According to its chemical composition, flocculants can be divided into inorganic flocculants and organic flocculants. Among them, inorganic flocculant includes inorganic coagulant and inorganic polymer flocculant; organic flocculant includes synthetic organic polymer flocculant, natural organic polymer flocculant and microbial flocculant.

The theoretical basis of flocculant is "coalescence" theory. Flocculant is mainly composed of groups with positive (negative) electricity and some particles with negative (positive) electricity in water which are difficult to separate. They are close to each other to reduce their potential and make them in an unstable state. By using their coalescence properties, these particles are concentrated and separated by physical or chemical methods.

Generally, the agent used to achieve this purpose is called flocculant. Flocculant is mainly used in the field of water supply and sewage treatment.

There are many kinds of flocculants, from low molecular weight to high molecular weight, from single type to compound type. The general trend is to develop in the direction of cheap, practical, non-toxic and efficient.