When it comes to the seemingly mysterious technologies in sewage treatment, anaerobic ammonification is definitely one of them. Think about it, generally we think that to deal with ammonia nitrogen, either we need to aerate it vigorously to turn it into nitrate (aerobic nitrification), or we need to find a carbon source to turn it into nitrogen (denitrification). However, anaerobic ammonification is not ideal. In an oxygen free environment, it can directly mix ammonia nitrogen with another substance called nitrite, and eventually turn it into nitrogen and escape. This process not only saves electricity for aeration, but also saves money on adding carbon sources. It is simply a "money saving expert" in the sewage treatment industry, so it has received special attention in recent years.
How does it actually work? We need to first get to know the 'hero' behind it - anaerobic ammonia oxidizing bacteria. This thing is not just any ordinary bacteria. It has a very strange temper, is afraid of both cold and heat, and is particularly "otaku". It likes to stay in anaerobic mud or biofilm, and grows very slowly, much slower than the bacteria in our common activated sludge. Sometimes it takes a week to reproduce. Because it grows slowly, it is difficult to recover once the bacteria are destroyed in the processing system.
The specific reaction process, to put it simply, is like a 'chemical reaction party'. Anaerobic ammonia oxidizing bacteria use ammonia nitrogen (NH4+) as "fuel" and nitrite (NO2-) as "oxidant", and the two react inside the bacteria. Among them, ammonia nitrogen loses electrons and is oxidized, nitrite obtains electrons and is reduced. Finally, the two "people" hit it off immediately, most of which turn into nitrogen (N2) and run into the air, while the remaining small part may turn into nitrate (NO3-) or water. The whole process is carried out in an anaerobic environment, without consuming oxygen or adding additional carbon sources such as glucose. Isn't it particularly efficient?
However, whether this' party 'can be held smoothly is influenced by many factors. Let's talk about them one by one.
Firstly, the temperature. The most comfortable temperature for anaerobic ammonia oxidizing bacteria is probably between 30 ℃ and 35 ℃, when they work the hardest. If the temperature is too low, such as dropping below 15 ℃, the activity of the bacteria will be greatly reduced and the reaction speed will significantly slow down; But if the temperature is too high, exceeding 40 ℃, the bacteria may be "scalded" or even die directly. So in actual operation, especially in sewage treatment plants in the north, it is necessary to find ways to insulate the reaction tank in winter, otherwise the treatment effect will definitely drop.
Then there is the pH value. This bacterium is particularly sensitive to acidity and alkalinity, with an optimal pH range of approximately 7.5 to 8.0, slightly alkaline. If the pH is too low, such as dropping below 6.5, or too high, exceeding 8.5, the activity of the bacteria will decrease. This is because pH changes can affect the activity of enzymes within the bacterial cells. Enzymes are like the "tools" of bacteria, and if the tools don't work properly, the work will naturally not be able to be done.
There is also dissolved oxygen, which is the "natural enemy" of anaerobic ammonia oxidizing bacteria. The word 'anaerobic' in its name indicates that it cannot see oxygen. Even a small amount of dissolved oxygen in water (such as exceeding 0.1mg/L) can inhibit bacterial activity, and in severe cases, even cause bacterial death. So oxygen must be strictly controlled in the reaction tank, generally ensuring that the dissolved oxygen concentration is below 0.1mg/L. This requires designing a reasonable water inlet method to avoid air entering, and sometimes nitrogen blowing is also necessary to remove oxygen.
Next is the substrate concentration, which is the concentration of ammonia nitrogen and nitrite. These two substances are the "food" of bacteria, but not too much, as too much will "support" them. Research has found that when the concentration of nitrite exceeds a certain limit (such as 200mg/L or more), it will inhibit the activity of bacteria; A high concentration of ammonia nitrogen is also not effective, as it can also have inhibitory effects. Moreover, the ratio of the two is also crucial. Generally speaking, the concentration ratio of ammonia nitrogen to nitrite is most suitable between 1:1 and 1:1.3, just like the appropriate ratio of salt and soy sauce when cooking, which makes dishes delicious.
In addition, the age of sludge is also important. As mentioned earlier, anaerobic ammonia oxidizing bacteria grow slowly, so the sludge in the system must be "kept for a longer time", that is, the sludge age must be long enough, usually kept at least 30 days, otherwise it will be discharged as soon as it grows a little, and the bacterial count in the pool will not be enough, which may not improve the treatment effect. This is also why anaerobic ammonification processes mostly use equipment such as sequencing batch reactors (SBR) or fluidized beds that can retain sludge.
Some inhibitors can also cause trouble, such as heavy metals (such as copper, zinc, chromium), certain antibiotics, and even toxic organic compounds brought in from wastewater, which can harm anaerobic ammonia oxidizing bacteria. So before entering the water, it is necessary to do a good job of pre-treatment and try to remove these harmful substances as much as possible, otherwise the bacteria that have been painstakingly cultivated may be "poisoned".
Finally, hydraulic retention time cannot be ignored. Simply put, it refers to the time that wastewater stays in the reaction tank. Because the anaerobic ammonia oxidation reaction is not particularly fast and bacteria grow slowly, sewage needs to stay in the pool for a while longer to allow bacteria enough time to "eat" ammonia nitrogen and nitrite. If the residence time is too short and the sewage is not completely treated before flowing away, the effluent quality will definitely not meet the standard.
Nowadays, anaerobic ammonification technology has been applied in many places, such as treating high ammonia nitrogen wastewater such as leachate from garbage and livestock farm wastewater, with particularly good results. But it is not omnipotent either. For example, when the nitrate in the wastewater is not enough, it needs to be combined with the previous short-range nitrification process. First, a part of the ammonia nitrogen is converted into nitrite, and then anaerobic ammonia bacteria are used for treatment.
Overall, anaerobic ammonification technology is like a "cost-effective and efficient" sewage treatment expert, but to make it work well, it is necessary to adjust the temperature pH、 Dissolved oxygen is well served, otherwise it would not be willing to contribute. With the increasing maturity of technology, we believe that more sewage treatment plants will use it in the future, making our water environment better.
