Abstract: A coagulator for receiving water bearing suspended solids and a coagulant and for coagulating the suspended solids includes a coagulation and sedimentation part and an upstream coagulation part. The coagulation and sedimentation part includes a second packing medium accumulation layer with accumulated packing media having a void volume, and a sludge accumulation zone positioned below the second packing medium accumulation layer and configured to receive solids that coagulate from the water and also having a sludge removal opening by which the received coagulated solids may be discharged. The water bearing the coagulant is allowed to flow upward through the second packing medium accumulation layer to coagulate and settle the suspended solids from the water in the sludge accumulation zone.

Abstract: A front packing medium accumulation layer (90) and a downstream packing medium accumulation layer (44) which are separated by a lower outflow prevention plate (40) are arranged within a coagulation tank (80). The superficial water passage velocity within the front packing medium accumulation layer (90) is so set as to be larger than the downstream packing medium accumulation layer (44). A floc drawoff pipe (98) is provided for drawoff flocs over the lower outflow prevention plate (40). The interface between a floc blanket which may be formed between the lower outflow preventing plate (40) and the upper packing medium accumulation layer (44) and water thereabove is detected by an interface meter (100) so that a flow regulating valve (102) is correspondingly controlled to regulate the flow rate of the flocs. This prevents any grown floc blanket from reaching the downstream packing medium accumulation layer (44). Thereby extending the cleaning interval.