Treatment plant effects on wastewater irrigation benefits: revisiting a case study in the Guanajuato River basin, Mexico.

In 1999 field research was carried out to explore the advantages and risks of urban wastewater use for 140 ha of crop production in the Guanajuato River basin. It was found that wastewater which was freely available to the farmers represented an important additional source of irrigation water, with secondary benefits including nutrients and the foregone cost of wastewater treatment. In 2002, the urban water supply and sanitation utility, a financially autonomous public utility, began to operate an activated sludge wastewater treatment plant in response to the imposition of legally mandated fines for the release of untreated wastewater to open water bodies. As follow-up to the 1999 study, this chapter is based on field visits and interviews and sets out to qualitatively answer the following research question: Does the introduction of wastewater treatment influence the crop production benefits of wastewater irrigation? The study found that because wastewater treatment was oriented to comply with environmental regulations, little attention was paid to the links with the land irrigated by wastewater. The presence of the treatment plant provides the utility with the option of selling treated wastewater, thus increasing its own economic benefits. Industrial users appear to be the most suitable potential customers; the utility would stand to receive US$0.43/m3 in estimated sale price plus saving the US$0.25/m3 fine. This transfer of water would introduce competition among water-use sectors, a process that is already leading to wastewater farmers’ uncertainty about their future share of irrigation water. However, to date no commercial transaction to transfer treated wastewater to non-agricultural users has taken place. For this reason the expected changes in impacts on wastewater farmers have been minimal. If this happens, however, the wastewater farmers stand to lose because only about 30% of the wastewater-irrigated land has a water concession title (linked to the land) issued by federal authorities.