Biological nitrogen removal requires the completion of two processes: nitrification and denitrification. The ammonia nitrogen in wastewater must be nitrified or converted into nitrite and nitrate, which are then reduced to nitrogen gas during denitrification, where nitrate serves as an electron acceptor in the process of oxidizing simple carbon compounds during cellular respiration. Therefore, denitrification aimed at removing nitrate necessitates the presence of readily biodegradable carbon sources. These sources include dissolved BOD in the influent, cellular debris from endogenous denitrification processes, and various types of supernatant returns. When the influent lacks sufficient dissolved organic matter and the nitrogen removal requirements are high, it becomes necessary to supplement with chemicals to provide the carbon source required for the denitrification process. This composite carbon source agent is a water-white, non-flammable liquid with a pH (1% aqueous solution) of 5.0-7.0. It is suitable for municipal sewage and industrial wastewater, supplementing the carbon source in sewage and adjusting the nutritional ratio required for microbial denitrification.