Nowadays, most wastewater projects are upgraded and transformed because biological treatment cannot reduce nitrogen and phosphorus in wastewater to the discharge standard. Therefore, MBR, denitrification filter and other processes are used. Today, let's talk about how nitrogen in wastewater is removed. The content is relatively large and very practical.
How to remove nitrogen from wastewater
Denitrification technology includes chemical and biological methods. Since chemical methods will produce secondary pollution and are costly, biological denitrification technology is generally used.
1. Biological denitrification
Biological treatment of sewage denitrification mainly ic compounds are first decomposed and converted into nitrogen NH4+ or NH3 under the action of microorganisms. This process is called “tion reaction"; nitrifying bacteria convertnitrogen into nitrates. This process is called "nitrification reaction"; denitrifying bacteria convert nitrates into nitrogen gas. This reaction is called "denitrification reaction". Nitrogen-containing organic compounds are finally converted into nitrogen gas and removed from sewage.
1. Nitrification process
The process of nitrifying bacteria converting nitrogen into nitrate is called nitrification process. Nitrification is a two-step process, which uses two types of microorganisms, nitrite bacteria and nitrate bacteria. These two types of bacteria are collectively called nitrifying bacteria. The carbon sources used by these bacteria are inorganic carbon such as CO32-, HCO3- and CO2.
In the first step, nitrite bacteria convert nitrogen into nitrite, and in the second step, nitrate bacteria convert nitrite into nitrate.
These two processes release energy, and nitrifying bacteria use this energy to synthesize new cells and maintain normal life activities. The conversion of nitrogen into nitrate nitrogen does not remove nitrogen but reduces its oxygen demand.
It takes about 4.3gO2 and 8.64gHCO3- (equivalent to 7.14gCaCO3 alkalinity) to oxidize 1g of a nitrogen.
Factors affecting the nitrification process:
1) Temperature: The most suitable temperature range for nitrification reaction is 30~35ºC. The temperature not only affects the specific growth rate of nitrifying bacteria, but also affects the activity of nitrifying bacteria.
2) Dissolved oxygen: Nitrification must be carried out under aerobic conditions. The dissolved oxygen concentration of 0.5~0.7mg/L is the limit that nitrifying bacteria can tolerate. When the dissolved oxygen is lower than 2mg/L, nitrogen may be completely nitrified, but a longer sludge retention time is required. Therefore, the dissolved oxygen concentration of the mixed solution should generally be maintained above 2mg/L.
3) pH and alkalinity: Nitrifying bacteria are particularly sensitive to pH. The optimal pH for nitrification is between 7.2 and 8. It takes about 7.14g of CaCO3 alkalinity to nitrify 1g of nitrogen. If the sewage does not have enough alkalinity for buffering, the nitrification reaction will cause the pH value to drop and the reaction rate to slow down.
4) Toxic substances: Excessive nitrogen, heavy metals, toxic substances and certain organic substances have an inhibitory effect on nitrification.
5) Sludge age: Generally speaking, the sludge age of the system should be more than twice the generation cycle of nitrifying bacteria, and generally should not be less than 3~5d. In winter, when the water temperature is low, the sludge age is required to be longer. In order to ensure sufficient nitrification reaction throughout the year, the sludge age is usually greater than 10d.
6) Carbon-nitrogen ratio: The ratio of BOD5 to TKN is C/N, which is an indicator of the ability of heterotrophic bacteria and nitrifying bacteria to compete for substrates and dissolved oxygen in the activated sludge system. Different C/N directly affects the denitrification effect. It is generally believed that when the BOD5 load of the treatment system is lower than 0.15BOD5/(MLVSS·d), the nitrification reaction can proceed normally.
2. Denitrification process
The denitrification process is the process of denitrifying bacteria dissimilating nitrates, that is, the process in which nitrates and nitrites produced by nitrifying bacteria are reduced to nitrogen gas under the action of denitrifying bacteria and overflow from the water. The denitrification process is mainly carried out under anoxic conditions, and the concentration of dissolved oxygen cannot exceed 0.2mg/L, otherwise the denitrification process will stop.
Denitrification is also divided into two steps. The first step is the conversion of nitrate into nitrite, and the second step is the conversion of nitrite intonand nitrogen.
Factors affecting denitrification:
1) Temperature: The most suitable temperature range for denitrification is 35~45ºC.
2) Dissolved oxygen: In order to ensure the progress of the denitrification process, a strict anoxic state must be maintained, and the redox potential must be maintained at -50~-110mV; in order to ensure the normal progress of the denitrification reaction, the dissolved oxygen in the suspended activated sludge system is maintained below 0.2mg/L; the attached biological treatment system can allow a higher dissolved oxygen concentration, generally below 1mg/L.
3) pH value: The optimal range is 6.5~7.5.
4) Carbon source organic matter: Sufficient carbon source needs to be provided. Different carbon source materials have different denitrification rates.
5) Carbon-nitrogen ratio: In theory, 2.86g of carbon source BOD5 is required to convert 1g of nitrate nitrogen into nitrogen.
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