Our colleagues in sewage treatment have probably encountered this headache: a few days ago, various indicators of effluent were stable, and ammonia nitrogen and COD were all below the qualified line, but suddenly a "load shock" hit. Within two days, the laboratory data turned red, and ammonia nitrogen rose rapidly. The leaders urged and environmental inspections were really two big things. Today, I will talk to everyone about how the load shock gradually caused the effluent ammonia nitrogen to exceed the standard. We have a clear understanding and should be prepared in advance.
Firstly, it must be clarified what is meant by 'load shock'? To put it simply, the "food intake" of the sewage treatment plant suddenly exceeded the standard. It could be that the upstream factory secretly discharged high concentration wastewater, or it could be that a large wave of pollutants was brought in by mixed flow in the pipeline network on rainy days, or it could be that the pump station did not control the water flow well, which suddenly stuffed too much "food" into the biochemical tank. Our biochemical system is like a cafeteria. Normally, everyone eats according to their appetite, but suddenly a group of big eaters rush in, causing chaos in the kitchen. This is the basic situation of load shock.
What changes will occur in the biochemical pool at the beginning of the load shock? The most obvious thing is that there is too much 'eating' and the microorganisms cannot keep up. We all know that the treatment of ammonia nitrogen mainly relies on nitrifying bacteria, which are very delicate. They need to be eaten in moderation and have a comfortable environment. Under normal circumstances, the concentration of ammonia nitrogen and organic load in the influent are stable, and nitrifying bacteria slowly multiply, which can convert ammonia nitrogen into nitrate. But once the load suddenly increases, such as when the concentration of ammonia nitrogen in the inflow drops from the usual 30mg/L to 80mg/L, or when the inflow doubles, the total amount of ammonia nitrogen per unit volume suddenly increases, and the "workload" of nitrifying bacteria suddenly doubles several times, they will first be "confused".
Immediately after, there was not enough dissolved oxygen. Microbial treatment of pollutants relies on oxygen assistance. When the load is high, microorganisms will desperately "breathe" to decompose organic matter, resulting in a sharp increase in oxygen consumption. Our aeration system has a maximum oxygen supply capacity, which is sufficient for normal use. When there is a shock, the food will stop. Just like a group of people running in a small room at the same time, there is definitely not enough oxygen, and everyone will gasp for breath. The dissolved oxygen concentration in the biochemical pool will rapidly drop from the usual 2-3mg/L to below 1mg/L, and even approach zero. Nitrifying bacteria are particularly sensitive to dissolved oxygen, and they need at least 1-2mg/L of dissolved oxygen when working. When there is a lack of oxygen, they will strike and the efficiency of ammonia nitrogen conversion will directly decrease. At this point, when measuring the dissolved oxygen in the biochemical tank, you will find that the value drops rapidly, and even when the aerator is turned to its maximum, it cannot withstand it. The bubbles on the water surface appear weak and lifeless.
Then the pH value will drop, which is even worse for nitrifying bacteria. When microorganisms decompose organic matter, they produce organic acids, and the higher the load, the more acids are produced. Meanwhile, the nitrification reaction itself also consumes alkalinity, requiring approximately 7.14g of calcium carbonate equivalent alkalinity for every 1g of ammonia nitrogen conversion. Under load shock, alkalinity is rapidly consumed without timely replenishment, and the pH value in the biochemical tank will drop from the usual 7.5-8.5 to below 7, or even to 6.5. Nitrifying bacteria are most suitable for working in neutral alkaline environments. As the pH decreases, their activity is like being frozen, and the reaction rate decreases significantly. At this point, when you go to measure pH, you will find that the value changes day by day and slowly drops, and the result measured by the alkalinity test kit will also be frighteningly low.
Even more troublesome is that load shock can cause conflicts within the microbial community. Our biochemical pool not only contains nitrifying bacteria, but also many heterotrophic bacteria that decompose organic matter. Heterotrophic bacteria are much more dominant than nitrifying bacteria, as they reproduce quickly and compete fiercely for food. Normally, everyone lives in peace, but once high concentrations of organic matter arrive, heterotrophic bacteria will multiply like hungry wolves, competing with nitrifying bacteria for oxygen and living space. Just like a group of strong men rushing to grab food in the cafeteria, the slow chewing nitrifying bacteria cannot compete and can only go hungry. At this point, microscopic examination will reveal that the bacterial colonies have become loose, protozoa have decreased, and the number of nitrifying bacteria, which originally accounted for a certain proportion, has sharply decreased. The structure of the entire microbial community has been disrupted.
As time goes by, the activity and quantity of nitrifying bacteria decrease. Due to the impact of load, they not only suffer from hunger, hypoxia, and pH discomfort, but may also die due to environmental degradation. The reproduction rate of nitrifying bacteria is already slow, with a generation cycle of several days, unlike heterotrophic bacteria which can reproduce several generations in a day. Once a large number of nitrifying bacteria die, it becomes difficult to recover. At this point, if you go to measure the ammonia nitrogen in the biochemical tank, you will find that the ammonia nitrogen at the inlet has not decreased much, and it is still high at the outlet, indicating that the nitrification reaction has almost stagnated. The value of ammonia nitrogen in the effluent starts to rise from this point on.
If the load impact lasts for a long time or the impact intensity is particularly high, the situation will be even worse. The nitrification system may completely collapse, and even if the influent load is reduced, the ammonia nitrogen cannot be reduced back. Because the nitrifying bacteria have almost died, the "main force" in the biochemical pool is gone and needs to be retrained. It's like the chef in the back kitchen of a cafeteria getting tired and running away. Even if there are fewer customers, no one can cook anymore, so we have to recruit and train new people. This process can take as short as one or two weeks, or as long as one or two months, and the effluent ammonia nitrogen will definitely continue to exceed the standard.
Then the pH value will drop, which is even worse for nitrifying bacteria. When microorganisms decompose organic matter, they produce organic acids, and the higher the load, the more acids are produced. Meanwhile, the nitrification reaction itself also consumes alkalinity, requiring approximately 7.14g of calcium carbonate equivalent alkalinity for every 1g of ammonia nitrogen conversion. Under load shock, alkalinity is rapidly consumed without timely replenishment, and the pH value in the biochemical tank will drop from the usual 7.5-8.5 to below 7, or even to 6.5. Nitrifying bacteria are most suitable for working in neutral alkaline environments. As the pH decreases, their activity is like being frozen, and the reaction rate decreases significantly. At this point, when you go to measure pH, you will find that the value changes day by day and slowly drops, and the result measured by the alkalinity test kit will also be frighteningly low.
Even more troublesome is that load shock can cause conflicts within the microbial community. Our biochemical pool not only contains nitrifying bacteria, but also many heterotrophic bacteria that decompose organic matter. Heterotrophic bacteria are much more dominant than nitrifying bacteria, as they reproduce quickly and compete fiercely for food. Normally, everyone lives in peace, but once high concentrations of organic matter arrive, heterotrophic bacteria will multiply like hungry wolves, competing with nitrifying bacteria for oxygen and living space. Just like a group of strong men rushing to grab food in the cafeteria, the slow chewing nitrifying bacteria cannot compete and can only go hungry. At this point, microscopic examination will reveal that the bacterial colonies have become loose, protozoa have decreased, and the number of nitrifying bacteria, which originally accounted for a certain proportion, has sharply decreased. The structure of the entire microbial community has been disrupted.
As time goes by, the activity and quantity of nitrifying bacteria decrease. Due to the impact of load, they not only suffer from hunger, hypoxia, and pH discomfort, but may also die due to environmental degradation. The reproduction rate of nitrifying bacteria is already slow, with a generation cycle of several days, unlike heterotrophic bacteria which can reproduce several generations in a day. Once a large number of nitrifying bacteria die, it becomes difficult to recover. At this point, if you go to measure the ammonia nitrogen in the biochemical tank, you will find that the ammonia nitrogen at the inlet has not decreased much, and it is still high at the outlet, indicating that the nitrification reaction has almost stagnated. The value of ammonia nitrogen in the effluent starts to rise from this point on.
If the load impact lasts for a long time or the impact intensity is particularly high, the situation will be even worse. The nitrification system may completely collapse, and even if the influent load is reduced, the ammonia nitrogen cannot be reduced back. Because the nitrifying bacteria have almost died, the "main force" in the biochemical pool is gone and needs to be retrained. It's like the chef in the back kitchen of a cafeteria getting tired and running away. Even if there are fewer customers, no one can cook anymore, so we have to recruit and train new people. This process can take as short as one or two weeks, or as long as one or two months, and the effluent ammonia nitrogen will definitely continue to exceed the standard.
Another easily overlooked point is that the sedimentation tank is also prone to problems after load shock, indirectly leading to an increase in ammonia nitrogen. Under impact, microbial activity is poor, and the coagulation effect of microbial flocs is not good. This can lead to sludge swelling and sludge leakage in the sedimentation tank. A large number of nitrifying bacteria flow out of the system with the sludge, and the microbial population in the pool decreases, naturally causing the treatment capacity to fall behind. At this point, if you go to the sedimentation tank and look, there will be a layer of fine sludge floating on the water surface, and a lot of sludge will also be carried out from the outlet weir. Measure the sludge concentration (MLSS), and you will find that it is much lower than usual.
Someone may ask, why hasn't the ammonia nitrogen decreased after the load shock passed? This is because the recovery of nitrifying bacteria takes time. Just like when a person is overworked and sick, it's not something that can be cured in just one day of rest, they need to take care of themselves slowly. Even if the inflow load returns to normal and environmental factors such as dissolved oxygen and pH are adjusted back, nitrifying bacteria will have to reproduce and accumulate again, which can take several days or weeks. During this recovery period, the effluent ammonia nitrogen will remain high until the nitrification system's function is fully restored.
Let's summarize this process: sudden increase in load → rapid increase in microbial oxygen consumption, insufficient dissolved oxygen → decomposition of organic matter to produce acid, alkalinity consumption, pH decrease → large proliferation of heterotrophic bacteria, occupying the living space of nitrifying bacteria → inhibition of nitrifying bacterial activity, reduction in quantity → significant decrease in ammonia nitrogen conversion efficiency → sludge runoff from sedimentation tank, intensified microbial loss → continuous increase in effluent ammonia nitrogen → even after the impact is over, the nitrification system still needs time to recover, and ammonia nitrogen remains high.
By understanding this process, we can better prevent and respond to load shocks in our daily lives. For example, strengthening water inflow monitoring to detect abnormal fluctuations in advance; Optimize the aeration system to ensure sufficient oxygen supply capacity; Reserve some alkaline agents and replenish them in a timely manner if necessary; Ensure proper control of sludge reflux to prevent sludge leakage and other issues. By doing these tasks well, we can minimize the impact of load shock on effluent ammonia nitrogen, making our sewage treatment system more stable and reliable.
