Wastewater can be reused after a treatment process and compliance with high quality standards that guarantee its safe use. The wastewater treatment plant of the Autonomous University of Aguascalientes (AUA), like others, uses primary, secondary, and tertiary processes. The tertiary process followed is chlorination and is used to eliminate microorganisms from the secondary process. Although water of acceptable quality is obtained with chlorine, there is evidence that toxic substances are generated when reacting with organic matter, so alternatives to the use of chlorination have been analyzed. In the present study, silver nanoparticles were synthesized from the aqueous extract of the Opuntia ficus indica fruit peel (OfAgNPs), by reducing a 2 mM solution of AgNO3. OfAgNPs were characterized by UV-visible spectroscopy, scanning electron microscopy, energy-dispersive X-ray spectroscopy, atomic absorption spectroscopy, and dynamic light scattering, in addition to his electrophoretic mobility. The OfAgNPs are spherical, with an average particle size distribution of 64.28 ± 11.82  nm, relatively stable at room temperature, negatively charged ( − 25.1 ± 0.03  mV), and composed of 61.29% silver. The activity of OfAgNPs was evaluated in water from the effluent of the AUA treatment plant, before and after chlorination, and inhibition of bacteria Escherichia coli var 1, Enterobacter aerogenes var 1, Citrobacter freudi var 2, atypical E. coli, and aerobic mesophilic microorganism was tested.