The wastewater treatment plant at a paper mill was discharging untreated effluent into the Loir River (during the river’s low-water period) subsequent to a malfunction of its digester. The effluent had been undergoing physicochemical treatment (screening, grit removal, acidogenesis tank residence), digestion and then biological treatment. The Classified Facilities Inspectorate estimated the quantity of pollution discharged at 400 kg of suspended solids, 10.5 tonnes of COD, 4.5 tonnes of BOD, 80 kg of P, and 10 kg of N. The mill was producing recycled paper intended for manufacturing corrugated cardboard. Notified by the operator on 16th August, the Classified Facilities Inspectorate conducted a same-day investigation. The digester malfunction was caused by works carried out in the plant’s grinding station, scene of the 1st stage of paste production from ‘secondary’ fibres. To comply with safety standards in this shop during the planned works, the operator covered some of the H2S generators, thereby modifying the quality of effluent entering treatment. The biolith present in the digester had absorbed a large quantity of sulphur and altered its mode of operations. After the accident, the operator reduced water consumption and ran the treatment plant in degraded mode for 1 week. Following physicochemical treatment, the effluent was purified via 3 distinct approaches: conventional treatment (25 cu.m/h), direct biological treatment (30 cu.m/h), and spreading (20 cu.m/h). As a complement to the first two approaches, effluent was treated using coccolith chalk and lightly chlorinated prior to discharge into the environment, in order to limit the development of filamentous bacteria. The inspectorate recorded its findings and proposed that the Prefect prescribe the spreading approach via an emergency measure order.