he 1996 Mexican environmental

regulation NOM-001-ECOL establishes the maximum amount of contaminants permitted in wastewater discharged into public water bodies or national property. This restriction aims to reduce wastewater disposal into the river and the negative impacts for health and the environment, through a fine of US $0.25 per cubic metre of untreated water that exceeds the permitted limits. However, the regulation also leads to a reduction in the nutrient values and forms a constraint to irrigation by waste-water. In accordance with this regulation, the public water-supply company of Guanajuato city, called SIMAPAG(2) constructed an activated sludge wastewater treatment plant, which started to operate in June 2002. In this article, the plant’s benefits are reviewed. The aim is to give some preliminary answers to the research question: How does the water-treatment plant influence the benefits of the use of wastewater for crop production? The need to assess these effects appears essential for future, since current national environmental laws and local policies will increase the volume of treated wastewater and actual will change the conditions of raw wastewater irrigation. In Guanajuato’s Hydraulic Plan for 2000-2025, an additional 47 % of wastewater is planned to undergo treatment, which together with the 16 % of wastewater already treated, makes for a total of 53 % of treated wastewater in the near future. The volume of residual waters generated in the 46 municipalities of Guanajuato total 207.13 million cubic meters per year. If this water could be used directly for agricultural purposes, it could irrigate around 20,500 ha, which is almost 5 % of the actual irrigated land in Guanajuato (416,690 ha). There are 16 treatment plants in the urban areas and another 26 wastewater-treatment systems in the rural areas. The lack of technical and administrative capacity prevents the water treatment programmes from being carried out satisfactorily