Gases with hydrogen sulphide (H2S) contents killed 2 employees at a plant producing algae-based natural food additives. The insoluble fractions stemming from the extraction of gelling agents with no direct usability were being treated on a porous soil (composed of perlite) and then pressed. The filter cakes were leached (to dissolve the salt) over a 0.5-ha zone prior to composting. The drippings were channelled into two sumps, one of which was fitted with an accelerator pump for the in-plant treatment of effluent. The discharge hose separated at times, thus requiring that the sump be drained and the pump be adjusted. The two employees were performing this task when the accident occurred. The warning was sounded 3 hours later, once it was confirmed that the two men had not returned; both of them would be found at the bottom of the sump. H2S concentrations in excess of 500 ppm were measured. The gendarmerie conducted an investigation into the matter, supported by an expert appraisal. This type of accident often goes underestimated and can arise from any anaerobic fermentation of sludge or compost in the presence of cavities that allow gas to accumulate in confined spaces. High contents (6,000 ppm and above) can overwhelm personnel to the extent that the sense of smell is lost and fainting happens almost instantaneously. In this case, heavy rainfall had prevented handling the accumulations and stimulated the formation of H2S; the proportion of soluble gas in the effluent created an additional hazard. The piping was altered to avoid access to the sump; furthermore, supervision of the filter cakes was optimised.