Transformation of lipids in activated sludge treatment plants is of interest for two reasons: lipids contribute 30-40% of the chemical oxygen demand (COD) in wastewater, and they may stimulate the growth of filamentous microorganisms in nutrient removal activated sludge plants. The transformation of lipids was investigated under aerobic and anoxic conditions by measuring the oxygen and nitrate uptake rates (OUR and NUR). The maximal OUR and NUR of long-chain fatty acid was found to be at the same level as acetate indicating that long-chain fatty acid was as easily consumable. However, the adsorption of long-chain fatty acid to surfaces of sludge flocs made it difficult to determine initial uptake rates of long-chain fatty acids. It was not possible to describe the hydrolysis rate of triacylglyceride by OUR and NUR to long-chain fatty acids because the hydrolysis rate was very slow. For a better description of the processes involved in transformation of lipids, a conceptual model was suggested. The processes in the suggested model were the adsorption/desorption of both triacylglyceride, and long-chain fatty acid onto surfaces of sludge flocs, hydrolysis of triacylglyceride by lipases and the uptake of long-chain fatty acid by bacteria under various conditions. The model can be helpful to structure design and evaluation of activated sludge experiment with lipids.