Analysis of Sewage Treatment Processes in Urban Wastewater Treatment Plants

With the acceleration of urbanization, the urban population continues to grow, leading to a steady increase in urban sewage discharge. The large amount of wastewater discharged by cities places significant pressure on the environment, as the organic matter, microorganisms, and heavy metals it contains pose serious threats to water quality and ecological balance.

Urban wastewater treatment plants serve as crucial facilities for ensuring water safety and protecting the environment. This paper explores the principles, characteristics, advantages, and disadvantages of sewage treatment processes in urban wastewater treatment plants.

1. Composition and Pollutants in Urban Sewage

Urban sewage consists of a complex mixture of organic matter, inorganic substances, microorganisms, and nutrients. Among these, organic pollutants—including phenols, acids, hydrocarbons, and fats—are the primary contaminants. The presence of organic matter promotes microbial growth and reproduction, leading to the formation of low-molecular-weight organic compounds and large amounts of sludge. Additionally, urban sewage contains significant amounts of nutrients such as nitrogen and phosphorus, which can disrupt aquatic ecosystems by promoting excessive growth of certain organisms.

2. Urban Sewage Treatment Processes

Urban sewage treatment generally involves four stages: pretreatment, primary treatment, secondary treatment, and advanced treatment.

2.1 Pretreatment

Pretreatment is the first and a crucial step in wastewater treatment. It primarily includes processes such as screening, grit chambers, and sedimentation tanks, which are designed to remove large debris and impurities from wastewater. Screening removes large objects such as leaves, paper, glass, and fabric. Furthermore, certain difficult-to-remove contaminants, such as sand and sludge, require removal through grit chambers.

2.2 Primary Treatment

Primary treatment primarily employs physical and chemical processes, including equalization tanks, grit chambers, and organic sludge tanks. This stage removes suspended solids (SS), chemical oxygen demand (COD), and biological oxygen demand (BOD) through sedimentation and particle settling. The choice of tank type depends on water quality and pollutant concentration. In urban wastewater treatment plants, rectangular and circular tanks are commonly used due to their wide adaptability. Primary treatment is relatively simple to operate and maintain, with stable operation, but its pollutant removal efficiency is relatively low.

2.3 Secondary Treatment

Secondary treatment mainly employs the activated sludge process. After sedimentation, wastewater is introduced into an activated sludge tank, where aerobic bacteria help further remove nitrogen, phosphorus, and other pollutants. Aerobic bacteria in the activated sludge interact with pollutants to achieve purification. The process requires multiple tank sections for aerobic, anoxic, and anaerobic conditions, making it more complex than primary treatment. Although secondary treatment is highly effective, it is more complicated and less adaptable to varying conditions.

2.4 Advanced Treatment

Advanced treatment is designed to remove high concentrations of heavy metals such as copper ions. This process relies on the bio-chelation of microorganisms within the sludge to capture and remove heavy metal ions. However, it also presents challenges in terms of operational complexity and maintenance requirements.

3. Conclusion

Sewage treatment processes continue to evolve, with new technologies emerging to enhance efficiency. However, challenges remain in addressing toxic and persistent pollutants, ensuring effective treatment in low-temperature conditions, and managing sludge disposal. The treatment of urban sewage is an ongoing process that requires collective efforts to achieve cleaner, smarter, and more sustainable cities.