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This study characterized the microfauna community during treatment of pulp and paper wastewater in a UNOX system aerated with pure oxygen, and with a high organic loading rate (0.4±0.06kg BOD/kg MLSSday), low sludge retention time (3.73±0.33 day), and high oxygen concentration (≤20mg O 2 /L) in comparison to municipal treatment systems. In the aeration tank, temperatures were high, averaging 35.7°C (March-May), then 38.9°C (June-August). Effluent quality was acceptable: 180±22mg COD/L, 7.2±2.1mg BOD 5 /L, and 33±5mg TSS/L. At 35.7°C, 5 taxa were identified in the activated sludge: small flagellates (flagellates <20μm), attached ciliates (Vorticella infusionum, Vorticella octava), crawling ciliates (Chilodonella uncinata) and free-swimming ciliates (Sathrophilus muscorum). During this period, the SBI (Sludge Biotic Index) was 6-7, corresponding to Quality Class II. At 38.9°C, two taxa co-dominated (Vorticella infusionum, Sathrophilus muscorum). When Vorticella infusionum dominated, the SBI was 5 or 7 (Quality Class III/II); when Sathrophilus muscorum dominated, the SBI was 0 (Quality Class IV). Slight changes in the abundance of two opposing Madoni keygroups impeded proper classification of activated sludge. However, effluent quality remained the same, showing that these indicators of activated sludge quality do not always reflect effluent quality., (Copyright © 2017 Elsevier GmbH. All rights reserved.)