A discharge of stormwater loaded with whitish particles stemming from a plasterboard plant polluted the Erier River. The overflow of a sedimentation tank for water containing glues into the site’s stormwater network during cleaning operations preliminary to the plant’s seasonal shutdown, gave rise to this pollution incident. Moreover, during the investigations performed, it appeared that the facility had discharged into the sewer network large volumes of effluent (up to 300cu.m/day, vs. 50cu.m/day authorised) at high temperature (47°C vs. 30°C authorised) with major hydrogen sulphide emissions (detection of 3 ppm H2S). This effluent stemmed primarily from the heat exchanger condensation water at the dryer outlet and should have been recycled into the process. Lastly, the presence of H2S was due, according to the operator, to decomposition in an anaerobic medium of gypsum particles generated from washing water and carried into the residual effluent network despite the sedimentation step. The Classified Facilities Inspectorate noted these findings and made workshop restart contingent upon compliance with the Prefectural order that originally authorised plant operations. As a consequence, the following measures were adopted: washing water was recovered in a tank and then removed by a specialist firm; drawings for the site’s underground water circulation network, poorly known given the age of the workshop (built in 1965), were established; condensation water was recycled back into the process; the industrial effluent network underwent cleaning and inspection in light of the corrosive nature of H2S; over the medium term, the operator planned on recycling 100% of washing water to avoid H2S formation in the pipelines.