Biological sulphate and metal removal from a copper and nickel mine effluent

A copper mine in Botswana produces amongst others, a metal and sulphate rich effluent, of which it plans to treat 7 m 3 /d with the aim to use the treated water for the irrigation of agricultural products. The results of the laboratory scale study showed that when applying the biological sulphate removal technology, the sulphate was removed consistently to a concentration of less than 450 mg/ℓ, when the effluent contained 1600mg/ℓ sulphate. Due to the production of alkalinity, the pH increased from 5.8 to 6.5. During this period, the Hydraulic Retention Time (HRT) varied from 24 to 12h. The nickel and zinc concentrations were reduced from 5.86 to 0.1mg/ℓ and from1.25mg/ℓ to 0.03 mg/ℓ, respectively. These results indicated that the metals were precipitated as Metal Sulphides (MeS). The treated mine water is of such quality that the copper and nickel mine management has decided to use this water for irrigation of agricultural crop, such as citrus fruit. This will benefit the rural population living in close proximity of the mine, resulting in poverty alleviation.