Background

Low-pH microbial Fe(II) oxidation can be incorporated into AMD passive treatment systems by enhancing the development of terraced iron formations (DeSa et al., 2010; Brown et al., 2011; Larson et al., 2014a and 2014b). For extremely difficult-to-treat AMD (very low pH, very high concentrations of Fe(II) and associated metals), an active treatment bioreactor is required by enriching acidophilic Fe(II)-oxidizing bacterial communities. This process can effectively change a high acidity, high metals discharge into a moderate acidity (still low pH), low metals discharge (Sheng et al., 2016).

Acidophilic aerobic Fe(II) oxidizers Acidithiobacillus spp., Leptospirillum spp., and Ferrovum myxofaciens have all been enriched in both suspended growth and fixed-film laboratory-scale bioreactors for AMD treatment (Hedrich and Johnson, 2012; Heinzel et al., 2009a and 2009b; Janneck et al., 2010; Tischler et al., 2013). For instance, Hedrich and Johnson (2012) designed an AMD remediation system that integrated low-pH Fe(II) oxidation and Fe removal in a multi-reactor system. A pure culture of the Fe(II)-oxidizer Ferrovum myxofaciens was enriched in first suspended-growth reactor. Heinzel et al. (2009a and 2009b), Janneck et al. (2010) and Tischler et al. (2013) all developed a natural mixed community of Fe(II)-oxidizers with porous fixed-film media in a pilot-scale reactor. A protocol of enriching mixed culture acidophilic Fe(II)-oxidizing bacteria in no-flow, fed-batch systems without fixed-film media is suggested here in a chemostatic bioreactor with controlled hydrogeochemical conditions (Sheng et al., 2016).