In rapid sand filtration, the turbidity contained in the raw water is physically removed with a filter medium, but if the particle size of the turbidity is small, it is difficult to remove with the filter medium, so the turbidity is removed with a filter medium with a flocculant. Aggregate to the size possible. Inorganic coagulants include aluminum-based coagulants such as PAC and iron-based coagulants such as iron chloride.
Aggregation is a method of electrically neutralizing the turbidity contained in raw water to form large flocs. Generally, a flocculant is added in a reaction vessel and the mixture is rapidly stirred for a certain period of time, and then slowly stirred in the agglomerating tank for a certain period of time to grow turbidity into sufficiently large flocs before filtration.
We also use multi-layer filtration using anthracite with large voids to eliminate the agglutination reaction tank and perform filtration. This method is called “microflock filtration method” or “coagulation filtration method”.

● Microflock filtration method
A method in which a flocculant is injected　and stirred in front of a filtration device to form fine flocs (microflocs)　and then immediately filtered. The suspension is filtered by the stirring　and coagulating action in the filter layer. ” It is said that the formed flocs have a small amount of hydrate　and become solid flocs in order to reduce the injection rate of the flocculant　and shorten the time for floc formation.

● Coagulation filtration method
After injecting the coagulant, the suspended substance is adsorbed on the surface of the filter medium by the action of the coagulant to grow. It is considered that the mechanism of neutralizing the potential of the suspended substance and the surface of the filter medium and the zeta potential generated by the passage of water works by the intervention of the flocculant. It has the advantage that the injection rate can be lower than that of the microflock filtration method. However, it is said that it is difficult to manage because the time from injection of the flocculant to reaching the filter layer affects the performance and the adsorbed turbidity is not sufficiently peeled off by washing.