If the nitrification system of activated sludge collapses, it is really a "big problem" in sewage treatment plants. Sometimes, when the data comes out the next day, the ammonia nitrogen index rises sharply, which can make the operators anxious. This nitrification system is like a delicate little girl, it can "go on strike" if it's a little uncomfortable. Today, let's talk about how it "collapsed" step by step.
First, let's talk about the "core employees" of this nitrification system - nitrifying bacteria. These little guys may not look impressive, but they are the main force in processing ammonia nitrogen, divided into two categories: one is nitrite bacteria that convert ammonia nitrogen into nitrite; Another type is nitrate bacteria that convert nitrite into nitrate. They are like workers on an assembly line, they need to work together seamlessly to do the job well. But these two bacteria have a common problem: they are afraid of cold, hunger, and fatigue. If the environment is slightly off, they will immediately abandon them.
First, let's talk about the most common "killer" - temperature. Think about it, the most comfortable temperature for nitrifying bacteria is 25 to 30 degrees Celsius, just like how we feel comfortable wearing a thin coat in spring. But once the temperature drops suddenly, for example, the heating is suddenly cut off in winter, or the cold water is poured into the biochemical pool by the rainstorm in summer, and the temperature drops below 10 degrees, these bacteria seem to be frozen stiff, and their activity is directly cut in half. What's even more deadly is that nitrate bacteria are more sensitive to cold than nitrite bacteria. When they slack off, nitrite will pile up in the pool. The nitrite bacteria in front of them see the "semi-finished products" accumulating more and more, and they don't have the heart to work anymore. The entire nitrification chain is stuck like this. There was a sewage treatment plant before, and suddenly there was a cold wave in autumn. The heating was not turned on in time at night, and the next day the ammonia nitrogen directly exploded on the meter. After checking, it was the temperature that caused the trouble.
Moreover, saying that something is wrong with the food can also bring down the system. The term 'eating' here mainly refers to the concentration and load of ammonia nitrogen in the influent. Sometimes factories suddenly discharge high concentrations of wastewater, causing ammonia nitrogen to skyrocket from the usual 50mg/L to 200mg/L. It's like stuffing a table of fat for nitrifying bacteria, which they can't digest at all. These bacteria usually eat slowly, but when they suddenly have such a big meal, they are either "stretched to death" or simply "hunger strike" in protest. There is also a situation where there is "no food to eat", such as when the factory stops production and the ammonia nitrogen in the inflow is almost zero. Nitrifying bacteria will "gnaw" on each other when they are extremely hungry, and their numbers will decrease. When the inflow is normal, even if there is food, there will not be enough bacteria to work, and the system will naturally collapse. Just like a person who has been hungry for several days, suddenly being asked to do heavy work is definitely unbearable.
Dissolved oxygen is also a key role. Nitrifying bacteria are "oxygen maniacs", and the dissolved oxygen in the biochemical pool must be maintained at 2mg/L or above in order for them to breathe freely. If the aeration equipment breaks down or the organic matter in the incoming water suddenly increases, aerobic bacteria will rush to eat the organic matter and consume all the oxygen, while nitrifying bacteria will be trapped in a sealed tank and suffocate in minutes. I have seen a small factory where the belt of the aeration fan was broken and not discovered in time. After two hours of no aeration, the dissolved oxygen in the tank dropped below 0.5mg/L. By the time it was fixed, the nitrification system had already collapsed and it took more than half a month to recover. What's even more frustrating is that sometimes dissolved oxygen levels fluctuate, like a person being held in water for a while and then pulled out to breathe. Bacteria simply can't adapt and their activity slowly diminishes.
PH value is also an 'invisible killer'. Nitrifying bacteria prefer neutral to alkaline environments, with pH values between 7.5 and 8.5 being the most active. But if the incoming water suddenly carries acidic wastewater, such as the waste acid discharged from a chemical plant, or if the effluent from an anaerobic tank flows back too much, causing the pH value in the tank to drop below 6, these bacteria will immediately wilt like vinegar. Once I visited a factory that processes chemical wastewater, and the neighboring workshop secretly discharged a batch of acid washing wastewater. The pH of the biochemical tank dropped from 7.8 to 5.2, and the nitrifying bacteria "collectively died". The ammonia nitrogen could not be reduced for several days, and in the end, we had to add new strains of bacteria to save it.
Another easily overlooked reason is the surprise attack of toxic substances. The "anti toxicity ability" of nitrifying bacteria is pitifully weak, such as heavy metals (such as copper and zinc), organic solvents (such as alcohol and acetone), and even some residual disinfectant water, which are highly toxic to them. Sometimes when factories clean equipment and discharge wastewater with disinfectant, even if the concentration is only a few ppm, it can completely destroy nitrifying bacteria. Even more insidious is that some toxic substances do not immediately cause the system to collapse, but rather accumulate slowly. They can cause some damage today and tomorrow, and by the time they are discovered, there are not many bacteria left. This "chronic poisoning" is more difficult to investigate than "acute poisoning".
In addition to these external factors, the system's own poor physical condition cannot withstand the turmoil. For example, if the age of the sludge is too short, nitrifying bacteria will naturally reproduce slowly, taking about 20 days to grow a new generation. If the age of the sludge is only 10 days, the newly grown bacteria will be expelled and there will be no way to save up the quantity. Also, the sludge concentration is too low, just like there are too few soldiers on the battlefield, even if we can fight, we cannot withstand the enemy's attack, and even a small fluctuation cannot withstand it. Some factories intentionally reduce sludge concentration in order to save costs, resulting in a slight impact on the load and the system crashing directly. Instead, they spend more money on repairs, which is like picking sesame seeds and losing watermelons.
Finally, there is a 'butterfly effect': once the system starts to malfunction, if not detected in a timely manner, small problems can turn into big problems. For example, if ammonia nitrogen is slightly elevated at the beginning and is not taken seriously, bacteria will lose their activity due to deteriorating "work results"; Subsequently, dissolved oxygen may appear "excessive" due to a decrease in bacteria, and beginners may mistakenly believe that aeration is sufficient and instead reduce aeration; By the time it was discovered, the sludge in the pool had turned black and smelly, and the nitrifying bacteria had already died off. At this point, it was impossible to recover after a month or so.
So, the collapse of the nitrification system is never sudden, like a domino effect where the first card falls (such as a sudden change in temperature), the following cards (decreased bacterial activity, accumulation of ammonia nitrogen, deterioration of sludge) fall along, and ultimately the entire system collapses completely. To ensure the stable operation of this system, one must be as careful as taking care of a child: constantly monitoring the temperature pH、 Dissolve oxygen, control the inflow load, and prevent toxic substances from entering. Observe the condition of the sludge regularly and make adjustments promptly if any abnormalities are found. After all, only by cultivating nitrifying bacteria can they effectively help us treat wastewater and ensure that the effluent meets the standards.
