What are short-range nitrification and short-range denitrification?

Preface

In the field of biological nitrogen removal, short-range nitrification and short-range denitrification are two important technological processes. They break through the traditional full-range nitrification and denitrification path and achieve efficient nitrogen removal by regulating the structure of the microbial community.

Short-range nitrification

Short-range nitrification refers to in the process of biological denitrification, through specific process control means, so that the oxidation process of ammonia nitrogen only proceeds to the nitrite stage, that is, ammonia nitrogen (NH₄⁺) is first oxidized to nitrite (NO₂⁻), and the nitrite is no longer further oxidized to nitrate (NO₃⁻). This process mainly relies on the strengthening of the activity of ammonia-oxidizing bacteria (AOB) and the effective inhibition of the activity of nitrifying bacteria (NOB). The advantage of short-range nitrification is that it reduces oxygen consumption and carbon source requirements, reduces the load in the subsequent denitrification stage, thereby improving the denitrification efficiency of the entire system.

Influencing factors and optimization strategies:

●DO concentration: DO needs to be strictly controlled at 0.5-1.5 mg/L to inhibit nitrifying bacteria and benefit ammonia-oxidizing bacteria.

●Temperature and pH value: Appropriate temperature (20-30°C) and neutral to weakly alkaline pH environment are conducive to the stable operation of short-range nitrification.

●SRT (sludge age): Appropriately shortening the SRT is conducive to selectively cultivating a short-range nitrification flora.

●Influent load: Maintain a stable ammonia nitrogen load to avoid damage to the short-range nitrification system due to impact.

Short-range denitrification

Short-range denitrification refers to directly reducing nitrate or nitrite to nitrogen (N₂), skipping the step of reducing NO₃⁻ to NO₂⁻ in the conventional denitrification process. This process usually requires special denitrifying bacteria, which can directly use nitrite as an electron acceptor for denitrification reaction under anoxic conditions. Short-range denitrification can not only avoid the intermediate product - nitric oxide (NO) produced in the traditional denitrification process, reducing the environmental pressure, but also improve the total nitrogen removal rate and save the input of organic carbon sources.

Influencing factors and optimization strategies:

●Anoxic environment: Strict anoxic conditions are the basis of short-range denitrification, and the DO should be controlled below 0.5 mg/L.

●pH value: pH between 6.0 and 8.0 is conducive to the growth and activity of short-range denitrifying bacteria.

●Temperature: Appropriate temperature (20-30°C) helps to improve the denitrification rate.

●Carbon source type and supply amount: Choose easily degradable and efficient carbon sources, and appropriately increase the carbon supply amount to promote the occurrence of short-range denitrification.

●Reactor internal flow pattern and mixing: Ensure uniform mixing in the reactor to avoid excessive oxygen concentration in local areas.

How to regulate the microbial community to achieve short-range nitrification and denitrification?

●Dissolved oxygen control: The realization of short-range nitrification depends on the inhibition of the activity of nitrifying bacteria (NOB). Usually, by strictly controlling the dissolved oxygen concentration in the reactor, it is maintained at a low level that can not only ensure the normal work of ammonia-oxidizing bacteria (AOB) but also be sufficient to inhibit the growth of NOB.

●Temperature and pH adjustment: Different microbial populations have different adaptabilities to environmental conditions. By adjusting the temperature and pH value of the reactor, the activity of target flora can be selectively promoted, such as some specific AOB having higher activity in a lower temperature or specific pH range.

●SRT (sludge age) and HRT (hydraulic retention time) optimization: Reasonably set the sludge age and hydraulic retention time of the biological reactor, which is helpful to selectively enrich AOB, while eliminating or inhibiting the proliferation of NOB.

●Alternate aeration and anoxic treatment: Using intermittent aeration or segmented treatment methods to create environmental conditions conducive to short-range nitrification and short-range denitrification, for example, first converting ammonia nitrogen to nitrite under aerobic conditions, and then quickly converting to anoxic or anaerobic conditions to directly reduce nitrite to nitrogen.

●Adding inhibitors: Some chemical substances can be used as inhibitors of NOB, such as methanol and isopropanol, which can effectively inhibit the activity of NOB, thereby realizing short-range nitrification.

●Carbon source dosing strategy: Appropriate carbon source dosing can regulate the denitrification process, so that nitrite is preferentially used as an electron acceptor instead of nitrate, realizing short-range denitrification.

To sum up, through a series of precise process control means, the short-range nitrification and short-range denitrification processes can be successfully induced and stably operated in the sewage treatment system, thereby improving the denitrification efficiency, reducing energy consumption, and reducing the generation of by-products.