With the rapid development of industrialization and urbanization, water pollution is becoming more and more serious, which has a great impact on human health, ecological environment and social economy. In order to effectively meet this challenge, countries all over the world have strengthened the construction of sewage treatment facilities and gradually promoted the progress and innovation of sewage treatment technology. This paper will introduce the key links in the process of modern sewage treatment and its role in environmental protection from four stages: pretreatment, primary treatment, secondary treatment and advanced treatment.

1. Pretreatment stage: The main purpose of this stage is to remove large particles and other substances that are easy to block the subsequent equipment, such as inorganic impurities such as sand and stones and grease pollutants. Common pretreatment facilities include grille, grit chamber and degreaser. This step can reduce the subsequent process load and improve the operation efficiency of the whole system.

2. Primary treatment (physical method): The pretreated sewage enters the primary treatment unit, where suspended solids are mainly precipitated by gravity to form sludge; At the same time, light floating objects are separated by flotation or air flotation. This stage can greatly reduce the turbidity of water body and create good conditions for subsequent biological treatment.

3. Secondary treatment (biochemical method): this is the core part of sewage treatment, which degrades dissolved organic matter through microbial metabolism. Activated sludge process is one of the most commonly used technologies, which can effectively remove BOD (biochemical oxygen demand), COD (chemical oxygen demand) and other indicators under aerobic conditions, and significantly improve the water quality. In addition, there are many methods to choose from, such as anaerobic digestion and biofilm reactor.

4. Advanced treatment/tertiary treatment: When higher discharge standards are required in some specific situations, tertiary or deeper purification measures are needed. These measures usually involve advanced technologies such as advanced oxidation, reverse osmosis and nanofiltration, aiming at further reducing the concentration of nutrients such as ammonia nitrogen and total phosphorus, and ensuring that the final discharged water quality meets the strict standards stipulated by the state.

In a word, sewage treatment is a complex and systematic work, and every step is very important. With the development of science and technology and the continuous improvement of social requirements for environmental quality, the future sewage treatment technology will be more perfect and mature, and make greater contributions to the realization of sustainable development goals.