In a case study located in suburban sectors of the metropolitan area of the Lerma Valley (Valle de Lerma), in the province of Salta (Argentina), 24 informal decentralized wastewater treatment systems (DWWTS) were evaluated. The analyzed systems had three general configurations: A, septic tank; B, septic tank combined with upflow anaerobic sludge blanket (UASB) reactor; C, septic tank combined with UASB and a final filtration step. Statistically significant differences (p < 0.05) were observed in effluent quality, measured as total coliforms, thermotolerant coliforms, and chemical oxygen demand (COD). Treatment A was the most inefficient, and was statistically different from B and C; there were no significant differences between the latter two. Thermotolerant coliform concentrations were high in all analyzed systems and did not comply with local discharge standards in soakaway pits or in the ground. The lack of a final disinfection step in these systems is thus a weakness that needs to be addressed. The formal inclusion of DWWTS in urban planning could reduce overall investment costs, as long as the best technologies are selected for each case. Incorporation of DWWTS in formal urban planning requires an open debate in which the social perspectives of all relevant users need to be considered.