Background ¿ Climate change and population growth are intensifying global water scarcity, necessitating innovative approaches to wastewater reuse, particularly in agriculture. Each year, an estimated 14.4 million m3 of nejayote, a strongly alkaline byproduct of the maize nixtamalization process with high organic load, suspended solids and calcium, are discharged into the water bodies in Mexico. Method ¿ The feasibility of coupling nejayote wastewater from small-scale tortilla producers with urban agriculture irrigation, through bioconversion treatment using an alkaliphilic microalgal-cyanobacterial consortium in an open HRAP photobioreactor, was evaluated. Integrating estimations of collection efficiency, treatment effectiveness, reuse viability, cost¿benefit ratio, and carbon footprint, the study assesses three collection scenarios using a novel Environmental Assessment Score (EAS) within the urban food system in Puebla, Mexico. Results ¿ Results indicate that all scenarios have potential for partial wastewater treatment and resource recovery, with treatment efficiency of 55% and a 160% improvement in soil water capacity retention. The 50% collection efficiency achieves the highest cost¿benefit ratio (91.1) suggesting that a 50% of HRAP system offers the most economically balanced configuration. This outcome likely reflects the optimization of operational costs relative to benefits where the system captures sufficient economies of scale to reduce unit treatment costs without incurring the logistical and energy inefficiencies associated with a 100% of nejayote recollection. Nevertheless, the 100% collection scenario achieved the highest overall EAS (77.63) since environmental and resource recovery benefits slightly outweigh the cost penalties at higher collection rates. Conclusion ¿ The assessment demonstrates that the HRAP-based fermented nejayote has potential as a treatment and valorization strategy for regional sustainability transitions by embedding circular economy practices in the tortilla production sector. Yet, for full-scale adoption, technological improvements in energy efficiency and process optimization will be crucial to ensure that scaling up collection does not inadvertently reduce overall sustainability performance.
