Total nitrogen is mainly composed of ammonia nitrogen, organic nitrogen, nitrate nitrogen, and nitrite nitrogen, among which ammonia nitrogen mainly comes from ammonia water and inorganic substances such as ammonium chloride. Organic nitrogen mainly comes from nitrogen-containing groups in some organic compounds, such as organic amines. Nitrate nitrogen is relatively stable in nature and has a high content. For example, industries such as mechanical chemistry use a large amount of raw materials related to nitrate as oxidants. At the same time, many wastewater also contains a large amount of nitrate after pre biochemical and nitrification. Because nitrate nitrogen is very stable and easily soluble in water, pollution is very serious and easily diffused.

So what are the reasons for the total nitrogen exceeding the standard? The following text will comprehensively analyze the nitrification denitrification process and propose solutions.

1. No analysis of organic nitrogen components in the raw water

Solution: Analyze the quality of raw water; Set up organic nitrogen conversion processes (such as anaerobic, hydrolysis, etc.)

2. Nitrification and denitrification

2.1 Insufficient carbon source in raw water and no additional carbon source added, resulting in insufficient carbon source during denitrification, making it difficult to completely remove total nitrogen

Solution: Accurately calculate the amount of carbon source added; Determine the variety of carbon sources.

2.2 Excessive reflux ratio of nitration solution leads to high dissolved oxygen in the denitrification tank, making it difficult to denitrify

Solution: Adjust the reflux ratio; Optimizing the structure of nitrification tanks; Set up an oxygen dissipation zone.

2.3 Calculation of nitrification and denitrification tank volume

Solution: Accurately calculate the volume of the nitrification and denitrification tank.

2.4 The calculation of air flow in the nitrification tank did not consider the oxygen demand of ammonia nitrogen, resulting in the inability of ammonia nitrogen to be converted into nitrate nitrogen

Solution: Accurately calculate the oxygen demand for ammonia nitrogen biological oxidation.

2.5 Too much foam in nitrification tank

Solution: Set up defoaming or spraying devices.

2.6 Temperature affects the progress of denitrification

Solution: Set insulation.

2.7 The issue of sludge age

Solution: Adjust the sludge discharge volume.

2.8 PH issues in nitrification tanks.

Solution: Set up a pH dosing device; Automatically control the pH value in the nitrification tank.

Wastewater contains organic nitrogen; Most wastewater containing organic nitrogen has characteristics such as difficult degradation, high nitrogen and low carbon, and complex composition. The effective decomposition of organic nitrogen in these wastewater directly affects the treatment efficiency of the entire system for total nitrogen. Therefore, the degradation of organic nitrogen in wastewater and the removal of ammonia nitrogen produced by degradation are the key and difficult points in the treatment of such wastewater. One of the biochemical denitrification pathways mainly relies on two types of bacteria, nitrification and denitrification, which have different living environments.

Nitrobacteria belong to the specialized autotrophic type, and they use the energy released during the ammonia nitrogen conversion process as their own metabolic energy. They are very sensitive to the environment and have low yields. The nitrification process only occurs when the sludge load is<0.15kg BOD/(kgSS.d). During the reaction, oxidizing 1kg of ammonia nitrogen consumes approximately 4.6kg of oxygen, while consuming approximately 7.14kg of calcium carbonate alkalinity. Nitrification can be divided into two categories based on the degree of separation of carbonization and nitrification functions: carbon oxidation and nitrification are carried out in the same reactor, called combined nitrification, and in different reactors, called separate nitrification.

Nitrification does not remove nitrogen, but converts ammonia nitrogen into nitrate nitrogen and nitrite nitrogen. The final removal of nitrogen needs to be completed through denitrification process.

There are many types of denitrifying bacteria, most of which are facultative anaerobic bacteria. In the absence of molecular dissolved oxygen, the energy generated by the reduction process of nitrate and nitrite is used as the energy source. In the presence of molecular dissolved oxygen, denitrifying bacteria will decompose organic matter to obtain energy. Therefore, the denitrification process should be carried out under anaerobic conditions, and the concentration of dissolved oxygen should not exceed 0.2mg/L, otherwise the denitrification process will stop.

The internal reflux mixed liquid carries dissolved oxygen of more than 2 mg/L, which will compete with NO3-N in the anoxic tank for carbon source and directly affect the denitrification effect of wastewater.

In the denitrification process, carbon containing organic compounds should be used as electron donors for reducing nitrate and nitrite. Practice has shown that ideal denitrification can be achieved when BOD₅/TKN>4 in wastewater, while the denitrification effect is not good when BOD₅/TKN<4.

From the perspective of biological denitrification, the organic carbon that can be utilized by denitrifying bacteria can be divided into three categories:

① Various organic compounds in sewage, such as organic acids, alcohols, and carbohydrates;

② Adding a carbon source, when BOD₅/TKN is too small, methanol and other carbon sources are often used in engineering. After decomposition, they produce carbon dioxide and water without leaving any intermediate products that are difficult to decompose;

③ The internal carbon source refers to the organic carbon released after microbial death and autolysis in activated sludge. The denitrification rate of the internal carbon source is very low, only 1/10 of the above two carbon sources.

During denitrification, for every 1kg of nitrate nitrogen reduced to nitrogen, theoretically 3.57kg of alkalinity can be recovered. In addition, for every 1kg of BOD₅ removed, 0.3kg of alkalinity can be produced. So in practical engineering, the denitrification section is referred to as the pre denitrification denitrification process before the nitrification section, that is, the A/O process.

The nitrification solution is refluxed for denitrification, which can utilize the organic matter in the original wastewater as an electron donor for denitrification, while providing partial alkalinity to offset the partial alkalinity consumption in the nitrification stage.

The improvement of denitrification rate in this process depends on increasing the reflux ratio, but the reflux ratio should not be too high, otherwise the DO carried in the reflux mixture will affect the anaerobic state of the denitrification section. In addition, increasing the reflux ratio will also increase operating costs.

Denitrification characteristics: During denitrification, for every 1kg of nitrate nitrogen reduced to nitrogen, theoretically 3.57kg of alkalinity can be recovered; Removing 1kg of BOD₅ can produce 0.3kg of alkalinity; Reducing 1kg of nitrate nitrogen to nitrogen consumes 2.47kg of methanol (approximately 3.7kg of COD); Reducing 1kg of nitrate nitrogen can provide 2.86kg of O2;

The advantages of pre denitrification:

① Energy saving

The oxygen demand of the pre denitrification biological denitrification system includes the oxygen required for the oxidation of carbon containing organic matter and the oxygen required for the nitrification of nitrogen-containing compounds.

Due to the setting of the anoxic tank before the aerobic tank, during denitrification in the anoxic tank, a portion of BOD in the influent is consumed as a carbon source for denitrification. At the same time, oxygen in NO₃-N can be utilized during denitrification, saving 2.86kg of oxygen demand per 1kg of nitrate nitrogen denitrification.

Therefore, according to the degree of denitrification, the amount of oxygen saved varies. Generally, the higher the denitrification rate, the greater the amount of oxygen saved, thereby saving more power required for aeration.

② Saving Medication

In the denitrification process, for every 1kg of nitrate nitrogen reduced to nitrogen, theoretically 3.57kg of alkalinity can be recovered. In the nitrification process, for every 1kg of NO₃-N oxidized, 7.14kg of alkalinity (calcium carbonate alkalinity) is required.

In pre denitrification, the alkalinity generated by denitrification can supplement about half of the alkalinity required for subsequent nitrification, greatly reducing the amount of alkalinity added to the entire A/O system, thereby saving on chemical and operational costs.

③ Comprehensive insulation measures

Although denitrifying bacteria are not as sensitive to temperature changes as nitrifying bacteria, the denitrification effect also varies with temperature changes.

The higher the temperature, the higher the nitrification rate, and DNR increases to its maximum at 30-35 ℃.

When the temperature is below 15 ℃, the denitrification rate will significantly decrease; At 5 ℃, denitrification will tend to stop.

Denitrification fully utilizes influent organic matter as a carbon source

Denitrifying bacteria belong to heterotrophic facultative anaerobic bacteria, which use NOx N as electron acceptor and organic matter (organic carbon) as electron donor under anaerobic conditions.

It can be seen that carbon source is an indispensable substance in the denitrification process, and the C/N of influent directly affects the biological denitrification effect as an important factor.

Generally, BOD/TKN=3-4, and the more abundant the organic matter, the faster the reaction rate. When the BOD/TKN in wastewater is less than 3, an additional carbon source is needed to achieve the ideal denitrification goal. Therefore, carbon sources have a significant impact on denitrification efficiency.

The carbon sources of denitrification can be mainly divided into three categories: first, the components of the wastewater itself, such as various organic acids, starch, carbohydrates, etc; The second is to add carbon sources during the wastewater treatment process, which can generally include nearby industrial by-products such as acetic acid, propionic acid, and methanol; The third is the carbon source released by the self death and autolysis of activated sludge, known as endogenous carbon.

The characteristics of nitrification reaction:

① The biological oxidation of NH3 requires a large amount of oxygen, and approximately 4.6kg of O2 is required for every 1kg of NO3-N removed;

② The cell yield during the nitrification process is very low and it is difficult to maintain high biological concentrations, especially in low temperature winter;

③ During the nitrification process, a large number of protons are generated. In order to facilitate the reaction, a large amount of alkali neutralization is required, which theoretically requires an alkalinity of 7.14 kg (calcium carbonate alkalinity) for every 1 kg of NH3-N oxidized.

Source: Environmentally friendly water treatment