Aggregation precipitation method

A method of aggregation precipitation treatment by adding an inorganic flocculant to water to be treated and agitating a resulting solution, which comprises:

• introducing the agitated solution into an influent section in a lower part of a first chamber of a separation tank where flocs are allowed to grow and subjected to solid-liquid separation in a floc blanket layer and, simultaneously, separated water is discharged out of the tank from an effluent section in an upper part of the first chamber above the floc blanket layer, allowing the flocs grown in the floc blanket layer of said first chamber and part of the separated water to flow down through a floc transfer pipe into a second chamber provided below the first chamber at a velocity higher than the settling velocity of the flocs as such, concentrating the grown flocs, discharging a concentrated slurry in the second chamber, and simultaneously withdrawing separated water formed by the concentration of the flocs and the separated water in the first chamber which has been introduced into the second chamber together with the flocs from the upper part of the second chamber as the separated water, and inhibiting the fragmentation of the flocs in the lower part of the floc transfer pipe by regulating the total discharge flow rate of the separated water and the slurry in the second chamber so as to render the flow velocity in the floc transfer pipe to 5 m/min or lower, or automatically controlling an effluent amount of the separated water in the second chamber with an index of the turbidity or suspended substance concentration of the separated water in the second chamber, so as to render the upward flow velocity above the lower end of the floc transfer pipe in the second chamber lower than the settling velocity of the flocs.