Should mega-cities of the developing world follow the development model of using municipal wastewater treatment technology of Western Europe and North America or is there an alternative “sustainable sanitation” approach? This paper tries to define the most efficient and cost-effective, minimum level of treatment needed to address one of the foremost problems of mega-cities, which is public health, putting forward the specific technological proposal of applying chemically enhanced primary treatment (CEPT). During the past ten years, the combination of low dosages of metal salts, such as ferric chloride, with very small dosages of anionic polymers, have resulted in an efficient single-stage treatment process known as CEPT. Some CEPT applications focus on increased suspended solids and/or BOD removal, increased flow capacity due to more rapid settling, while others are concerned with effluent disinfection by chlorine or ultraviolet light or with high levels of phosphorus removal through precipitation. A number of case studies with which the authors have been associated are used to illustrate a wide range of CEPT options that are particularly suitable for rapidly growing mega-cities. Examples are provided from Mexico City, Southern California, Hong Kong, Sao Paulo, and Rio de Janeiro.