Tannery effluent as a carbon source for biological sulphate reduction

Tannery effluent was assessed as a carbon source for biological sulphate reduction in a pilot-scale upflow anaerobic sludge blanket (UASB), stirred tank reactor (STR) and trench reactor (TR). Sulphate removals of between 60–80% were obtained in all three reactors at total sulphate feed levels of up to 1800 mg l−1. Sulphate removal in the TR (400–500 mg SO4 l−1 day−1) and UASB (up to 600 mg SO4 l−1 day−1) were higher than those obtained in the STR (250 mg SO4 l1 day−1). A change in operation mode from a UASB to a STR had a large impact on chemical oxygen demand (COD) removal efficiencies. COD removal rates decreased by 25% from 600–700 mg COD l−1 day−1 to 200–600 mg COD l−1 day−1. The TR had an average COD removal rate of 500 mg COD l−1 day−1. Large quantities of sulphide were produced in the reactors (up to 1500 mg l−1). However due to the elevated pH in the reactor, only a small amount was in the form of H2S and thus the odour problem normally associated with biological sulphate reduction was not present.