Wastewater sludge and pig manure used as culture media for bioleaching of metal sulphides

Precious and base metal recovery from leaching metal sulphide concentrates or low-grade ores is often based on the activity of bacteria, mainly Acidithiobacillus ferrooxidans, which converts insoluble metal sulphides into soluble sulphates. This study explored the possibility of using organic wastes as nutrients during the growth of A. ferrooxidans. Flask bioleaching tests using 10% (v/v) pulp density of a pyritic mine waste concentrate have shown that the liquid fraction of municipal sewage sludge, paper mill sludge and pig manure can support the growth of the leaching bacteria and allow metal solubilisation almost like a synthetic mineral medium. Dilution of these organic wastes is required to reduce the dissolved organic carbon (DOC) concentration, which inhibited the bacterial growth activity. The toxicity of the DOC appeared when the concentration of pig manure supernatant and sewage sludge filtrate was higher than 180 and 500 mg/L, respectively. A lag period has been noted before the active growth of A. ferrooxidans in the presence of pig manure and sewage sludge. In contrast to sewage sludge and pig manure, there is no toxicity detected (no bacterial activity inhibition and lag phase period), using paper mill sludge supernatant containing DOC ranging from 40 to 350 mg/L. An important decrease of the DOC (23–94%) has been measured during all bioleaching tests. The organic matter was probably removed by heterotrophic microorganism activity.