Bioremediation of acid-rock drainage by sulphate-reducing prokaryotes: A review

Acid-rock drainage (ARD) is a widespread environmental problem that causes adverse effects to the quality of ground water and surface water through acidification, high concentration of the iron, sulphate, and elevated levels of soluble toxic metals. Active treatment technologies are often expensive and require regular attention resulting in increased overall costs due to operation and maintenance expenses. One of the effective treatment methods is to use sulphate-reducing prokaryotes (SRP) in bioreactors. They offer advantages such as high metal removal at low pH, stable sludge, very low operation costs, and minimal energy consumption. Sulphide precipitation is the desired mechanism of contaminant removal; however, many mechanisms including adsorption and precipitation of metal carbonates and hydroxides also occur in passive bioreactors. Several factors influencing the performance of the bioreactors are reviewed. The fundamental biochemical and microbiological reactions that occurs in the bioreactors has been dealt in detail. The present review presents performance of bioreactors, chemical characterisation of organic substrates for successful treatment of ARD. Moreover, design parameters, longevity and future scope of the study on bioreactors is also discussed in this review.