Characterization and coagulation-flocculation treatability of Mexico City wastewater applying ferric chloride and polymers

Physical, chemical and bacteriological characterization of two principal Mexico City wastewater flows are reported. COD and TSS variations were in the ranges of 290 to 680 mg/l and of 50 to 380 mg/l, respectively. The wastewater was strongly alkaline with marked bacteriological characteristics. High concentrations of chlorides, sulfates, Na, K, Ca, Mg and Si were found. Ascaris counts were between 24 and 84 eggs/l. Zeta Potential was from -20 mV to -40 mV. Conventional bench-scale tests were done to study the treatability of the wastewater by coagulation-flocculation with FeCl₃ and polymers. The tests of FeCl₃ performance showed that at a dosage of 80 to 100 mg/l, TSS and COD removals of between 60 to 68% can be obtained. Best performing flocculants were the cationic acrylamid copolymers, but good results were obtained with some anionic flocculants, too. The removal rates achieved with 20-30 mg/l of FeCl₃ and a series of anionic polymers at doses 0.2 to 1.5 mg/l were 74 to 84% for TSS, 48-55% for COD and up to 77% for turbidity and color. Coliforms were not removed, however helminth eggs could be removed completely with low reagent doses.