Removal of Suspended Solids — Coagulation, Sedimentation, and Filtration: Intercepting Fine Particles for Crystal Clear Water
June 16, 2026
I. Characteristics of Suspended Solids (SS) in Secondary Effluent Suspended solids (SS) in secondary treatment effluent exhibit the following characteristics:
Composition: Primarily consists of biological flocs with particle sizes ranging from several millimeters down to 10 μm, along with some uncoagulated colloidal particles.
Organic Nature: The particles are almost entirely organic, with 50%~80% of the BOD originating from these suspended particles.
Bimodal Distribution: The particle size exhibits a bimodal distribution — smaller particles concentrate around 1~15 μm, while larger particles concentrate around 50~150 μm. This bimodal distribution directly influences the selection of filtration mechanisms.
Technology Selection Principles: Determined by the state and particle size of the SS. Particles > 1 μm are generally removed by sand filters, microfiltration machines, etc.; whereas colloidal particles require coagulation and sedimentation.
II. Coagulation and Sedimentation
1. Characteristics of Coagulation and Sedimentation Coagulation and sedimentation are among the most commonly used technologies in advanced treatment, primarily for removing fine suspended organic and inorganic pollutants, and colloids. Additionally, it can remove some dissolved substances such as Mg and As, as well as nitrogen, phosphorus, bacteria, and viruses.
Differences from water supply coagulation: The colloids and zoogloea (bacterial floc) particles in secondary effluent have a strong affinity for chemicals, allowing the coagulation process to be completed in a very short time after dosing. This is significantly different from the coagulation in natural water, which primarily targets silt and sand.
