Our friends who deal with sewage treatment, who deal with activated sludge every day, know that this thing is like a group of "foodie soldiers" who rely on organic matter in the water to work. But you know what? If these soldiers want to work well, having "food" alone is not enough, they also need various "nutritional supplements", among which phosphorus is a particularly crucial one. If there is a lack of phosphorus in the water, the activated sludge will not be lively, and there will be a lot of troubles in the future. Today, let's break it apart and talk about this matter.
First of all, we need to understand what phosphorus does for microorganisms in activated sludge. Think about it, microorganisms need to grow, reproduce, and synthesize various enzymes to decompose organic matter, all of which rely on phosphorus. Let's talk about the genetic material DNA in cells, it cannot be synthesized without phosphorus; There are also substances responsible for transmitting energy, such as ATP and phosphorus, which are core components. Simply put, phosphorus is like the "energy battery" and "growth building material" of microorganisms. Without it, microorganisms would find it difficult to survive, let alone work on treating wastewater.
If phosphorus is really lacking, what changes will occur in activated sludge first? The most intuitive thing is that the settling performance of sludge has deteriorated. Originally, the activated sludge was fine, and after sedimentation, the supernatant was clear and the sludge layer was compact. However, when phosphorus was deficient, you would find that the sedimentation tank was filled with scattered sludge, like a layer of "cotton fluff" floating on it, and the supernatant became turbid. Why is this? Due to phosphorus deficiency, microorganisms are unable to synthesize cell walls and capsule structures normally, making the cell walls fragile and prone to secreting some poorly viscous polysaccharides. In this way, the sludge particles cannot gather well together and become "scattered soldiers", naturally unable to sink. Sometimes there may be a situation of "sludge swelling", where the sludge in the entire aeration tank is floating. When measured with a sludge concentration meter, the concentration looks not low, but it is all virtual and has no combat effectiveness.
Moving on to the processing effect, this is what we are most concerned about. Phosphorus deficiency first affects the efficiency of organic matter removal. Microorganisms originally ate organic matter as easily as we eat, but after phosphorus deficiency, they couldn't even synthesize enzymes that break down organic matter, and the organic matter they ate couldn't be completely decomposed, only staying in the intermediate product stage. You can test the COD (Chemical Oxygen Demand) to find out. The influent COD is quite high, but the effluent COD still cannot be reduced. Sometimes, the effluent COD may even be higher than before - it's not that the microorganisms are not working, it's that they can't decompose and even discharge some organic matter from their bodies.
In addition to organic matter, nitrogen removal can also be affected. Nowadays, many sewage treatment plants need to carry out denitrification, relying on nitrifying bacteria and denitrifying bacteria. Nitrifying bacteria convert ammonia nitrogen into nitrate nitrogen, while denitrifying bacteria convert nitrate nitrogen into nitrogen and release it. But both types of bacteria are particularly sensitive to phosphorus, especially denitrifying bacteria. When phosphorus is deficient, the respiration of denitrifying bacteria is inhibited, and they cannot use nitrate nitrogen as an electron acceptor. As a result, the nitrate nitrogen content in the effluent skyrockets, and the denitrification effect is directly "halved". Sometimes you may find that the dissolved oxygen in the aeration tank is sufficient, but the removal rate of ammonia nitrogen cannot increase. After checking, it is very likely that the lack of phosphorus leads to the activity of nitrifying bacteria
Let's talk about the amount and properties of sludge. Phosphorus deficiency can slow down the growth and reproduction rate of microorganisms. Originally, they could reproduce one generation per day, but after phosphorus deficiency, it may take two or three days to reproduce one generation, which will reduce the growth of sludge. Some friends may think that "less sludge is good, saving the need to treat sludge", but in fact, it is not the case - less sludge means less microorganisms that can work, which in the long run will lead to a "shortage of forces" in the entire activated sludge system. When encountering a slight impact load, such as a sudden increase in organic matter in the inflow, the system cannot withstand it and is prone to collapse. Moreover, the properties of sludge with phosphorus deficiency will also change, such as an increase in moisture content. Originally, the moisture content of sludge cake could be controlled below 80% after dehydration, but after phosphorus deficiency, it may rise to over 85%. The subsequent cost of sludge disposal will directly increase, whether it is landfill or incineration, which will require more money.
Another easily overlooked point is that phosphorus deficiency can make the activated sludge system particularly "fragile" and have poor impact resistance. Under normal circumstances, even if there are small fluctuations in the incoming water quality, microorganisms can slowly adapt; But when phosphorus is deficient, microorganisms themselves are in a state of "malnutrition", with slight fluctuations such as changes in pH, temperature drops, or the presence of trace amounts of toxic substances in the water, making it easy for microorganisms to die in large numbers. You will find that the foam in the aeration tank suddenly becomes more and more, with a fishy smell. Take some sludge for microscopic inspection, and you can see the bodies of many microorganisms. The active protozoa (such as bell worms and rotifers) are almost invisible, but they are all metazoans (such as nematodes) with strong pollution resistance, which indicates that the system has gone wrong.
How can we determine if there is a phosphorus deficiency? There are actually a few simple methods. Firstly, the phosphorus content in the influent and activated sludge can be measured. Generally speaking, the phosphorus content in activated sludge (calculated as dry sludge) should be between 1.5% and 2.5%. If it is lower than 1%, it is highly likely that there is a phosphorus deficiency. In addition, the appearance of sludge can also be seen. The phenomena of poor sedimentation, turbid supernatant and more foam just mentioned can be used as a reference. Another thing is to test the treatment effect. If the removal rates of COD, ammonia nitrogen, and nitrate nitrogen suddenly decrease at the same time, after excluding factors such as dissolved oxygen, pH value, and temperature, it is necessary to check if there is insufficient phosphorus.
If you really lack phosphorus, don't panic. The most direct way is to supplement phosphorus. The commonly used phosphorus supplements include superphosphate, potassium dihydrogen phosphate, and disodium hydrogen phosphate. The specific amount to be supplemented depends on the phosphorus content of the influent, sludge concentration, and treatment objectives. For example, if you want to increase the phosphorus content in activated sludge to 2%, you can calculate how much phosphorus needs to be added based on the dry weight of the sludge, and then add it in several times - don't add too much at once, otherwise it may lead to excessive phosphorus in the effluent, which is even more troublesome. In addition, it can also be controlled from the source, such as checking if there is too much industrial wastewater in the water and the phosphorus content is already low. If upstream enterprises can be coordinated to adjust the drainage water quality and fundamentally solve the problem of phosphorus deficiency, it would be even better.
Overall, although the proportion of phosphorus in activated sludge systems is not high, it is definitely a key nutrient that can have a ripple effect on the whole body. Lack of phosphorus, microorganisms
