In San Luis Potosí, the right mix of local conditions and political support combined with a creative project funding and design that incentivized wastewater reuse, which made wastewater treatment economically and environmentally sustainable. This case study focuses on the largest wastewater treatment plant in the area: the Tenorio-Villa de Reyes wastewater treatment plant (WWTP) that treats 45% of the total wastewater generated by the city and is an example of how a well-implemented plan can benefit both the private and the public sector and have environmental benefits at the same time.

Water is scarce in the state of San Luis Potosí, Mexico. With more than 2.7 million inhabitants in the metropolitan area (as of 2015) and intensive industrial and economic development, the region gets less than 400 mm of rainfall a year. The state government has been a pioneer in understanding wastewater as a resource to utilize rather than a waste only to dispose. Since the 1990s, the city’s development has been tied to water conservation efforts and wastewater reuse. Given the area’s fast growth, the aquifer was being depleted, with extractions doubling its natural recharge.

To protect the aquifer and to promote the use of wastewater for non-potable uses such as for agriculture and industry, the state government implemented an Integrated Plan for Sanitation and Water Reuse. Since then, seven wastewater treatment plants have been built in the region, treating most of the wastewater from the metropolitan area and reusing 100 percent of the wastewater treated.

The innovative aspects of the Tenorio Project are the multiquality use of the treated wastewater to meet the different end users’ needs, and the contractual agreement with the industrial user, which ensure a constant revenue stream and enhance the financial sustainability of the project.

The treated wastewater is used for three purposes: (a) for cooling purposes in a nearby thermal power plant instead of using groundwater; (b) for agricultural irrigation instead of using wastewater and groundwater; and (c) for environmental enhancement to improve the quality in the Tenorio tank, which has been modified to perform as a wetland.

In addition to the economic benefits for both the industry and farmers who rely on a continuous flow of wastewater, the environmental and social benefits include an indirect augmentation of drinking water resources and rehabilitation of the aquifer, a larger biodiversity as a consequence of the improved water quality in the Tenorio tank wetland (especially migratory birds), health improvements (decrease of gastrointestinal and skin diseases rate in the area previously irrigated with raw wastewater), an improved living standard for the population near the wastewater treatment plant as the ecological environment of the zone was enhanced, and the creation of a successful reference case of wastewater reuse.

After six years of operation, groundwater extractions were reduced by 48 million cubic meters, which is equivalent to nearly 20,000 Olympic-sized swimming pools.

Photo by Ivan Bandura on Unsplash