Gypsum in animal slurry systems enhances generation of hydrogen sulphide and increases occupational exposure hazard.

Hydrogen sulphide gas (HS) produced by sulphate reducing bacteria (SRB) in stored animal slurry is highly toxic and, if emitted into poorly ventilated confined spaces, can build up to concentrations capable of causing asphyxiation. Therefore it is important to understand factors influencing HS emission from slurry. Powdered gypsum (hydrated calcium sulphate) may be used as animal bedding and, if it enters slurry systems, could be metabolised by SRB and further increase HS generation. Cattle slurry and cattle bedding collected from farms was used in laboratory-scale experiments sealed in 20litre vessels fitted with mechanical stirrers. HS was monitored in head space above the slurry using real-time gas detectors before and after stirring, and before and after adding 1% of two sources of gypsum powder. In one set of experiments, gypsum was already present in the slurry having been used in bedding on the farm. HS monitoring continued daily for up to 25days. Before stirring, HS levels in head spaces were minimal. After stirring, even without gypsum, maximum head space HS levels with slurry or bedding ranged from 330 to 1190ppm. By comparison, the UK short-term (15min) Workplace Exposure Limit is 10ppm. Statistically significant increases in HS levels were associated with gypsum addition, as high as 1772ppm with slurry and 3940ppm with bedding. Emissions peaked at around day 15 with slurry and bedding to which gypsum was freshly added, but within 5days when added to slurry already containing farm-added gypsum. Levels of HS produced from stirred slurry would constitute a hazard to anyone exposed to it, and adding gypsum further increased emission levels. Therefore, if gypsum residues enter slurry it could increase the risk of HS accumulation in confined spaces associated with slurry systems. It is important therefore to take this into account in managing risk.