If you have been to a sewage treatment plant, you must have seen clusters of brown "mud" in the aeration tank - this is activated sludge. Don't underestimate it, there are thousands of microorganisms hidden inside, such as bacteria, fungi, and protozoa, which are the "main force" in treating sewage. By relying on them to decompose organic matter and remove pollutants, sewage can become clean. But if cyanide mixes into the sewage, these "environmental experts" will be in trouble, and the entire activated sludge system may go on strike directly. Today, let's talk in plain language about how cyanide really affects activated sludge.
Let's first understand: What does activated sludge rely on for "work"?
Before discussing the hazards of cyanide, it is necessary to first understand the "working principle" of activated sludge. Its core is the "microbial community", and these little ones have clear division of labor: some bacteria are responsible for gnawing on organic matter in sewage, turning them into carbon dioxide and water; Some can decompose nutrients such as nitrogen and phosphorus; Protozoa, on the other hand, will consume excess bacteria to maintain the balance of their microbiota. The entire process is like a miniature 'sewage treatment plant', and the normal metabolism, respiration, and reproduction of microorganisms are the key to the operation of this system.
Simply put, microorganisms can only work when they are alive. Once they encounter problems, activated sludge transforms from a "purification artifact" into useless "dead mud". Cyanide, on the other hand, is precisely the "deadly nemesis" of microorganisms.
Cyanide on Microorganisms: Precise Strike on 'Respiratory Organs'
Cyanide targets microorganisms not just by randomly punching the master, but by precise sniping - specifically targeting the cytochrome oxidase in microbial cells. What is this thing? You can consider it as the "breathing valve" of microorganisms, responsible for the final step of cellular respiration, combining oxygen and electrons to produce the necessary energy (ATP) for microbial survival.
Just like how people suffocate without oxygen, microorganisms lose their normal respiratory function, and energy supply is instantly cut off. For example, bacteria were busy decomposing organic matter in wastewater. As soon as cyanide came in, the "breathing valve" got stuck, and the bacteria immediately "couldn't breathe". Not to mention decomposing pollutants, it was even difficult for them to survive. In a short period of time, a large number of bacteria died and the number of "effective microbial communities" in activated sludge plummeted.
And cyanide is particularly toxic, even if the concentration in sewage is only a few milligrams per liter (mg/L), it can cause harm to sensitive microorganisms; If the concentration exceeds 20mg/L, many activated sludge systems will simply collapse. What's even more frustrating is that unlike some pollutants that slowly decompose, once they enter the aeration tank, they quickly spread and the entire microbial community suffers.
Activated sludge 'exposed': changes from inside out
Once microorganisms die, the characteristics and functions of activated sludge will immediately change, and knowledgeable sewage treatment workers can tell at a glance that something has happened. These changes are particularly intuitive, let's talk about them one by one:
1. Deterioration of sludge properties: from "clumping" to "scattering sand"
Normal activated sludge is flocculent, like small cotton fibers, which can stably settle at the bottom of the tank (this is called "good settling ability"), making it convenient for subsequent separation. But after cyanide poisoning, bacterial death releases substances such as polysaccharides and proteins in the cells, causing the sludge flocs to become loose, and some even break into small particles.
The result is that the water in the aeration tank becomes turbid, the sludge cannot settle down, and the effluent from the secondary sedimentation tank carries a large number of sludge particles, directly deteriorating the water quality. Sometimes, "sludge bulking" will occur, and the sludge volume will become larger, floating on the water like foam, which can not be handled normally.
2. Sharp decline in processing capacity: pollutants cannot be cleared away
The core task of activated sludge is to degrade pollutants, but the microorganisms are almost dead, who will do the work? The most obvious thing is the sharp drop in COD (chemical oxygen demand, a measure of organic matter) removal rate - originally able to remove 80% of COD, after poisoning, only 30% may remain, and the effluent COD seriously exceeds the standard.
If there is still nitrogen and phosphorus in the sewage, the situation will be even worse. The bacteria responsible for nitrification (converting ammonia nitrogen into nitrate) are particularly sensitive to cyanide, and even at low concentrations, the nitrification process will first "stop", leading to a surge in ammonia nitrogen in the effluent. At this point, the sewage treatment plant has basically lost its purification capacity, and the discharged water is "unqualified".
3. Microbial community 'big overhaul': less useful, more useless
The microbial community in activated sludge is "survival of the fittest", and under normal circumstances, beneficial bacteria that decompose organic matter dominate. But when cyanide comes, most beneficial bacteria cannot withstand it, leaving only a few miscellaneous bacteria that are resistant to cyanide (such as certain pseudomonas). Although these miscellaneous bacteria can survive, their ability to decompose pollutants is very poor. Even if the bacterial population gradually recovers, the purification efficiency of activated sludge cannot return to before.
What's more troublesome is that some resistant bacteria can produce sticky substances, making sludge more difficult to handle and even breeding odors, making the entire aeration tank smelly.
Acute poisoning vs. chronic poisoning: two different types of torment
The impact of cyanide on activated sludge can be classified into "acute" and "chronic", just like how a person's illness can occur suddenly or slowly.
Acute poisoning: sudden onset, quick death
Most of the time, it is due to the illegal discharge and leakage of industrial wastewater, which causes high concentrations of cyanide to rush into the sewage treatment plant. This situation is like "suddenly pouring a pot of poison on the activated sludge", and obvious changes can be seen within a few hours: more foam in the aeration tank, lighter sludge color (from brown to grayish yellow), and turbid effluent. If not detected in time, the entire system may crash within half a day, and it may take several days or even weeks to recover.
Previously, a local electroplating factory secretly discharged cyanide containing wastewater, causing the activated sludge from nearby sewage treatment plants to "fail" overnight. The effluent COD skyrocketed from tens to hundreds, and finally had to urgently stop the inflow and add new sludge strains.
Chronic poisoning: hidden deep, destroyed for a long time
This is more covert, for example, some factories discharge low concentrations of cyanide for a long time, and the concentration is not enough to immediately kill microorganisms, but over time, the damage to the microbial community gradually becomes apparent. At the beginning, it may only be a slight decrease in COD removal rate or a deterioration in sludge sedimentation, which can easily be mistaken for other problems (such as insufficient aeration or changes in water temperature).
But over time, the activity of microorganisms will decrease, and the structure of the microbial community will be completely destroyed. Even if cyanide emissions are stopped later, it will be difficult for activated sludge to recover. Just like a person who consumes trace amounts of toxins for a long time, their body gradually collapses and it is particularly difficult to recover.
How to deal with it? Anti toxic measures for sewage treatment plants
Since cyanide is so harmful, sewage treatment plants cannot sit idly by. There are two commonly used methods now: "prevention in advance" and "rescue after the fact".
1. Pre prevention: Keep cyanide outside the door
The most fundamental solution is to prevent high concentration cyanide containing wastewater from entering the sewage treatment system. Industrial enterprises (such as electroplating, mining, and pharmaceutical factories) must first treat cyanide containing wastewater themselves and reduce the cyanide concentration to a range acceptable to the wastewater treatment plant (generally requiring less than 0.5-1mg/L) before discharge.
The sewage treatment plant will also install online monitoring devices at the inlet to monitor the cyanide concentration in real time. Once it exceeds the standard, an alarm will be immediately triggered, and the inlet valve will be closed to prevent "toxic water" from entering the aeration tank. This is like a security check at the entrance of a residential area, where 'dangerous goods' are first stopped.
2. Post rescue: detoxify and prolong the life of activated sludge
If there is really a cyanide leak, we need to rescue it quickly:
-Emergency dilution: Add a large amount of clean water or reflux sludge to the aeration tank to reduce cyanide concentration and give microorganisms a chance to catch their breath.
-Add detoxifying agents such as sodium hypochlorite and hydrogen peroxide to oxidize cyanide into harmless carbon dioxide and nitrogen gas; Alternatively, ferrous sulfate can be added to combine with cyanide to form insoluble ferrous cyanide precipitate, which can then be filtered off.
-Supplementary bacterial strains: After the cyanide concentration drops, add new activated sludge or efficient bacterial strains to help restore the vitality of the bacterial community. This process is like administering intravenous fluids and supplementing nutrients to a sick person, which requires slow adjustment.
3. Long term domestication: cultivating an "anti toxic bacterial community"
For wastewater treatment plants that frequently come into contact with low concentrations of cyanide (such as wastewater from electroplating wastewater concentration areas), active domestication of activated sludge can be initiated. Simply put, it is to gradually increase the concentration of cyanide in wastewater, allowing microorganisms to adapt and screen out bacterial communities that can resist or even decompose cyanide.
Just like exercising immunity, adding extremely low concentrations of cyanide at the beginning and slightly increasing it after microbial adaptation, over time, the sludge can tolerate higher concentrations of cyanide while maintaining its purification ability. However, this process needs to be taken slowly, as rushing can actually lead to sludge poisoning.
Finally, to be honest
Activated sludge is the "core weapon" of sewage treatment, and cyanide is the "precision strike weapon" against it. Nowadays, with the rapid development of industry, the risk of cyanide containing wastewater discharge has always existed. If enterprises are not responsible and treatment plants are not vigilant, it is easy to encounter problems.
At the end of the day, protecting activated sludge from cyanide damage is not only the responsibility of sewage treatment plants, but also requires polluting enterprises to follow the rules and pre treat wastewater well. After all, if the sewage is treated well, our rivers and soil will not be polluted. This matter concerns everyone. I hope there will be less news about "cyanide leaks" in the future, so that those microorganisms can do their job as "sewage treatment workers" safely and steadily~
