<p>With the increase in industrial and agricultural activities, huge amount of wastewater containing nitrogen is produced. Biological denitrification is currently the most widely used, sustainable and cost-effective process to remove nitrogen from wastewater. In biological denitrification, application of external carbon sources for denitrification is important because it is the way to boost the denitrification process and it is necessary for wastewater treatment plants that have to meet very stringent effluent nitrogen limits. This review study focused with treatment of wastewater using biological denitrification as advantages. This also deals with advantages of different external carbon sources with emphasis on biological denitrification as the cost of biological denitrification is controlled by the carbon source because the cost of biological denitrification is controlled by the carbon source.. Hence, use of alternative carbon sources such as agricultural wastes, and food waste is reviewed in this paper. This study recommend other waste material such as textile waste can be a part of denitrification because carbon source such as methanol, ethanol and acetic acid/acetate can be recovered from textile waste.</p>