H2S Production & Corrosion Potential
H2S and corrosion control within the SWD system is critical for quality of life and economic reasons. H2S and corrosion are primarily associated with the production of hydrogen sulfide gas. Hydrogen sulfide (H2S) is produced only by sulfate-reducing bacteria (SRB) under anaerobic conditions using sulfate as the final electron acceptor.
The SRB are a unique physiological group of bacteria in that they are capable of using sulfate as the final electron acceptor in respiration. Bacteria outside of this group cannot use sulfate as the final electron acceptor in respiration and do not produce H2S external to the cell, ever. The SRBs are a very hardy group of anaerobic bacteria in that they are capable of limited oxygen metabolism, have a broad range of metabolic capabilities, and many are motile(capable of or demonstrating movement by independent means). Most of the SRB species are mesophilic (Bacteria that are best active at median temperatures), capable of growth between 68-107 degrees Fahrenheit and can be found growing in pH ranges of 4.0-9.5. Desulfovibrio sp. are SRBs that are incapable of forming spores, motile through single polar flagella, and have a generation time of around 180 minutes. Desulfotomaculum sp. are SRBs that are capable of forming spores, motile through peritrichous flagella (flagella projecting in all directions), and have a generation time of around 540 minutes.
The principal end products of SRB respiration using sulfate are H2S and CO2 at a ratio of 1:2, respectively, in cultures where acetate accumulates from lactate catabolism (the set of metabolic pathways that break down molecules into smaller units and release energy). Most SRBs are also capable of using many other compounds as a final electron acceptor. Some Desulfovibrio and Desulfobulbus species are able to utilize nitrate as an electron acceptor producing ammonia and water as end products. Depending on the species, in the presence of both electron acceptors either sulfates or nitrate may be the preferred electron acceptor, or both may be reduced at the same time. The SRB’s have considerable capability and diversity in using various compounds for electron donors. In the absence of sulfate, certain strains of SRB can use a single carbon compound as both the electron donor and an electron acceptor by a process called dismutation. The SRB’s are generally thought to be obligate anaerobes (microorganisms that live and grow in the absence of molecular oxygen) but recently it has been determined that they can tolerate oxygen for short periods of time and even proliferate in the presence of oxygen. This gives SRBs a slight advantage over more oxygen-sensitive anaerobes. However, oxygen respiration has not been found to be long-term, in fact only one doubling has been observed in homogenously aerated systems.
Certain formations contain high sulfate concentrations (ca. 100 to 1000 ppm) relative to organic carbon concentrations, but almost no nitrate or nitrite (Ito et al., 2002). Thus, SRBs thrive in this environment while other anaerobic bacteria struggle with the low oxygen and nitrate levels and Thiobacillus sp. become dominant in aerobic areas. The SRBs become the dominant bacteria in the anaerobic parts of the system and begin to generate H2S in quantities sufficient to create malodorous conditions and Thiobacillus sp. become dominant in aerobic parts of the system and create a corrosive environment.
H2S Production & Corrosion Control w/ LIVE MICROBES Technology
One huge benefit of bioaugmentation of SWD produced water with LIVE MICROBES is that these bacteria do not contain the enzymatic pathways necessary for use of sulfate as the final electron acceptor during anaerobic growth conditions. None of the bacteria within the LIVE MICROBES treatment are SRBs or thiobacilli. Thus, there is never an external production of H2S or conversion of H2S to H2SO4 by LIVE MICROBES. The LIVE bacteria found in the LIVE MICROBES formulation primarily use sulfur in the form of sulfate through assimilatory sulfate reduction. During assimilatory sulfate reduction, the sulfate is first actively transported into the cell and activated by adenosine triphosphate (ATP) sulfurylase. The direct uptake of sulfide is not feasible for most microorganisms because of the very high toxicity of H2S. Once internalized, the sulfate is reduced to H2S and then immediately incorporated into the amino acid cysteine using the Oacetylserine pathway. This avoids H2S toxicity within the cell. Cysteine, in turn, is the source of sulfur for other organic molecules such as methionine, coenzyme A, and acyl carrier protein that are used internally within the cell and are not released to the environment. SRBs are the only group of bacteria that can utilize sulfate as an electron acceptor and in doing so, release H2S to the environment.
The organisms present in the LIVE MICROBES treatment are facultative anaerobes. Many of them are introduced in a resting stage as spores. They can grow anaerobically by using nitrate as the final electron acceptor, aerobically using oxygen as the final electron acceptor, and at any stage in between. Thus, they have a competitive advantage over the SRBs but only if they are added at a higher level than occurs under normal untreated conditions. The continuous addition of the highly concentrated LIVE MICROBES treatment, at multiple points within the collection system, is necessary to counteract the constant inoculation of SRBs through produced water collection activities.
Our selection of LIVE MICROBES have a faster growth rate than the SRBs mainly because of their ability to use oxygen as the final electron acceptor under aerobic conditions but also because of their flexibility under reduced-oxygen and anoxic conditions. Thus, it can grow under different concentrations of oxygen in a manner that allows it to out-compete SRBs for the available nitrogen and carbonaceous compounds necessary for its proliferation.
Our LIVE MICROBES are accustomed to living in low nutrient conditions as their natural environment and many are termed strategists. An strategist relies on high reproductive rates for continued survival within the community. An strategist microorganism is one that, through rapid growth rates, takes over and dominates situations in which resources are temporarily abundant. An additional benefit of Our LIVE MICROBES consortium is that during low nutrient conditions these bacteria can revert to abundant and resistant spores for dispersion and survival. This is a considerable advantage for bacteria in a SWD collection system.
In summary, by the continuous addition of Our LIVE MICROBES suspension to a SWD, one is able to repopulate the collection system with bacteria that are incapable of producing external H2S or converting H2S to H2SO4, are more flexible in their oxygen requirements, and are better adapted to the SWD environment than SRBs are introduced at every Produced Water Delivery to the SWD treatment systems and thrive under anaerobic untreated environments or thiobacilli that thrive in aerobic low pH environments in the presence of dissolved H2S. Our LIVE MICROBES treatment decreases odor in a SWD collection system by preventing the proliferation of the organisms causing the H2S gas and decreases corrosion by greatly reducing an essential part of the metabolism of thiobacilli. A continuous addition of Our LIVE MICROBES treatment allows gradual repopulation of the biofilm that lines the collection system by nonsulfate-reducing bacteria and prevents the proliferation of H2S oxidizing bacteria.