Sewage as a Mixed Organic Substrate for Desulfurization Bacteria

Objectives:The objective of this research is to 1) investigate the kinetics of sulfate reduction to sulfides by sewage-fed sulfate desulfurization bacteria (SRB), and 2) evaluate potentials for consequent sulfide toxic inhibition to SRB. Research results are applicable to cases where SRB organisms are employed in sulfate-containing waters for organic and heavy metal removals such as for certain industrial treatment systems, vertical flow wetlands, and use mine pool waters for reduction of heavy metal discharges.Theory and Background:Biological sulfate reduction is carried out by a distinctive and unique group of anaerobic bacteria commonly called sulfate-reducing bacteria (SRB). There are numerous genera and species of SRB which are ubiquitous in nature, having been found in supercooled brackish pools in the Antarctic; in deep Pacific hydrothermal vents; in fresh, marine, and brackish waters; in soils; in the stomachs of ruminant animals and in systems managing mine pools and acid mine/acid rock discharges.SRB share an ability to carry out sulfate reduction as a principal component of their bioenergentic processes employing sulfate as a terminal electron acceptor to oxidize a host relatively simple organic substances such as organic alcohols and acids. Little information is available on the ability of dilute sewage to be used as an SRB mixedorganic “food source”. Reduced sulfur from this reaction is released into the external environment as sulfide ion and hydrolyzes to either HS− or free H2S, depending upon the pH.A generalized equation for this heterotrophic reaction is:organic matter + SO42− → H2S/HS− + HCO3− (1)This reaction takes place at reduced redox potentials when all dissolved oxygen and oxygen from nitrites and nitrates are used up. In general, SRB organisms can predominate a culture when the general environmental conditions for SRB are oxidation reduction potential (ORP) < - 100 mV and 5 < pH < 9. The presence of sulfate will maintain the ORP in a range suitable for SRB desulfurization, while the absence of inorganic bound oxygen will cause further depression of the ORP which can result in an anaerobic environment leading to methanogenesis.Produced sulfides from biological desulfurization can interact with available trace metals to form insoluble metal sulfides, which are consequently precipitated and removed from the aqueous phase. This may be of particular value when dealing with mining or industrial wastewaters where trace metals are of environmental, regulatory or health significance. Examples of such potential interactions include systems where excess stormwaters can be impounded and some biological degradation can occur of diluted sewage in underground mine pools; organic degradation found in wetlands; beneficial use of certain sulfate containing mine wastewaters and biodegradation of some organics in municipal landfills.