An “in-lake” reactor system was developed to treat acidic mining lakes. The reactor uses the microbial processes of sulfate reduction and iron reduction followed by precipitation of iron sulfides to remove acidity, sulfur, and iron from the lake water. The basic reactor design is a straw-filled tube, which was vertically installed in the water column of an enclosure in the lake. Bottom water was pumped through the reactor, and ethanol was continuously fed as substrate for the microbial processes.

Microbial sulfate reduction and iron reduction took place inside the reactor, even under acidic conditions. Overdosage of substrate led to the accumulation of the potentially toxic intermediates H₂S and acetate. Leakage of ethanol led to anoxic conditions in the entire enclosure, followed by accumulation of H₂S in the water column. Sulfides were not precipitated because the pH was never above 3.8. Mixing of the water column in autumn introduced oxygen into the system and led to reoxidation of the H₂S. Future designs of in situ reactors to treat acidic mine drainage should consider that the limiting step is not the microbial formation of alkalinity but the fixation of the alkalinity gain as pyrite.