In the aeration tank of a wastewater treatment plant lies a bustling "microscopic food city." Here, there are no Michelin-starred chefs, no food delivery couriers, and even ready-made "boxed meals" are scarce. The autotrophic microorganisms in the activated sludge are the most unconventional "diners" in this culinary hub—while others eagerly compete for organic debris in the wastewater, they stubbornly rely on their own efforts, extracting energy from "air and stone," and ingeniously transform inorganic matter into a "feast of Chinese and Western delicacies." Today, we’ll uncover how these "self-sufficient dining kings" turn "hard times" into "rich second-generation" living in the wastewater.
Let me start by giving you a bit of background: Activated sludge, in plain terms, is a "eco-friendly team" composed of billions of microorganisms. Their ultimate mission is to purify wastewater by breaking down pollutants. This team is divided into two major factions: heterotrophic microorganisms are the "gluttonous eaters," specializing in feasting on readily available organic matter (such as starch, protein, and grease) in the wastewater—they feast with great appetite. Meanwhile, autotrophic microorganisms are the "hardcore artisans," naturally equipped with a "kitchen buff," and simply disdain those "leftover scraps." They refuse to rely on others' leftovers and insist on sourcing raw materials themselves, crafting "organic feasts" from scratch. Do you think they're arrogant? No, they've evolved the superpower of "not needing to compete for food." In the "resource competition" environment of wastewater, they've forged a path to "self-sufficiency" in feeding themselves.
When it comes to the "top-tier foodies" among self-sustaining microorganisms, the nitrite bacteria family undoubtedly takes the crown—these little guys are the "chemical chefs of sludge," excelling in the art of "inorganic oxidation" to cook. You might wonder: inorganic matter? Doesn’t that include "hardcore" stuff like stones, ammonia, and sulfides? What we find unpalatable is actually the most delicious raw material in the eyes of nitrite bacteria—more tempting than even "Buddha jumps over the wall.".
In the family of nitrifying bacteria, there are two core "chefs": ammonia-oxidizing bacteria and nitrite-oxidizing bacteria. They are the "perfect duo," working with clearly defined roles, much like appetizer and main-course chefs in a restaurant. First to appear is the ammonia-oxidizing bacteria, whose "signature skill" is treating ammonia nitrogen (NH3) in wastewater as an "appetizer." Don’t underestimate this ammonia nitrogen—it isn’t highly concentrated in wastewater, but it carries a pungent "toilet smell." Heterotrophic bacteria tend to avoid it, yet the ammonia-oxidizing bacteria treat it as a treasure. Using its "oxidase tools" inside its body, it gradually "breaks down and oxidizes" the ammonia nitrogen. This process is like lighting natural gas with a lighter—though there’s no open flame, it releases continuous energy, which serves as their "cooking fuel.".
But the "culinary skills" of ammonia-oxidizing bacteria stop at the appetizer stage. After oxidizing ammonia nitrogen into nitrite (NO2-), they satisfy their own hunger and thirst, then hand over the remaining "half-finished product" of nitrite to their "good brother" nitrite-oxidizing bacteria. This "main dish master" is even more formidable, as it can further oxidize nitrite into nitrate (NO3-), releasing another wave of energy. Do you think they go through all this trouble just for their own benefit? No, while oxidizing these inorganic compounds, they are also secretly "engaging in a side business"—using the released energy to treat carbon dioxide (CO2) from the air as "flour," and scavenging water (H2O) and inorganic salts (such as potassium, phosphorus, and iron) from wastewater as "seasonings," synthesizing glucose and proteins into "organic delicacies" within their bodies. This operation is equivalent to while others are scrambling for mantou, they have already grown wheat, milled flour, and steamed buns, achieving a "self-sufficiency ceiling" in the microbial world.
Even more impressive is the fact that nitrifying bacteria, these "master chefs," are particularly "hardy." Although their "cooking efficiency" isn't particularly high—synthesizing 1 gram of organic matter may require oxidizing dozens or even hundreds of grams of ammonia nitrogen—they thrive on their "unfussy and uncompetitive" nature. In activated sludge, when heterotrophic bacteria have nearly exhausted the organic matter in wastewater, they grow hungry and even resort to "competing for food." Meanwhile, nitrifying bacteria can leisurely "slow work to perfect results," transforming inorganic matter into their own sustenance using the residual ammonia nitrogen in wastewater and carbon dioxide from the air.
It's like after a food festival, while everyone else picks up snack crumbs from the ground, they instead take out their own tools to process the "air and stones" on site into a feast. Such survival wisdom is truly admirable.
In addition to the "chemical cooking school" of nitrifying bacteria, there are another group of autotrophic "foodies" in the activated sludge—phototrophic autotrophic microorganisms, such as cyanobacteria and purple sulfur bacteria, who can be called "sunlight chefs." However, their role in wastewater treatment plants is less prominent than that of nitrifying bacteria, as the light in the aeration tank is not abundant, and the constant agitation from the aeration system makes it difficult to enjoy a quiet sunbath. Yet, despite these challenges, they excel with a "positive mindset and skilled hands." Even with minimal light, combined with "special ingredients" like hydrogen sulfide (H2S) and ferrous ions (Fe²⁺) in the wastewater, they can activate "Photosynthesis 2.0 mode." While ordinary plants use carbon dioxide and water to synthesize organic matter with light energy, releasing oxygen in the process, these phototrophic autotrophic microorganisms are more down-to-earth. They can replace water with hydrogen sulfide, use light to convert carbon dioxide and hydrogen sulfide into organic matter, and simultaneously extract sulfur from hydrogen sulfide, storing it as elemental sulfur particles within their bodies. This is like cooking while simultaneously "stockpiling.".
You can imagine this scene: in the corner of the aeration tank, a faint beam of light shines in, and the blue bacteria immediately become "energetic", spreading their "photosynthetic wings" (photosynthetic layers) one after another, like a group of "little chefs" holding solar panels, sunbathing and charging, while treating the pungent hydrogen sulfide in the sewage as "soy sauce" and carbon dioxide as "rice". With a fierce operation, they make a delicious "organic meal" and also treat the odorous pollutant hydrogen sulfide - solving the problem of eating and achieving environmental KPI, which is simply a model of "no delay in dry rice work".
However, speaking of which, although these autotrophic microorganisms are "highly skilled", they are not "otherworldly". When they make "organic meals", they also need some "trace element seasonings" - such as minerals such as iron, manganese, and zinc absorbed from sewage, which are equivalent to the salt and MSG we use for cooking. Without them, the "meal" they make will have no taste, and microorganisms will not grow on their own. And their "cooking rhythm" is particularly slow. Heterotrophic bacteria may only need a few hours to eat a meal, while nitrifying bacteria may take several days or even weeks to "eat and drink enough to grow their bodies". So in activated sludge, although they don't have to compete for food, they still need to develop in a low-key manner to avoid being "accidentally harmed" by heterotrophic bacteria (after all, heterotrophic bacteria are abundant and reproduce quickly, sometimes occupying their living space).
But don't underestimate these 'slow paced cooks', they are the' invisible heroes' of sewage treatment plants. Think about it, the ammonia nitrogen in sewage is a big problem. If not treated, it can lead to eutrophication of the water body when discharged into the river, resulting in a large number of algal blooms and red tides that can poison fish and shrimp. And the group of "chemical chefs" such as nitrifying bacteria can gradually convert ammonia nitrogen into nitrate, which is then converted into nitrogen by denitrifying bacteria and discharged into the air, effectively turning the "toxic waste" in sewage into a "harmless gas". If it weren't for them, the sewage treatment plant wouldn't have been able to complete the "denitrification" task, and our river water might have turned into "green soup" long ago.
What's even more interesting is that these autotrophic microorganisms are particularly 'united'. The two "masters" of nitrifying bacteria - ammonia oxidizing bacteria and nitrite oxidizing bacteria - never "fight alone", they always come together to form a "nitrifying bacterial community". The nitrite produced by ammonia oxidizing bacteria happens to be the "food" for nitrite oxidizing bacteria; And nitrite oxidizing bacteria eat nitrite, which can prevent the accumulation of nitrite in the environment, and in turn protect ammonia oxidizing bacteria (high concentrations of nitrite are toxic to ammonia oxidizing bacteria). This tacit understanding of "you cook, I wash dishes, you produce, I consume" is simply a "model couple" in the microbial community. No wonder they can firmly establish themselves in sewage and become "evergreen trees" in the dry food industry.
Sometimes I really admire these micro world 'rice eaters': they have no eyes, no mouth, and even no brain, but they can accurately find the' raw materials' they need, use the most primitive chemical reactions to turn inorganic substances into organic substances, and turn sewage into clean and dry water. They don't eat whatever they see like heterotrophic bacteria, but insist on "doing it themselves and being well fed". While others are competing for food, they silently cultivate their "cooking skills" and ultimately occupy an indispensable place in the "micro food city" of activated sludge.
In fact, thinking about it, the "dry food philosophy" of these autotrophic microorganisms is quite worth learning: they do not rely on external "ready-made resources", but cultivate their own "core skills", create value through their own abilities, not only solve their own "food problems", but also contribute to the entire ecosystem (sewage treatment plants). They are like a group of craftsmen silently working in the sewage, not seeking grandeur, but seeking self-sufficiency, steadfastness, and reliability.
So the next time you pass by a sewage treatment plant and smell the faint "disinfectant smell", you may want to think about those autotrophic microorganisms in the aeration tank - they may be busy oxidizing ammonia nitrogen, sunbathing to synthesize organic matter, or "collaborating" with their peers to make a big meal. These inconspicuous' micro rice cooks' are writing the legend of self-sufficiency in sewage in their own unique way, and also using their 'cooking skills' to protect our water environment. Salute to the "king of self-sustaining dry rice" in these activated sludge, after all, they can turn "air and stones" into a feast. Looking at the entire microbial community, they are the only ones who can do it!
