When we talk about sewage treatment, activated sludge is an unavoidable protagonist. It's like a super powerful small factory, inhabited by various microorganisms. These little guys work together to break down the dirt in the sewage and make it clean. Today, let's talk about the principle of using activated sludge to remove COD and ammonia nitrogen, as well as the intricate relationships between them.
Let's first talk about COD, which stands for Chemical Oxygen Demand. Simply put, it refers to the amount of reducing substances in water that can be oxidized by chemical oxidants. These substances are mostly organic compounds, such as various food residues and excreta in domestic sewage, and various organic compounds in industrial wastewater. For sewage treatment, reducing COD means treating these organic compounds.
The removal of COD by activated sludge mainly relies on the metabolic process of microorganisms. There are many types of microorganisms in activated sludge, including bacteria, fungi, protozoa, etc. Among them, bacteria are the main force in treating COD. These bacteria are like super foodies, absorbing organic matter from the surrounding environment into their bodies through cell membranes. For example, it's like sucking milk tea into our mouths with a straw. The organic matter entering the cell undergoes a series of complex chemical reactions under the action of various enzymes, which is like opening a chemical plant in the bacterial body. Ultimately, organic matter is decomposed into carbon dioxide, water, and other small molecular substances. Carbon dioxide goes into the air, leaving water in the treated water. Some small molecule substances are used by bacteria to synthesize their own cellular substances for growth and reproduction, while others continue to participate in other metabolic reactions. Excess biological sludge can be discharged from the system through sludge discharge equipment. During this process, a large amount of COD in the sewage is consumed, and the water quality is purified.
Let's talk about ammonia nitrogen again. Ammonia nitrogen refers to the nitrogen present in water in the form of free ammonia (NH3) and ammonium ions (NH4+). Domestic sewage and many industrial wastewater contain a significant amount of ammonia nitrogen. If directly discharged, it can cause serious harm to water bodies such as eutrophication. Therefore, removing ammonia nitrogen is also an important task in sewage treatment. The removal of ammonia nitrogen by activated sludge mainly relies on two processes: nitrification and denitrification, which are completed by different types of microorganisms working together. The nitrification process is the conversion of ammonia nitrogen into nitrate nitrogen (NO3-), which is mainly carried out by nitrifying bacteria. Nitrifying bacteria can be divided into two types. One type is nitrifying bacteria, which first oxidizes ammonia nitrogen into nitrite nitrogen (NO2-). This process is like adding a little extra layer to a small house; Another type is nitrifying bacteria, which further oxidize nitrite nitrogen into nitrate nitrogen, equivalent to covering a house. Nitrifying bacteria require oxygen for their work, so this process is carried out under aerobic conditions.
The denitrification process is the opposite of the nitrification process, which converts nitrate nitrogen back into nitrogen and releases it into the air. The ones doing this job are denitrifying bacteria, which use organic matter in wastewater as electron donors in anaerobic environments to gradually reduce nitrate nitrogen to nitrite nitrogen, nitric oxide, nitrous oxide, and ultimately nitrogen gas. This process is like demolishing a previously built house bit by bit, and finally turning it into air that disappears. The denitrification process not only removes ammonia nitrogen, but also utilizes organic matter in wastewater to reduce COD, which can be described as killing two birds with one stone.
Next, let's talk about the relationship between COD and ammonia nitrogen removal. The mutual influence and synergy between them are quite interesting. From the perspective of mutual influence, firstly, COD has an impact on the removal of ammonia nitrogen. As mentioned earlier, the denitrification process requires organic matter as an electron donor. If the COD content in the wastewater is too low and there is not enough organic matter to provide energy and electrons, denitrifying bacteria cannot work properly, and the removal rate of ammonia nitrogen will naturally be affected. For example, if there is very little COD in the sewage, denitrifying bacteria are like workers who haven't eaten enough and don't have the energy to work. Nitrate nitrogen cannot be smoothly converted into nitrogen gas, and ammonia nitrogen cannot be removed.
Conversely, the process of ammonia nitrogen removal also affects the removal of COD. During the nitrification process, although nitrifying bacteria mainly oxidize ammonia nitrogen, this process also consumes a certain amount of organic matter. Because the growth and metabolism of nitrifying bacteria also require energy, a portion of which comes from the decomposition of organic matter. So, the level of ammonia nitrogen content and the intensity of nitrification reaction will indirectly affect the removal of organic matter (COD).
Let's talk about their synergistic effect. In the activated sludge system, the microorganisms that remove COD and ammonia nitrogen are not independent, they form a collaborative ecosystem. In the aerobic zone, heterotrophic bacteria provide a suitable living environment for nitrifying bacteria while removing COD. They consume a large amount of organic matter in wastewater, reducing the BOD (biochemical oxygen demand, which is also an indicator of organic matter content and closely related to COD) in the water, so that nitrifying bacteria will not be at a disadvantage due to competition for dissolved oxygen with heterotrophic bacteria. In the anoxic zone, denitrifying bacteria utilize the intermediate products or residual organic matter generated during the COD removal process as a carbon source to complete the denitrification process and remove ammonia nitrogen, while further reducing COD. It's like different processes on a production line, where everyone works together to complete the big task of sewage treatment.
The process of removing COD and ammonia nitrogen from activated sludge is like a carefully choreographed dance, with microorganisms showing their own abilities. The removal processes of COD and ammonia nitrogen are intertwined, influenced, and synergistic. Only by deeply understanding these principles and relationships can we better control the sewage treatment system, ensure its efficient and stable operation, turn dirty water into clean water resources, and protect our environment.
