Combining the action of SRBs and sulfide oxidizing microbes

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Overall biological process.

Sulfate reduction by SRBs. A typical overall conversion equation is (neglecting the small amount of organic material required to produce biomass):

SO42- + CH3COOH + 2 H+ → HS- + 2 HCO3- + 3 H+

Eight electrons are transferred from acetic acid to sulphate in order to produce sulphide. The reaction equation shows that in the same process alkalinity is also produced. This leads to an increase in the pH of the water, often to a near neutral value.

Biological sulfide oxidation. In order to remove more sulfate than the equivalent of dissolved metals, additional sulfate can be converted into dissolved sulfide. The sulfide is subsequently converted into elemental sulfur with a 2nd step:

HS- + ½ O2 → S0 + OH-

Thiobacillus species typically perform this conversion.

The conversion results in a further increase in pH, typically to ~8. The solid, hydrophilic elemental sulfur can be removed from the water through settling.

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