Biological sulfuric acid transformation: Reactor design and process optimization

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Abstract
As an alternative to the current disposal technologies for waste sulfuric acid, a new combination of recycling processes was developed. The strong acid (H2SO4) is biologically converted with the weak acid (CH3COOH) into two volatile weak acids (H2S, H2CO3) by sulfate‐reducing bacteria. The transformation is possible without prior neutralization of the sulfuric acid. The microbially mediated transformation can be followed by physiochemical processes for the further conversion of the H2S. The reduction of sulfate to H2S is carried out under carbon‐limited conditions at pH 7.5 to 8.5. A fixed‐bed biofilm column reactor is used in conjunction with a separate gas‐stripping column which was installed in the recycle stream. Sulfate, total sulfide, and the carbon substrate (in most cases acetate) were determined quantitatively. H2S and CO2 are continually removed by stripping with N2. Optimal removal is achieved under pH conditions which are adjusted to values below the pKa‐values of the acids. The H2S concentration in the stripped gas was 2% to 8% (v/v) if H2SO4 and CH3COOH are fed to the recycle stream just before the stripping column. Microbiol conversion rates of 65 g of sulfate reduced per liter of bioreactor volume per day are achieved and bacterial conversion efficiencies for sulfate of more than 95% can be maintained if the concentration of undissociated H2S is kept below 40 to 50 mg/L. Porous glass spheres, lava beads, and polyurethane pellets are useful matrices for the attachment of the bacterial biomass. Theoretical aspects and the dependence of the overall conversion performance on selected process parameters are illustrated in the Appendix to this article. © 1993 John Wiley & Sons, Inc.