This study shows that the major organic fraction in activated sludge effluent that exhibited a high adsorption in activated carbon columns is a fulvic acid-like material with a molecular weight ranging from approximately 100 - 10,000. A high molecular compound with a molecular weight larger than 50,000 absorbed poorly in carbon columns. This compound consists of carbohydrates and proteins and resembles a humic acid-like compound. This fraction also enhances flocculation of bacteria and is found to prevent virus adsorption in diatomaceous earth filters coated with polyelectrolytes. Another fraction in activated sludge effluent that absorbed poorly in carbon columns consists of polar compounds, e.g., volatile free fatty acids with a molecular weight smaller than 100.