Optimization of the chemical pretreatment process prior to tertiary filtration of municipal secondary effluent was studied. Jar tests involving coagulation with alum, flocculation, settling and filtration were conducted on secondary effluent samples and these established the occurrence of two distinct coagulation mechanisms: the mechanism of charge neutralization, and the mechanism of sweep coagulation at different alum dosages and pH conditions. An optimum design region on the alum coagulation diagram also was established where the removals of various wastewater parameters were the highest. This region was bound by alum dosages of 55 to 60 mg/l and pH values of 6 to 6.5. In the above domain, the BOD5, TOC, turbidity, total-P, and the orthophosphate levels of the secondary effluent were reduced by 73%, 57%, 98%, and 99% respectively after tertiary filtration. Particle size analysis of the settled and filtered effluent also was conducted in both the charge neutralization and the sweep coagulation zones. A wide difference was observed in particle sizes between the effluents after settling and after filtration in the charge neutralization zone. In comparison, the difference in particle size between the settled and the filtered effluents was smaller in the sweep coagulation zone.