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Where there is a variation in quality and quantity of sewage, advanced primary treatment (APT) is a useful means of treating wastewater. Mexico City, which has a combined sewage network and variations of flow rates ranging from 45 m(3) s(-1) during the dry season to 300 m(3) s(-1) during the rainy season, is just such a case. The data presented in this paper show the suitability of APT for Mexico City, enabling the continued reuse of such waters in a very arid region of 85 000 ha of poor quality soils. Given the agricultural nature of the irrigation area, the objective of this treatment is determined mainly by health requirements (helminth eggs and bacterial removal) instead of the organic and nutrient contents of the effluent, which are needed for the agricultural purposes of the irrigated area. From tested conditions, alum in a dose of 100 mg l(-1) and 0.2 mg l(-1) of an anionic polymer produce an effluent with a helminth egg content of <1 egg l(-1), which allows its use for irrigation for any kind of crop. Removal efficiencies attained were of 63.87% for TSS, 28.36% for COD, 17.42% for N, 20.43% for P, and 97.7% for helminth eggs. However because faecal coliform content is 1.2 X 10(6) NMP/100 ml(-1), an additional disinfection process is needed.