The generation of H₂S in wastewater results principally from the biochemical decomposition of the waste components.
Bacteria remove electrons (oxidation) from the organic molecules, gaining energy. The electrons are then passed to an acceptor. This results in reduction of the acceptor. The electron acceptor can be an inorganic or organic substance.
Under aerobic conditions, free molecular oxygen (O₂) is used by the microbes as the electron acceptor in a process that produces water.
C₆H₁₂O₆ + O₂ → CO₂ + H₂O + biomass (aerobic heterotrophic bacteria)
Under anaerobic conditions, various compounds or ions can be used by different groups of microbes as the electron acceptor. If neither oxygen nor nitrate (NO3-) is present, microbes that use sulfate (SO4-₂) as an electron acceptor (sulfate reducing bacteria) predominate and these microbes generate sulfide.
C₆H₁₂O₆ + NO₃⁻ +H⁺ → CO2 + H₂O + N₂ +N2O +biomass (denitrifying bacteria)
C₆H₁₂O₆ + SO₄⁻² +H⁺ → CO₂ + H₂O + HS- +biomass (sulfate reducing bacteria)
(Note: equations are illustrative and not meant to show stoichiometry; H+ on the left side indicates acid is consumed in the reaction while H⁺ on the right indicates acid generation.)
Microcat®-ANL Odor Control Bioformula is a blend of strict and facultative anaerobic microbes selected for their ability to oxidize sulfides to elemental sulfur (collectively referred to as sulfur bacteria).
Thus, as sulfate is normally present in wastewater, the sulfur cycle becomes a critical step in the breakdown of waste under anaerobic conditions. Sulfides are present in three forms: hydrogen sulfide (H₂S) at low pH, hydrosulfide ion (HS-) at neutral pH and sulfide ion (S-₂) at high pH. The ionic forms stay dissolved in water while the neutral H₂S is easily volatilized. The oxidation of H₂S under highly aerobic conditions, e.g. in the biofilm above the waterline in sewers, generates corrosive sulfuric acid.
HS- + O₂ → SO₄⁻² + H⁺
In contrast, the sulfur bacteria oxidize sulfide to elemental sulfur without the generation of acid.
H₂S + CO₂ → S⁰ +CH₂O (organic cellular components)
Elemental sulfur is not used by sulfate reducing bacteria, so the cycle is disrupted.
Microcat®-ANL Odor Control Bioformula is a blend of strict and facultative anaerobic microbes selected for their ability to oxidize sulfides to elemental sulfur (collectively referred to as sulfur bacteria). Because it is a blend of several species and strains, some components of the blend can function when oxygen is present, some can use alternate electron acceptors such as nitrate, others only function in the absence of oxygen, and some are photosynthetic. Fatty acids are also metabolized by the strains in Microcat -ANL.