Abstract: Uncontrolled discharge of untreated or incompletely treated nitrogenous pollutants has caused a great deal of harm to the environment. The traditional technologies are inefficient for treating medium -low ammonium -nitrogen. Using residual activated sludge from sewage plant modified 2 h by ferric chloride as adsorbent, wastewater containing medium -low ammonium -nitrogen was treated. The experimental results indicate that the removal rate of ammonium-nitrogen can attain above 95 % with 0.15 mol/L ferric chloride, 5 g/L dosage (pH=9) after 40 min reaction at room temperature. The adsorption process accords with pseudo second-order kinetics equation. Under the experimental conditions, initial ammonium-nitrogen concentration and COD being 102.68 mg/L and 362 mg/L, was reduced to 9.2 mg/L and 83 mg/L for NH₄⁺ and COD respectively, after adsorption treatment in actual industrial wastewater. The discharged waste water has achieved < Integrated wastewater discharge standard>(NH₄⁺ < 15 mg/L and COD < 100 mg/L).