Screw-type separation device, casing, wastewater treatment system, and cleaning method of screw-type separation device

A screw-type separation device comprising:

• a casing including
• an object discharging port provided on one end part side and discharging an object having been dehydrated, and
• a separated liquid discharging port provided on another end part side and discharging separated liquid having been separated from the object by dehydration;
• a screw shaft provided inside the casing and extending in an extending direction that is a direction from the one end part to the another end part;
• a first screw blade extending spirally on an outer peripheral surface of the screw shaft;
• and
• a second screw blade extending spirally on the outer peripheral surface of the screw shaft such that a predetermined gap is formed with respect to the first screw blade in the extending direction, wherein
• a groove is formed on an inner peripheral surface of the casing.