US 5,076,927 AGranted

Biocatalyzed partial demineralization of acidic metal sulfate solutions

OwnerRobert M. Hunterinventor

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Abstract

This invention comprises the steps of (1) acid phase anaerobic digestion of biomass to produce volatile acids and a stabilized sludge, (2) use of the volatile acids as the carbon source and electron donor for biological sulfate reduction for removal of acidity, metals and sulfate from acid mine drainage, and to produce acetate, (3) use of the acetate solution as feed for methane phase anaerobic digestion to produce methane and to reduce the organic content of the effluent of the process, (4) and use of the methane to satisfy the energy requirements of the process. Key to the process is the use of kinetic control (i.e., a relatively short mean cell residence time) to ensure partial oxidation of higher molecular weight volatile acids (e.g., propionic, butyric, valeric) and production of acetate during the sulfate reduction step. In this way, the higher molecular weight volatile acids produced during acid phase anaerobic digestion can be used both as electron donors for sulfate reduction (during which they are converted to acetate) and as substrates for the subsequent methane production step.

Claims23

Dependent claims

A method for culturing sulfate-reducing bacteria and methane-producing bacteria in at least two reactors in series comprising the steps of:

delivering to a first reactor a liquid substrate comprising a dissolved sulfate and more moles of at least one dissolved electron donor with a molecular weight greater than that of acetic acid than are required by sulfate-reducing bacteria to biologically reduce at least a portion of the dissolved sulfate,
fermenting the substrate in the first reactor under anaerobic conditions at a pH between about 6 to 8, at a temperature between about 30 degrees C. to 40 degrees C. and at a mean cell residence time less than that required for essentially complete oxidation of acetic acid by sulfate-reducing bacteria at the selected pH and temperature to produce sulfide and a first liquid effluent, and
reducing the concentration of the sulfide generated by the fermentation process in the liquid in the first reactor,
delivering the first liquid effluent from the first reactor to a second reactor,
fermenting the first liquid effluent in the second reactor at a pH of between about 6.8 to 7.4, at a temperature between about 20 degrees to 65 degrees C. and at a mean cell residence time greater than the mean cell residence time required for essentially complete utilization of acetic acid by methane-producing bacteria to produce methane and a second liquid effluent,
removing the methane from the second reactor, and
removing the second liquid effluent from the second reactor.

4 more elements

A method for culturing microorganisms comprising:

continuously delivering a semisolid biomass feed to an acid phase digester at a loading rate of about 1 to 10 pounds of volatile solids per cubic foot per day,
fermenting said feed in the acid phase digester under anaerobic conditions at a temperature between about 20 and 40 degrees C. to produce a first effluent in an acid phase digestion step,
removing the first effluent and separating said first effluent into a first semisolid portion and a first liquid portion,
conveying the first liquid portion to a reactor comprising sulfate-reducing bacteria,
disposing of the first semisolid portion,
continuously delivering a wastewater comprising dissolved sulfate to said reactor comprising sulfate-reducing bacteria,
fermenting a mixture of the wastewater and the first effluent in the reactor at a pH between about 6 and 8 and a temperature between about 20 to 40 degrees C. to produce hydrogen sulfide and a second effluent in a sulfide reduction step,
removing the hydrogen sulfide from the reactor comprising sulfate-reducing bacteria,
removing the second effluent and separating said second effluent into a second semisolid portion and a second liquid portion,
conveying the second liquid portion to a separate methane phase digester,
fermenting the second liquid portion in the methane phase digester at a pH between about 6.8 and 7.4 and a temperature between about 20 and 65 degrees C. to produce methane and a third effluent in a methane phase digestion step,
removing the methane, and
removing the third effluent from the methane phase digester to process a semisolid component and a liquid component of the effluent.

10 more elements

Dependencies are inferred from the claim text.