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The WWTP of Piedras Negras is located in the state of Coahuila (Mexico). The works began in 2000 in the framework of Cross-border cooperation between the USA and Mexico. The wastewater from the city of Piedras Negras was poured into the Rio Bravo, which in this area forms the natural border between the two countries and later, the river enters again in the USA territory, and at that moment carried with him the polluted waters due to its passage through Piedras Negras.

The WWTP of Piedras Negras has the following treatment steps, which are listed below following the water line:

- Pumping station network outside of the plant (distributed in the urban area)

- Headworks (where the different sewers pour the waste water into the plant)

- Pre-treatment channels (gross screening, fine screening and grit-grease removal)

- Distribution tank to biological treatment

- Anoxic chamber

- Biological carrousel tanks

- Aeration system for biological tanks

- Process control units

- Recirculation and purge of excess sludge

- Secondary settling

- Outlet of treated water and disinfection

- Sludge thickening

- System of sludge conditioning and mechanical dewatering

- Technical buildings and control room