Municipal sewage generated from daily urban life contains soluble organic matter, nitrogen and phosphorus pollutants, as well as a large number of colloidal particles, fine suspended solids and sediment impurities. These particles feature extremely small particle sizes and high dispersibility, which cannot be removed by natural sedimentation. They are the primary causes of sewage turbidity, excessive chemical oxygen demand (COD) and deteriorated water appearance. As a core unit of the physicochemical treatment of municipal sewage, the flocculation process is based on the classic chemical colloid stability theory. It serves as a key procedure to realize solid-liquid separation and water purification, and is widely applied in the secondary and tertiary treatment processes of urban sewage treatment plants.
From the perspective of chemical engineering principles, colloidal particles in sewage generally carry the same negative charge. Subjected to the electrostatic repulsion effect, the particles repel each other and fail to aggregate, thus stably dispersing in water and forming a steady colloidal dispersion system. The core principle of flocculation treatment is to break the stable state of colloids by dosing special flocculants. Through three major mechanisms including charge neutralization, adsorption bridging and sweep flocculation, tiny particles are aggregated into large flocs, which can be rapidly separated via sedimentation and filtration.
Flocculants commonly used in municipal sewage treatment are divided into two categories: inorganic flocculants and organic polymer flocculants, which are adapted to different water quality working conditions. Represented by polyaluminum chloride (PAC) and polymeric ferric sulfate (PFS), inorganic flocculants are polynuclear hydroxy complexes. After dissolving in water, they hydrolyze to produce a large number of positively charged hydroxyl metal ions. These ions can rapidly neutralize negatively charged colloidal particles in sewage, eliminate electrostatic repulsion, and achieve particle destabilization and collision aggregation. They are applicable to the preliminary turbidity removal, heavy metal ion removal and partial colloidal organic matter removal of sewage.
Domestically produced organic polymer flocculants in China are mainly polyacrylamide (PAM), which is an indispensable auxiliary agent in municipal sewage treatment. Its molecular chain is equipped with a large number of active amide groups, endowing it with excellent adsorption bridging performance. After dosing, the long molecular chains can adsorb multiple destabilized micro flocs simultaneously, connecting and overlapping dispersed particles like bridges to form dense, coarse and well-settling alum flocs. Meanwhile, polymer flocs produce a sweep flocculation effect during sedimentation, entrapping residual fine impurities and suspended particles in water to settle down synchronously, which greatly improves water purification efficiency. In practical engineering, the combined process of inorganic and organic flocculants is usually adopted to achieve low-cost and high-efficiency water purification.
In the practical treatment process of municipal sewage, the flocculation procedure consists of three stages: mixing, reaction and sedimentation, which require strict chemical and hydraulic process conditions. The first stage is rapid mixing, where high-speed stirring is adopted to realize instantaneous and uniform diffusion of flocculants in sewage, completing chemical hydrolysis and colloidal charge neutralization. The second stage is slow flocculation reaction, in which the stirring flow velocity is reduced to provide a stable hydraulic environment for particle collision and bridging aggregation and prevent floc breakage. Finally, the mixture flows into the sedimentation tank to realize solid-liquid separation by gravity. The supernatant enters the subsequent biochemical treatment and disinfection processes, while the settled sludge is delivered to the sludge dewatering system for further treatment. As the core process of sewage physicochemical treatment, flocculation can efficiently remove suspended solids, turbidity and chroma from sewage, and effectively reduce the content of COD, total phosphorus and heavy metals in water, significantly lowering the operating load of the subsequent biochemical treatment unit. Compared with other treatment technologies, the flocculation process has the advantages of fast reaction rate, simple equipment, low operating cost and wide adaptability to different water qualities. It is a core chemical technology for municipal sewage treatment that balances economy and stability, and serves as an important foundation for ensuring the standard discharge of urban tail water and maintaining the stability of water environment ecology.