treatment plant in Mexico’s Hidalgo province was published on 12 May, with proposals due in on 7 October. The proposed $615 million plant is the jewel in the crown of the government’s ambitious programme to treat 100% of Mexico City's wastewater by 2012. Less than 10% of Mexico City's 3.5 million m3/d wastewater flows are currently treated, with most of the untreated effluent being pumped directly to the nearby state of Hidalgo.

"It's going to be one of the biggest plants in the world," explains Miguel Guevara Torres, the man in charge of water and sewage provision in the Valley of Mexico. "Most other plants of that size were built with a smaller capacity that was later increased. Because the need to treat wastewater in the Valley of Mexico has been neglected, this plant will have to be gigantic from the word go."

The Atotonilco plant will offer wide-ranging opportunities for private companies. "We are totally open to new technology, and the tendering companies will have the freedom to make their best suggestions," Guevara said.

A total of 49% of the construction funding is expected to come from the National Infrastructure Fund, with the remaining 51% to come from private sector sources, split roughly 20% equity and 31% debt. The government subsidy is capped, however, and given the ongoing decline of the Mexican peso against the US dollar, there are concerns among prospective bidders that the project company may ultimately struggle to service the debt taken out to cover the construction costs through tariffs alone.

The plant will employ a two-phase treatment process: a secondary biological stage to treat 1.99 million m3/d at regular capacity, and a physical-chemical stage to boost capacity to more than three million m3/d, including stormwater flows during Mexico's rainy season.

The treated water will be reused primarily for agriculture, and given that farmers in Hidalgo already use untreated wastewater from Mexico City for irrigation purposes, the main difference will be to give farmers the possibility to irrigate land with treated wastewater, which will allow different, export-grade crops to be cultivated. To further boost the penetration of water reuse in Mexico, legislative changes are going through which will require the use of treated wastewater for agricultural purposes where potable-grade water is not strictly necessary.

Federal backing from the National Infrastructure Fund for the Atotonilco WWTP was authorised on 13 April, and although there is no formal prequalification stage, the first official site visit for interested parties is planned for 21 May, with the first technical clarification meeting scheduled for 5th June.

Despite the federal government’s backing for the construction of the plant, the local Federal District (DF) government has dragged its feet over the funding to pay the operators of the plant. Negotiations between the two levels of government are still underway, and while Guevara admits frustration with the ongoing impasse, he insists that the problems will not impede the progress of the Atotonilco project.

"It would have delayed things if the decision had not been taken to release federal funds, but now we can go ahead with the project despite the delay in signing the agreement," he said.