Biological treatment device, carrier capture device for biological treatment device, water treatment method, and method for modifying biological treatment device

The purpose of the present invention is to provide a biological treatment device capable of suppressing adhesion of a carrier to a screen. A biological treatment device (2) is provided with: a biological reaction tank (2A) in which a carrier, in which microorganisms for purifying water to be treated are immobilized, flows; a partition wall (18) that is provided in the biological reaction tank (2A) and separates the biological reaction tank (2A) into a first region (19) that includes a supply unit (22) for water to be treated, and a second region (20) that communicates with the first region (19) at the lower part of the biological reaction tank (2A) and has an outflow unit (25) for treated water treated in the biological reaction tank (2A) formed only at the upper part; and a carrier trapping device (26) provided with a screen (27) provided to the outflow part (25), the screen (27) preventing the carrier from flowing out of the biological reaction tank (2A). 本发明的目的是提供一种能够抑制载体向筛网的附着的生物处理装置。生物处理装置(2)具有：生物反应槽(2A)