The degradation of pollutants in sewage treatment is a process of graded removal and progressive purification, with the core steps divided into four stages: pretreatment, primary treatment, secondary treatment, and tertiary treatment, each targeting different types of pollutants
The primary purpose is to remove large particulate impurities to prevent blockages in subsequent equipment. Through a bar screen (to filter out floating debris such as branches and plastic bags) and a grit chamber (to separate inorganic particles like mud and stones via gravity), the physical pollutants in the wastewater are initially reduced, alleviating the burden on subsequent treatment processes.
The focus is on suspended solids (SS) and partial COD, with the core facility being the sedimentation tank. The wastewater flows slowly within the tank, and suspended particles settle to the bottom under gravity, forming primary sludge. Simultaneously, floating oil is removed via an oil separator. This step can remove approximately 30% of COD and 60% of SS, but it cannot degrade soluble organic matter or nitrogen and phosphorus.
Utilizing microbial metabolic processes to decompose soluble organic matter, nitrogen, phosphorus, and other pollutants is a key step in wastewater purification, with mainstream processes divided into three stages: anaerobic, anoxic, and aerobic
Anaerobic stage: In an oxygen-free environment, anaerobic bacteria break down macromolecular organic matter (such as proteins and starch) into small-molecule organic acids and methane, reducing the load on the subsequent aerobic stage.
Anaerobic zone: Facultative bacteria complete denitrification under conditions without molecular oxygen but with nitrate, converting nitrate into nitrogen gas released into the air.
Aerobic stage: Aeration and oxygenation, where aerobic bacteria (such as flocculants and nitrifying bacteria) proliferate in large numbers, oxidizing and decomposing small-molecule organic matter into CO₂ and H₂O; nitrifying bacteria can also convert ammonia nitrogen into nitrates, achieving nitrogen removal.
After the biochemical reaction, microorganisms combine with pollutants to form activated sludge, which is separated by sedimentation in the secondary clarifier. The supernatant enters subsequent treatment processes, while part of the sludge is recirculated to maintain the microbial population in the system and part is discharged for further treatment.
To further purify residual trace nitrogen and phosphorus, suspended solids, and refractory organic matter in secondary effluent to meet higher discharge standards, common processes include:
Chemical phosphorus removal (adding coagulants such as PAC and PFS to form phosphate precipitates);
Membrane filtration (e.g., MBR membrane, intercepting residual suspended solids and microorganisms);
Disinfection (addition of sodium hypochlorite, UV disinfection, and pathogen elimination).
The final treated water can be directly discharged or reused (e.g., for landscaping or toilet flushing).
