A large number of studies have proved that nitrogen in sewage is one of the main causes of water eutrophication. Conventional wastewater treatment technologies mainly remove organic matter and suspended solids, but the removal efficiency of nitrogen is low. Many countries have limited the nitrogen content in the sewage, and require the sewage treatment plant to meet the requirements of nitrogen removal.
The technology of wastewater denitrification can be divided into physical method, chemical method and biological method. Relatively speaking, biological nitrogen removal technology has the advantages of less investment, simple operation and easy treatment of secondary pollutants, so biological nitrogen removal method is widely used.
Nitrogen removal by activated sludge process
The principle of nitrogen removal by activated sludge process is to create aerobic and anoxic conditions, through two steps of nitrification and denitrification, and use some specific bacteria such as nitrifying bacteria and denitrifying bacteria to realize the transformation of nitrogen form. The basic methods include the traditional biological denitrification method with multi-stage sludge return system and the single-stage biological denitrification method with only one sludge return system. The single-stage biological nitrogen removal process can be considered as an improvement on the traditional biological nitrogen removal process, including a / O process, bardenpho process and other forms. The oxidation ditch, SBR process and other processes with nitrogen removal function also belong to the single-stage biological nitrogen removal system.
1. Traditional biological nitrogen removal process
The traditional biological nitrogen removal process is a three-stage activated sludge system. In this process, the oxidation of carbonaceous organic matter, nitrification of nitrogenous organic matter, nitrification of ammonia nitrogen and denitrification of nitrate are carried out in three structures respectively, and independent sludge return systems are maintained. Both aeration tank and nitrification tank should be aerated to maintain aerobic state, while denitrification tank should maintain anoxic state without aeration. Only slow stirring is used to make sludge in suspension state and keep good mixing with sewage. The carbon source for denitrification is methanol / glucose.
The advantage of this process is that aerobic bacteria, nitrifying bacteria and denitrifying bacteria grow in different structures respectively, and can maintain their own suitable growth environment, so the reaction speed is fast, and good BOD5 removal effect and nitrogen removal effect can be obtained. In addition, different types of sludge are separated in different sedimentation tanks, and have their own independent sludge return system, so the operation flexibility and adaptability are better. Its disadvantages are long process, many structures, and high operation cost due to the addition of methanol / glucose as carbon source. A certain amount of methanol / glucose is often retained in the effluent.
2. Bardenpho process
Bardenpho process is composed of two anoxic / aerobic (A / O) processes in series, with a total of four reaction tanks, so it is sometimes called four stage bardenpho process, and its process flow is shown in the figure below.
In the primary A / O process, the nitrate nitrogen in the reflux mixture is denitrified in an anoxic tank by using the carbonaceous organic matter in the original sewage as the carbon source under the action of denitrifying bacteria. After the denitrified effluent enters an aerobic tank, the carbonaceous organic matter is oxidized, and the carbonaceous organic matter realizes ammoniation and nitrification of ammonia nitrogen. At the same time, the nitrogen produced by denitrification in an anoxic tank is oxidized in an anoxic tank The aerobic tank is released by aeration.
In the two-stage A / O process, after the mixed liquor from the first aerobic tank enters the second anoxic tank, denitrifying bacteria use the endogenous metabolites in the mixed liquor for further denitrification. The N2 produced by denitrification is released by aeration in the second aerobic tank to improve the sedimentation performance of sludge. At the same time, the ammonia nitrogen produced by endogenous metabolism can also be nitrified in the second aerobic tank.
Bardenpho has two denitrification processes, and the denitrification efficiency can reach 90% - 95%.