Sulfur Cycle Technology

Microbial protein production from biogas: focus on carbon fixation and sulfur upcycling by MOB and SOB to produce S-protein 

Researcher: Ms. Marica Areniello

Project: The research is focused on the possibility to synthesize microbial protein (MP) by supplying biogas to a mixed bacterial culture, made by methane oxidizing bacteria (MOB) and sulfur oxidizing bacteria (SOB), thereby enabling the conversion of the organic carbon (CH4) into MP-biomass, coupled with sulfide oxidation (of the H2S present in biogas) and conversion into additional MP biomass, potentially rich in sulfur amino acids.

Relevance and impact: Microbial protein represent a valid tool to recover resources and upcycle nutrients stored in waste streams. Indeed, despite the conventional bioenergy applications, organic waste could potentially be valorized into higher quality applications by strategies aimed at achieving a more efficient resource recovery and valorization process through carbon capture and nutrient upcycling. In addition, MP can address one of the key challenges of the 21st century that is meeting the food and protein demand of the ever-increasing global population, which is causing an unprecedented pressure over the conventional agri-food chain. Microbial protein (MP) indeed, constitute a valid and sustainable alternative protein source for both feed and food applications since feed and food protein additives obtained from bacterial biomass can be produced with a lower environmental footprint as compared with other plant or animal-based alternatives.

Microbiological suitability of elemental sulfur disproportionation for metal recovery

Researcher: Dr Anna Florentino

Project: Although heavy metal-containing wastewaters generated by metal and mining industries have become an increasing global environmental problem, they represent a potential resource of valuable metals when their recovery is possible in an economically feasible way. Sulfidogenic microorganisms are of scientific and biotechnological interest, as they extend the horizon of operation for biotechnological processes, such as metal recovery. One ancient and relevant process in the sulfur cycle is the disproportionation of elemental sulfur. Although this important conversion is common in nature and has much potential for biotechnological precipitation and recovery of metals from industrial waste streams, its microbiological suitability for such purpose has received very little or no attention. This project focuses on the microbiological suitability of elemental sulfur disproportionation for metal recovery, as well as on the machinery required by the microorganisms to execute the process.

Relevance and impact: Biological sulfidogenesis by sulfate reduction is the most applied technology for the treatment of metal-containing wastewaters. However, a critical bottleneck for widespread application of high-rate biogenic sulfide technologies is the cost of the electron donors. The ability to disproportionate elemental sulfur, generating sulfate and sulfide without the addition of external electron donor is a cost-effective alternative to treat metal-rich streams.

Nitrification-autotrophic denitrification of wastewater using sulphur compounds as electron donors

 Researcher: Ms Federica Carboni

Project: Nitrogen removal from wastewaters is an important aspect to avoid the presence of nitrates in drinking water that lead to an increasing risk of severe diseases such as methemoglobinemia, toxicity problems to fish species of the receiving water bodies and eutrophication. Denitrification can be conducted by autotrophic bacteria that use inorganic electron donors in stead of organic carbon, like inorganic sulfur compounds. Sulfide (S2- ) and pyrite (FeS2) can be used for autotrophic denitrification. The purpose of this project is to optimise autotrophic denitrification with these two electron donors treating a high salinity wastewater and to determine the best operating conditions in order to optimise the process.

Relevance and impact: Autotrophic denitrification is advantageous for groundwater or wastewater, e.g. those from the leather and fertilizer-processing industry, landfill leachates and livestock, that contain little or no organics and thus have a low C/N ratio, since no external carbon source needs to be added compared to the conventional denitrification carried out by heterotrophic bacteria for which an organic substrate is required.

Application of elemental sulfur (S⁰) as biosorbent for heavy metal removal and recovery from wastewater

Researcher: Ms Sudeshna Saikia

Project: Heavy metal pollution has become a global concern in the last decades owing to toxicity and insusceptibility to the environment. Despite the availability of numerous techniques for the treatment of effluents with heavy metals, biosorption is constantly viewed as a highly efficient and cost-effective alternative due to the presence of various functional groups on the surface of the biosorbent, onto which the metal can attach. This project focuses on the application of elemental sulfur as a biosorbent for heavy metal removal and recovery from industrial watewater.

Relevance and impact: Elemental sulfur (S0) is an important intermediate in the biological sulfur cycle. Incomplete oxidation of sulfide in gaseous streams under oxygen-limiting conditions by S-oxidizing microorganisms leads to the production of biogenic S0 (or biosulfur). Biogenic S0 globules, generated by different strains of bacteria, are hydrophilic, with a structure made of orthorhombic S0 crystals surrounded by a hydrated layer of long-chain polymers or polythionates. The chemical composition and small particle size of biogenic S0 globules affects the S0 (bio)chemical reactivity and makes S0 a potent candidate for metal recovery technologies.

Role of sulfur compounds in biofuel production

Researcher: Dr Chiara Cassarini

Project: The research focuses on the development of biotechnological platforms, specifically aiming at combining wastewater treatment to the production of valuable products. The research will focus on the treatment of wastewater rich in organic matter and sulfur compounds to produce valuable products, such as hydrogen, liquid fuels (e.g. butanol and ethanol) and elemental sulphur. This will be investigated by the combination of different subprojects: (1) quantification of sulfur intermediate compounds, (2) role of elemental sulfur and polysulfides in microbial metabolism, (3) co-culturing of microorganisms for production of valuable compounds and (4) production of fine chemicals from carbon monoxide.

Relevance and impact: Sulphur compounds such as sulphate, thiosulfate, sulphide, sulfite and dithionite are common contaminants discharged in fresh water due to industrial activities. Biological anaerobic sulphate reduction has been successfully applied for the treatment of these sulphur compounds. One of the main limiting factors is the cost of the electron donor, which can be reduced with the use of a cheaper and more available carbon source, such as methane or carbon monoxide. Combining wastewater treatment with the production of biofuels and/ or other valuable compounds (acetate and elemental sulfur) from waste will have a positive economical and societal impact by improving societal health, well-being and reducing the cost of fuel production.

Assessment of lignocellulosic materials as slow release electron donor for sulfate reduction coupled to metal recovery from acid mine drainage

Researcher: Rachel Biancalana Costa

Project: Acid mine drainage (AMD) represents an environmental liability. Biological sulfate reduction based technologies are a possibility to associate metal recovery with the mitigation of impacts. This strategy allies biological and chemical processes to achieve metal recovery and the neutralization of AMD simultaneously. Sulfate-reducing bacteria demand the addition of an electron donor, which cost could impair the applicability of this technology. Hence, we suggest using lignocellulosic materials, often available as solid waste, as a slow release electron donor for sulfate reduction. This alternative allows using low-cost materials, such as potato peel, crabshell or agro-industrial wastes, which are by-products of industrial processes. Moreover, hydrolytic activity is necessary to slowly release electron donors to be used by sulfate reducers, which is expected to create supersaturated areas with sulfide, thus enhancing the conditions for metal sulfide precipitation with good settling properties.

Relevance and impact: Biological sulfate-reduction treatment of acid mine drainage allows obtaining added-value products (metal sulfides) and neutralization of the wastewater. The use of a lignocellulosic material as a low-cost electron donor will make the process cheaper, combined with achieving metal sulfide precipitates with good settling properties, which are easier to recover. Hence, we intend to recover metal sulfides, while mitigating the acid waste from mining activities, and using a cheap and largely available solid waste (lignocellulosic materials) as electron donor for sulfate reduction.

Microbial protein production from biogas: focus on carbon fixation and sulfur upcycling by MOB and SOB to produce S-protein