People usually flush the toilet or pour vegetable wash water, but they may rarely think about how many pollutants are hidden in these sewage. Today we will talk about a super critical indicator in sewage treatment - COD (Chemical Oxygen Demand), and how a group of silent "microbial cleaners" clean up these pollutants.
First, understand what COD is?
The term COD sounds quite professional. Simply put, it refers to the total amount of substances in wastewater that can be "burned" by chemical oxidants. Organic substances such as leftovers from the kitchen and organic wastewater from factories contain a large amount of carbon elements, which can be oxidized by oxidants. The amount of oxygen consumed in this oxidation process is COD. The higher the value, the more "dirty things" there are in the sewage. If it is directly discharged into the river, the fish and shrimp in the water will suffer.
Microorganisms: the "main force" of sewage treatment plants
The sewage enters the treatment plant, and the real workers are not the tall treatment equipment, but the microorganisms that can only be seen under the microscope. They are like a group of small workers with clear division of labor, some are good at "eating meat in big bites" to decompose large organic molecules, while others can slowly chew difficult compounds into harmless substances. These microorganisms are mainly divided into three categories: bacteria, fungi, and protozoa. The most skilled "workers" are still bacteria. Let's focus on them.
The "three-step" strategy for microbial decomposition of COD
Step 1: Adsorption and Absorption - "Bring the takeout into the house"
The organic matter in the sewage is either like large chunks of fatty meat (high molecular weight organic matter) or small diced meat (low molecular weight organic matter). Microorganisms do not grab food with their hands like we do. They secrete a sticky substance on the surface of their cells, like putting a "sticky coat" on them. When sewage flows through, small organic molecules can directly penetrate into cells, while large organic molecules will be "stuck" to the cell surface, waiting for the next step of decomposition.
For example, it's like ordering takeout, where small molecule organic matter is a pre cut fruit platter that can be stuffed directly into your mouth; High molecular weight organic matter is a whole watermelon that needs to be sliced open before eating.
Step 2: Intracellular decomposition - "Let's eat! Chew and digest the food"
After large organic molecules are adsorbed onto the surface of microorganisms, they will "spit out" something called extracellular enzymes. These enzymes are like sharp knives that can cut starch into glucose, protein into amino acids, and fat into fatty acids and glycerol. After decomposition, small organic molecules can smoothly pass through the cell membrane and enter the interior of microbial cells.
Small organic molecules entering cells undergo a series of complex chemical reactions, similar to digestion in our bodies. The most common reaction is respiration, where microorganisms "burn" the carbon elements in organic matter, releasing energy to sustain life activities. This process requires oxygen assistance, which is commonly referred to as aerobic treatment. However, some microorganisms are very unique and can work in environments without oxygen, which is anaerobic treatment.
Aerobic respiration: aerobic version of "burning organic matter"
In an oxygen rich environment, microorganisms are like driving a "turbocharged engine", decomposing organic matter at a particularly fast speed. They will 'burn' glucose and oxygen, producing carbon dioxide, water, and a large amount of energy. Part of this energy is used to synthesize substances needed for microbial growth, such as proteins and nucleic acids; The other part is used to maintain the daily activities of microorganisms, such as "swimming" to search for food.
Anaerobic respiration: anaerobic version of "alternative digestion"
When there is no oxygen, microorganisms also have ways. They will look for other substances that can replace oxygen, such as nitrates and sulfates. These substances can also accept electrons released during the decomposition of organic matter and complete "respiration". However, anaerobic treatment is slower than aerobic treatment and produces odorous gases such as methane and hydrogen sulfide (so anaerobic tanks sometimes have a foul odor). But it also has benefits, it can handle stubborn organic matter that aerobic microorganisms cannot handle, and it can also produce biogas as an energy source.
Step 3: Synthesis and Transformation - "You're full, it's time to grow your body"
Microorganisms decompose organic matter not only to obtain energy, but also to 'grow their bodies'. They will use the intermediate products generated during the decomposition process to synthesize the cellular substances needed for their own growth and reproduction. Simply put, it means turning the "food" you eat into your own "meat". As microorganisms continue to multiply, the number of microorganisms in sewage increases, forming visible "small groups" that we often refer to as activated sludge or biofilm.
When microorganisms eat and drink enough, the COD in sewage also decreases. After treatment, the water undergoes sedimentation to separate microorganisms from the water. Clean water can be discharged or reused, while "fat growing" microorganisms will be further processed and turned into fertilizer or sludge for landfill.
The 'little temper' that affects the efficiency of microbial work
Although microorganisms are the main force in work, they also have a "temper" and do not work well in unsuitable environments:
-Temperature: Most microorganisms prefer an environment of 20-35 ℃. If it is too cold, they will freeze; if it is too hot, they will suffer from heatstroke, and their decomposition efficiency will decrease.
-PH value: A pH value between 6.5-8.5 is most suitable, as excessive acidity or alkalinity can damage the cellular structure of microorganisms.
-Nutrient ratio: Microorganisms also need to have a "balanced nutrition" when working, in addition to organic matter, they also need an appropriate amount of nitrogen, phosphorus and other elements, just like how people eat meat and vegetables in a balanced manner.
-Toxic substances: Heavy metals, chemical agents, and other substances are like "poison" to microorganisms, and too high a concentration can directly "poison" them.
Future: Making Microorganisms Work More Efficiently
Scientists have been studying how to make microorganisms better remove COD. For example, by genetically engineering microorganisms to decompose more difficult to treat pollutants; Develop new sewage treatment processes that combine aerobic and anaerobic treatment to improve treatment efficiency. Maybe one day, these small microorganisms can still help us deal with big problems such as plastic pollution and oil spills!
Next time you pass by a sewage treatment plant, don't underestimate those inconspicuous pools and pipelines, which hide billions of "microbial cleaners" who are constantly cleaning dirty water day and night. Although they are small, they are guarding our water environment and are definitely the most powerful "environmental guardians" on Earth!
