The potential of a cyanobacterium Phormidium sp., for the tertiary treatment of piggery wastewater chemical oxygen demand greater than or equal to 3000 mg.l(-1), using an aerobically stabilized secondary effluent, was studied. Batch cultures were carried out in 2-1 glass reactors and in a 30-1 glass-fibre carousel reactor. The nutrient removal efficiency as well as biomass production were compared in synthetic mineral medium and in different concentrations of aerobically stabilized piggery effluent. The best performance of Phormidium sp. ocurred in diluted stabilized secondary effluent (1:1). Removal efficiencies were 100% for P-PO4-3, 50% for N-NH4+ and 35% for N-NO3- in small-volume cultures; and 31% for P-PO4-3, 100% for N-NH4+ and 70% for N-NO3- in an open carousel reactor. Biomass production on the aerobically stabilized effluent was six times higher in the 2-1 reactors and 1.7 times higher in the carousel reactor when compared to the synthetic medium.