This paper summarizes a series of experiments carried out on nutrient removal from raw sewage and pretreated cattle-manure. The immobilization capacities of three different eucaryotic algae (Chlorella vulgaris, Chlorella kessleri and Scenedesmus quadricauda) were evaluated using internal immobilization with sodium alginate and k-carrageenan and external immobilization using polystyrene and polyurethane foams. The effect of flow pattern was studied and light sources were compared for both internal and external modes of immobilization. The use of sodium alginate pellets of Chlorella vulgaris and Chlorella kessleri gave good results with raw sewage. Higher efficiencies were obtained with the utilization by Chlorella vulgaris in an expanded-bed when the columns operated under natural light. Internal immobilization and the use of an upflow pattern failed with pretreated cattle manure due to the dark colour of this substrate, but good efficiencies were obtained at hydraulic loadings in the range of 0.5-2.5 I/l-day using downflow columns. (C) 1996 Elsevier Science Ltd.