Preventing A Sewage Flood!
Saving Mexico City’s Sewers

By  Pedro Sanchez

In October 2007, a catastrophic flood occurred in the city of Villahermosa in Mexico’s state of Tabasco, creating a state of emergency that placed thousands of lives at risk. The flood had many repercussions in the capital, Mexico City, where 20 million people were living with the potential for a similarly deadly flood. The city’s aging sewer system had not been maintained for 35 years due to time and cost considerations. But after the devastating experience in Villahermosa, the government decided they could no longer ignore it. Because of the potential for unpredictable decimation, the authorities decided to inspect the city’s main sewer system to assess the situation.

“Sistema de Aguas de la Cuidad de México,” the public office in charge of the city’s sewer system, hired a team of engineers to inspect the 60 km long, 6.5m diameter, 35-year-old sewer. First, to access all areas of the pipe, the personnel in charge of the inspection used a boat to go through the pipe to evaluate the tunnel’s condition and the damage in the pipe concrete walls. Later on, when the waste water was drained, they used an all-terrain car named “unimog.”  Based on the engineers’ findings, the authorities decided to repair and recoat the concrete pipe with Belzona 5811Immersion Grade sprayable epoxy.

Not only is a functioning sewer a municipal concern, in Mexico City, it is also geographic. For many years, this city has had to fight against continuous flooding due to the nature of its location. Founded on land dredged from an old lake, the necessity of draining water out of the city forced authorities to construct a massive system to channel the flow of water outside city boundaries. This elaborate system also meant that parts of the sewer can be diverted, which in turn meant that the coatings project would not have to be worked “live” over raw sewage. The coatings crew, a local team called Ivresse, was given a tight 28 day window in which to repair and recoat 12,000 square meters of deteriorated concrete. Fortunately, they also worked during the “dry” season when the level of water in the system is at its lowest. 

In order to access the pipes, located 70 to 80 meters underground, crew members first checked in with the safety monitor. They then boarded an elevator and, reaching their underground job site, constructed a special cart, the wheels of which ran at a 45° angle along the pipe’s diameter, adequate to easily move the equipment and the crew, and structured to help the applicators reach the top of the pipe. All the personnel – engineers and applicators --were also required to wear safety harnesses and full PPE including 3M half-mask respirators, goggles, suits, and gloves. Further, anyone entering the sewer had to be trained in confined space safety and was required to log in and out with a safety monitor. Gas detectors constantly monitored the air for gases and fans on the surface connected to pipes in the sewer insured that fresh air was constantly available.

As in every sewer, the top half of the concrete pipeline had been attacked by acid gases generated by the decomposition of organic materials transported in the water. The acid contributes to reducing the pH and drives a consequent risk of passivation (when a material becomes “passive” in relation to another material) on the reinforcing steel, as well as creating a sulfates and alkali-aggregate reaction. All of these factors generate expansive reactions that destroy the concrete and the reinforcing steel.

The decomposition of sewer waters produce sulfuric acid, high amounts of methane, and other chemicals such as hydrochloric acid and sulfuric acid, all of which constitute chemical attacks on the pipe. These chemicals cause damage to both the concrete and the steel.

In this instance, the lowest level of water in the pipe was 50cm and, on occasion, the pipe was under pressure.  Furthermore, the transportation of all sorts of debris caused abrasion on the concrete walls. This abrasion led to cracks and exposed steel supports which could compromise the structural integrity if not promptly treated.

The crew first pressure washed the concrete using 5,000 psi high-pressure water jetting to clean the substrate and to create a surface profile of three mils minimum. Once the surface had been prepped, they spray-applied Belzona’s 5811 Immersion Grade two-component epoxy using a Graco plural component system. Applied at a 3:1 ratio by volume at 10 mils per coat, the team applied two coats, the first a gray, the second a beige.

An appealing feature of this product is its capability to be applied under the “wet” conditions of the substrate and the environment. Durability was essential in selecting the right product since the repair needed to be low maintenance, show resistance to the existing variety of acids in different concentrations, and be cost-effective in the long term. Another feature of this product is its ability to resist negative hydrostatic pressure from the soil.

Thanks to this coatings application, the city has hopefully not only avoided a flood, but also extended the operational life of its sewer system. Of course, this unique drainage system must be maintained to avoid higher maintenance costs due to possible major failures that can lead to serious consequences for the city. However, the recently applied protective coating should ensure the permanence of the structural integrity of the system, extending the service life for at least 10 years as well as reducing overall costs to the Mexican nation.

Pedro Sanchez is a Technical Service Engineer for Belzona Inc. He may be contacted at 2000 NW 88 Court, Miami, FL, (305)594-4994, www.belzona.com, psanchez@belzona.com.

 

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