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[Source: China Water Treatment Network]

Water treatment technology:

Biological denitrification can be divided into three steps: ammoniation-nitrification-denitrification. Because ammoniation reaction is very fast and can be completed in general wastewater treatment facilities, the key of biological denitrification is nitrification and denitrification.

Biological denitrification is a process of converting organic nitrogen and NH3-N into N2 and N2O gases under the action of microorganisms.

There are organic nitrogen, NH3-N and NOx-N in wastewater, among which NH3-N and organic nitrogen are the main forms. In the process of biological treatment, organic nitrogen is oxidized and decomposed by heterotrophic microorganisms, that is, NH3-N is converted into NH3-N by ammoniation, and then into NO-N by nitrification. Finally, NO-N is converted into N2 by denitrification and escaped into the atmosphere.

Therefore, biological denitrification can be divided into three steps: ammoniation, nitrification and denitrification. Because ammoniation reaction is very fast and can be completed in general wastewater treatment facilities, the key of biological denitrification is nitrification and denitrification.

1. Ammoniation

Ammoniation refers to the process of converting organic nitrogen compounds into NH3-N, also known as mineralization. The bacteria involved in ammoniation are called ammoniating bacteria. In nature, there are many kinds of them, including aerobic Pseudomonas fluorescens and Prodigibacter, facultative Proteus and anaerobic Clostridium putrefaciens. Under aerobic conditions, there are two main degradation modes, one is oxidative deamination catalyzed by oxidase [2]. For example, amino acids produce ketoacids and ammonia:

Another is that some aerobic bacteria can hydrolyze denitrification under the catalysis of hydrolase. For example, urea can be hydrolyzed by many bacteria to produce ammonia. Urea-decomposing bacteria include Urea Octacoccus and Urea Bacillus. They are aerobic bacteria. The reaction formula is as follows:

Under anaerobic or anoxic conditions, anaerobic microorganisms and facultative anaerobic microorganisms ammoniate organic nitrogen compounds through three ways: reduction, hydrolysis and dehydration.

2. Nitrification

Nitrification refers to the biochemical reaction of oxidizing NH3-N to NO 2-N, which is accomplished by nitrite bacteria and nitrate bacteria, including two steps: nitrification and nitrification. The reaction process is as follows:

Nitrite bacteria include Nitromonas, Helicobacter nitrite and Nitrococcus. Nitric acid bacteria are Nitrobacter and Nitrococcus. Nitrite bacteria and nitric acid bacteria are collectively called nitrifying bacteria [22]. When nitrification occurs, bacteria obtain energy from the oxidation of NH3-N and NO2-N, respectively. The carbon source comes from inorganic carbon compounds, such as CO32-, HCO-, CO2, etc. Assuming that the cell composition is C5H7NO2, the stoichiometric relationship of nitrifying bacteria synthesis can be expressed as follows:

Three important characteristics of nitrification can be seen from the formula.

_Biological oxidation of N H3 requires a lot of oxygen, and about 4.2 gO 2 is needed for each removal of 1g of NH3-N.

(2) Cell productivity in nitrification process is very low, and it is difficult to maintain a high concentration of substances, especially in low temperature winter.

(3) A large number of protons (H+) are produced in the nitrification process. In order to make the reaction proceed smoothly, a large amount of alkali neutralization is needed. In theory, the alkalinity of N H3-N is about 5.57g (in NaCO3).

3. Denitrification

Denitrification is a biological reaction in which NO_x-N and other nitrogen oxides are used as electron acceptors to be reduced to nitrogen or other gaseous oxides of nitrogen under anaerobic or anoxic conditions (DO < 0.3-0.5mg/L). This process is accomplished by denitrifying bacteria [3-4]. The reaction course is as follows:

[H] can be any substance that can provide electrons and reduce NO - N as nitrogen, including organic matter, sulfide, H + and so on. The main bacteria responsible for this kind of reaction are Proteus, Micrococcus, Pseudomonas, Bacillus, Alkali-producing Bacillus, Flavobacillus and other facultative bacteria, which widely exist in nature. In the presence of molecular oxygen, oxygen is used as the ultimate electron acceptor to oxidize organic matter and breathe; in the absence of molecular oxygen, NO 2-N is used to breathe. The results show that the conversion between molecular oxygen and NO_x-N is easy, and even frequent exchange will not inhibit denitrification.

Most denitrifying bacteria can assimilate NO_x-N into NH3-N for cell synthesis while denitrifying, which is also called assimilation denitrification. Only when NO_x-N is the only available ammonia source for denitrifying bacteria can NO_x-N assimilation and metabolism occur. If NH3-N exists simultaneously in wastewater, denitrifying bacteria can make limited use of NH3-N to synthesize.

4. Assimilation

In the process of biological denitrification, a part of nitrogen (NH3-N or organic nitrogen) in wastewater is assimilated into a part of heterotrophic cells. Microbial cells were expressed by C60H87023N12P. According to the dry weight of cells, the nitrogen content in microbial cells was about 12.5%. Although the endogenous respiration and cytolysis of microorganisms can return some of the cell nitrogen to wastewater in the form of organic nitrogen and NH3-N, the nitrogen in the cell and endogenous respiratory residues of microorganisms can be removed from the wastewater in the secondary sedimentation tank.