Water & Solutions Forum – Water Science, Processing and Maintenance



Biological nitrification



Ammonia oxidation-nitrification

 The activity (reaction rate, growth rate) of nitrifying bacteria is easily affected by environmental conditions such as water temperature, pH, DO, etc. In general, the nitrification reaction is represented by the following formula.

① NH4+ + 3/2O2 → NO2 + 2H+ + H2O
② NO2 + 1/2O2 → NO2

③ NH4+ + 2O2 → NO3 + 2H+ + H2O
Figure Example of daily changes of nitrogen and pH in a saltwater fish tank with circulating biological filtration (indoor closed system, 15° C)
 アンモニアから亜硝酸を生成するには、ニトロソモナス属(Nitorosomonas sp.)、ニトロコッカス属(Nitrosococus sp.)等の硝化細菌の作用によって進む。次に、亜硝酸から硝酸を生成する反応は、ニトロバクター属(Nitrobacter sp.)等の硝化細菌の作用によって進む。
  The production of nitrite from ammonia proceeds by the action of nitrifying bacteria such as Nitorosomonas sp and Nitrosococus sp. Next, the reaction of producing nitric acid from nitrous acid proceeds by the action of nitrifying bacteria such as Nitrobacter sp.
  Oxygen O2 (2mol) is required to oxidize ammonia NH4+(1mol) to NO3 (1mol) , and hydrogen ion H+ (2mol) is generated and the pH is lowered.
 In the actual nitrification reaction, it is desirable to keep the pH at 6.8 to 8.5. In the floating culture reaction, for example, the activated sludge method, the sludge retention period is set to 7 days or more. In the biofilm reactors in which microorganisms are adhered and cultured on a carrier or the like, the sludge retention days required for nitrifying bacteria are sufficiently maintained. The optimum temperature is 20 to 30 ° C, but the nitrification rate decreases significantly below 15 ° C.
 If the concentration of organic substances is high (about 30 mg/L or more), the nitrification reaction hardly progresses, so secure a sufficient treatment time (hydraulic retention time) to keep the concentration of organic substances low in the reaction tank. The dissolved oxygen concentration is preferably 2 mg/L or more.


Posted: May 28, 2020
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