Let's first understand one thing: In sewage treatment plants, activated sludge is not just ordinary mud. It is the home of a group of "sewage treatment experts" - microorganisms such as bacteria, fungi, and protozoa live inside, relying on the organic matter in the sewage to survive. At the same time, they decompose pollutants into harmless water and carbon dioxide, which makes the sewage clean. But once the activated sludge is' poisoned ', these little experts will wilt, the processing capacity will plummet, and even the entire system may collapse. Today, let's break it apart and talk about how this poisoning gradually caused the breakdown of processing ability.
Firstly, it is important to clarify what is meant by 'activated sludge poisoning'? In fact, it's just that there are inappropriate "toxins" mixed into the sewage - it could be heavy metals discharged from factories (such as copper, chromium, mercury), strong acids and bases, or difficult to degrade organic compounds (such as components in certain pesticides and chemical wastewater). Once these things enter the system, they become "deadly killers" for microorganisms, which will then trigger a series of chain reactions and naturally reduce their processing capacity.
The first step, and also the most direct, is to kill the "main players" - bacteria with the poison. We all know that the core of activated sludge is heterotrophic bacteria, which are the main force in decomposing organic matter. The cell membranes and enzyme systems of these bacteria are all "weak spots". For example, heavy metals, when they come into contact with bacteria, will stick to the cell membrane, breaking the structure of the cell membrane, which is equivalent to smashing the "protective shell" of the bacteria. The cytoplasm and nucleus inside will leak out, and the bacteria will die directly; There are also some toxins that compete with enzymes in the bacterial body for space. Enzymes are originally "tools" that help bacteria decompose organic matter. When toxins occupy space, enzymes cannot work, and even if bacteria are alive, they cannot "eat" and can only slowly starve to death.
Think about it, the pool was originally filled with active bacteria, but after poisoning, they died and collapsed, and the number of bacteria that could work suddenly decreased by half. Previously, 100 bacteria could process 100 parts of organic matter, but now there are only 20 live ones left. Isn't the remaining 80 parts of organic matter piled up in the pool? The effluent quality must have exceeded the standard, so the treatment capacity has decreased.
The second step is that even if some bacteria do not die immediately, they will still be "too scared to move" - enter a "dormant mode", or change their metabolic mode and no longer properly decompose pollutants. Many microorganisms have a "stress response". Once they feel danger in the environment (such as the presence of toxins), they will stop normal growth, reproduction, and metabolic activities, and instead synthesize some "protective substances" to wrap themselves up, just like animals hibernating, saving their lives first.
For example, if high concentrations of phenolic compounds (commonly found in chemical wastewater) suddenly mix into sewage, many bacteria that decompose organic matter will "strike" and no longer decompose COD (chemical oxygen demand, representing the amount of organic matter in water). Instead, they will activate a special metabolic pathway to try to break down phenols to save their lives. But in this way, they don't have the energy to deal with the organic matter that should have been treated, and the COD in the pool cannot be reduced, so the treatment effect naturally deteriorates. Moreover, this' dormancy 'is not temporary. If toxins persist, bacteria may not recover their activity and their processing capacity will be sluggish for a long time.
The third step is that the "structure" of activated sludge will be disrupted, transforming from "flocculent small groups" to "scattered soldiers" and unable to settle properly, further dragging down the treatment system. Normal activated sludge is flocculent, like small cotton balls, and these flocs are coated with a large number of bacteria, with particularly good settling performance - in the secondary sedimentation tank, they can quickly settle to the bottom of the tank, clear water flows away from above, and sludge can also be returned to the aeration tank for continued use.
But once poisoned, the situation changes. On the one hand, after the bacteria die, the "skeleton" in the flocs (mainly the viscous substances such as polysaccharides and proteins secreted by the bacteria) loses support, and the flocs will break down into small particles; On the other hand, some toxins can destroy the sticky substances secreted by bacteria, causing flocs to lose their stickiness and unable to gather together, only floating in water. These broken small particles cannot sink in the secondary sedimentation tank and will flow away with the effluent, forming the phenomenon of "mud running".
Think about it, all the sludge has run away, the amount of activated sludge in the aeration tank is decreasing, and there are fewer microorganisms that can work, forming a vicious cycle. And the sludge that runs out will also make the effluent turbid, with COD and SS (suspended solids) exceeding the standard. The treatment system is like adding insult to injury. Can the treatment capacity not decrease?
The fourth step is to disrupt the "ecological balance" of microorganisms, causing beneficial bacteria to disappear and harmful bacteria to grow rapidly, further interfering with the treatment process. In normal activated sludge, microorganisms cooperate with each other: bacteria decompose organic matter to produce small molecule substances, protozoa (such as nematodes and rotifers) eat bacteria, control the number of bacteria, and secrete viscous substances to help form flocs. Everyone plays their own role in maintaining system stability.
After being poisoned, this balance is disrupted. Because different microorganisms have varying tolerance to toxins - beneficial bacteria that decompose organic matter often have poor tolerance to toxicity and die first; However, some highly toxic miscellaneous bacteria (such as certain actinomycetes) survived and began to reproduce in large numbers. These miscellaneous bacteria not only do not decompose pollutants, but also compete with the remaining beneficial bacteria for nutrients and oxygen, and even secrete substances that inhibit the growth of beneficial bacteria.
For example, sometimes after poisoning, a large number of filamentous bacteria will appear in the aeration tank. These bacteria grow thin and long, and will wrap around the activated sludge flocs, causing a decrease in the settling performance of the flocs (commonly known as "sludge swelling"), which can also cause sludge leakage. Moreover, filamentous bacteria do not decompose organic matter very much. When there are too many of them, the living space of beneficial bacteria is compressed, and their processing capacity naturally cannot be improved.
Lastly, the "recovery ability" of activated sludge will weaken after poisoning, and even if the toxins are removed later, the treatment capacity cannot be immediately restored. Because the growth and reproduction of microorganisms require time, dead bacteria cannot be revived, and the remaining few bacteria need to reproduce to their original numbers, rebuild floc structures, and restore ecological balance. This process may take several days or even weeks. During this period, the processing capacity of the system has been at a low level. If there are any errors in the middle (such as fluctuations in the inflow load), it may even completely collapse and require the addition of sludge to restart.
To sum up, the reason why activated sludge poisoning leads to a decrease in treatment capacity is essentially a "toxin attack on microorganisms" - first killing or inhibiting the core decomposing bacteria, then destroying the floc structure and microbial ecological balance of the sludge, and finally leading to the inability of organic matter to decompose, sludge running away, and a vicious cycle in the system. So the biggest fear for sewage treatment plants is the sudden mixing of toxic substances into the incoming water. They usually have to monitor the quality of the incoming water and take measures immediately if any abnormalities are found. Otherwise, the painstakingly built treatment system may go on strike due to a single poisoning.
