Biotechnological Applications

Application of sulfate reduction with methane in the Biotechnology

Sulfur compounds are cycled between the earth’s soils, oceans, atmosphere and living matter in the so-called “natural sulfur cycle”. However due to human activities the emissions of sulfur compounds to surface waters and atmosphere have increased dramatically, causing major environmental problems like the acidification of surface waters, the release of heavy metals from sediments, the increasing salinity of freshwaters and the production of toxic sulfide in anaerobic soils.

There are two important sources for these emissions:

· The first source is the mining and metallurgical industry. The mining wastewater, called acid mine drainage, contains besides the sulfate large quantities of heavy metals. Heavy metals are toxic for humans and have a devastating effect on ecosystems.

· The second source of sulfurous emissions is the petrochemical industry. Fossil fuels (like coal, oil and gas) contain hydrogen sulfide or organic S-compounds; their combustion results in the emission of sulfur dioxide, a major compound in the acid rain formation. Therefore, sulfur dioxide has to be removed from the off-gas or sulfur compounds have to be removed from fuels prior to combustion, both processes result in the generation of a waste stream containing the sulfur compounds.

Waste streams containing sulfur compounds can be treated using the biological sulfur-cycle (figure 1). Oxidized sulfur compounds can be converted into sulfide by biological sulfate reduction. Sulfide can subsequently be used to precipitate metals. Metal sulfides can be removed from the water phase by sedimentation and reused in the metallurgical industry. Sulfide can also be oxidized to elemental sulfur. Elemental sulfur is a safe, storable and reusable product. It can be removed from the water phase by sedimentation.

Figure 1: The biological processes of the Sulfur-cycle

For sulfate reduction an electron donor is required. The electron donor forms the major running cost of sulfate reduction processes. Cheap electron donors like organic waste streams are not easily degradable and often contain some inert material, therefore pre or post treatment is required. Fully degradable pure bulk chemicals are therefore a better option. Ethanol and methanol are interesting electron donors on smaller and middle scale. On large scale the best electron donor is hydrogen (van Houten, 1996).

At the Zinifex zinc factory in Budel (the Netherlands), hydrogen is used as electron donor for biological sulfate reduction (figure 2). The hydrogen is produced from the methane in natural gas by steam reforming.

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Figure 2: Simplified flow-scheme of the Zinifex process (Budel, the Netherlands)

However, due to the low efficiency of the steam reformer and the high temperature and pressure required for this physical-chemical conversion, the emission of CO₂ and the costs of the wastewater treatment would be greatly reduced if methane could be used directly as electron donor for biological sulfate reduction. In this project the possibilities to use methane directly as electron donor for sulfate reduction are investigated.

Reference:

van Houten, R.T. (1996) Biological sulphate reduction with synthesis gas. PhD Thesis Wageningen University

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