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About the calculation of carbon source and basicity of nitrification and denitrification!

I. nitrifying bacteria


Nitrification process: under aerobic conditions, ammonia nitrogen is oxidized by nitrifying bacteria to nitrite and nitrate. There are two basic reaction steps: Nitrosomonas SP is involved in the reaction of transforming ammonia nitrogen into nitrite; Nitrobacter SP is involved in the reaction of transforming nitrite into nitrate. Both nitrobacteria and nitrobacteria are chemoautotrophic bacteria. They use CO2, CO32 -, HCO3 - as carbon source, and through the oxidation-reduction reaction of NH3, NH4 +, or NO2 - Get energy. The nitrification process needs to be carried out under the condition of aerobic or oxic, with oxygen as the electron acceptor and nitrogen as the electron donor. The corresponding reaction formula is:


Nitrosation equation:


55NH4++76O2+109HCO3→C5H7O2N﹢54NO2-+57H2O+104H2CO3


Nitrification equation:


400NO2-+195O2+NH4-+4H2CO3+HCO3-→C5H7O2N+400NO3-+3H2O


Total reaction formula of nitrification process:


NH4-+1.83O2+1.98HCO3→0.021C5H7O2N+0.98NO3-+1.04H2O+1.884H2CO3


According to the material balance calculation of the above reaction process, in the nitrification process, the oxidation of 1G ammonia nitrogen to nitrate nitrogen requires 4.57g aerobic (including 3.43g oxygen consumption for nitrosation and 1.14g oxygen consumption for nitrification) and about 7.14g bicarbonate (calculated by CaCO3) alkalinity.


In the nitrification process, the transformation of nitrogen experienced the following processes: NH4 - → NH2OH → Noh → NO2 - → NO3 -.


Denitrifying bacteria


Denitrification process: under the condition of anoxia, denitrifying bacteria are used to reduce nitrite and nitrate to nitrogen and escape from anhydrous water, so as to achieve the purpose of nitrogen removal.


Denitrification is the process of reducing nitrate and nitrite produced in the nitrification process to nitrogen. Denitrifying bacteria are a kind of heterotrophic facultative anoxic microorganism. In the presence of molecular oxygen, denitrifying bacteria oxidize and decompose organics, using molecular oxygen as the final electron acceptor. In the absence of molecular oxygen, denitrifying bacteria use N3 + and N5 + in nitrates and nitrites as electron acceptors, O2 - as hydrogen acceptor to generate water and oh alkalinity, and organics as carbon source to provide electron donor to provide energy and obtain oxidation stability It is known that denitrification should be carried out under anoxic conditions. The reduction process from NO3 - to N2 is as follows:


NO3-→NO2-→NO→N2O→N2


In the denitrification process, denitrifying bacteria need organic carbon sources (such as carbohydrate, alcohol, organic acid) as electronic donors, and oxygen in NO3 - is used for oxygen deficient respiration. The reaction process can be simply expressed as follows:


NO3 - + 4H (electronic donor organics) → 1 / 2n2 + H2O + 2OH-


NO2 - + 3H (electronic donor organics) → 1 / 2n2 + H2O + Oh-


Carbon containing organic compounds in wastewater are used as electron donors in denitrification process. It can be seen from the above formula that for each conversion of 1gno2 - to N2, 1.71g of organic matter (in BOD) is required; for each conversion of 1gno3 - to N2, 2.86g of organic matter (in BOD) is required. At the same time, 3.57g bicarbonate alkalinity (calculated by CaCO3) is produced.


If the sewage contains dissolved oxygen, in order to make denitrification complete, the required carbon source organic matter (expressed in BOD) is calculated by the following formula:


C = 2.86ni + 1.71n0 + do0


Among them:


C is the demand of organic matter in denitrification process (expressed in BOD), mg / L;


Ni is the initial nitrate nitrogen concentration (mg / L)


N0 is the initial nitrite nitrogen concentration (mg / L)


Do0 is the initial dissolved oxygen concentration (mg / L)


If the concentration of carbon source organics in sewage is insufficient, the carbon source organics (methanol, ethanol or sugar) that are easy to biodegradation should be added. Take methanol as an example, then


NO3-+1.08CH3OH+0.24H2CO3→0.056C5H7O2N+0.47N2↑+1.68H2O+HCO3-


If NO2 - is present in water, the following reactions occur:


NO2-+0.67CH3OH+0.53H2CO3→0.04C5H7O2N+0.48N2↑+1.23H2O+HCO3-


It can be seen from the above formula that 1.53g and 2.47g methanol need to be consumed for each reduction of 1gno2 - and 1gno3 - respectively.


When there is dissolved oxygen in the water, the reaction formula of oxygen consuming methanol is:


O2+0.93CH3OH+0.056NO3-→0.056C5H7O2N+1.64H2O+0.056HCO3-+0.59H2CO3


To sum up, the formula of the amount of organic carbon source (methanol) required for denitrification process is as follows:


Cm=2.47Ni+1.53N0+DO0


Among them:


Cm is the methanol concentration required in denitrification process (mg / L)


The rest of the symbols are the same as above


In conclusion, 4.57g of oxygen and 7.14g of alkalinity are consumed per 1g of ammonia nitrogen oxidized in the nitrification reaction, which shows a decrease in pH value. In the denitrification process, the nitrate nitrogen is removed while the carbon source is removed, which is equivalent to do2.6g. In addition, 3.57g of alkalinity is compensated in the denitrification process.

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