NOVELTY - A biological treatment reactor (10) comprises biological treatment adjuvant holding part (11) with which biological treatment adjuvant is filled, sludge retention tank (12) which accommodates biological treatment adjuvant holding part, inflow part (13) which accepts sludge of sludge return path of water treatment plant, discharge part (14) which returns accepted sludge to water treatment plant, circulation mechanism (16) which circulate-supplies sludge flowing in from inflow part to biological treatment adjuvant holding part, and filter mechanism (17) for capturing foreign substance.    USE - A biological treatment reactor.    ADVANTAGE - Processing tank is not equipped with a large opening which makes the reactor easy to install, even if it is a water treatment plant which processes organic wastewater, such as excrement and purification-tank sludge. Operation method of reactor can prevent biological treatment adjuvant accommodating part from being obstructed with foreign substances, even if reactor is not maintained over a long term.    DETAILED DESCRIPTION - A biological treatment reactor comprises biological treatment adjuvant holding part with which biological treatment adjuvant which dominates specific microorganisms group in sludge is filled, sludge retention tank which accommodates biological treatment adjuvant holding part, inflow part which accepts sludge of sludge return path of water treatment plant in sludge retention tank, discharge part which returns accepted sludge to water treatment plant, circulation mechanism which circulate-supplies sludge which flows in from inflow part to biological treatment adjuvant holding part within sludge retention tank, and filter mechanism for capturing foreign substances which may entrain to sludge and discharged when sludge is released from discharge part while capturing foreign substances from sludge circulate-supplied by biological treatment adjuvant holding part with the circulation mechanism. INDEPENDENT CLAIMS are included for:    (1) water treatment plant comprising pretreatment facility which removes solid foreign material from organic wastewater, single-or-multiple biological treatment tank which biologically treats organic wastewater from which solid foreign material is removed with pretreatment facility, and solid-liquid-separation tank which separates into solid and liquid the biologically treated organic wastewater in which the reactor is integrated in sludge return path of returning part of sludge extracted out from water treatment plant to water treatment plant; and    (2) operation of biological treatment reactor which involves stopping inflow of sludge after making sludge of predetermined amount flow in into sludge retention tank from water treatment plant which biologically treats organic wastewater, capturing foreign substances in upstream of sludge cycle flow to biological treatment adjuvant holding part while circulate-supplying sludge which flows in at sludge inflow step to biological treatment adjuvant holding part accommodated in sludge retention tank, and discharging sludge from sludge retention tank with foreign substances captured by sludge circulating step.    DESCRIPTION OF DRAWING(S) - The drawing shows a schematic diagram of the biological treatment reactor.    Biological treatment reactor (10)    Biological treatment adjuvant holding part (11)    Sludge retention tank (12)    Inflow part (13)    Discharge part (14)    Circulation mechanism (16)    Filter mechanism (17)