Elemental sulfur as electron donor and/or acceptor: Mechanisms, applications and perspectives for biological water and wastewater treatment

Biochemical oxidation and reduction are the principle of biological water and wastewater treatment, in which electron donor and/or acceptor shall be provided. Elemental sulfur (S0) as a non-toxic and easily available material with low price, possesses both reductive and oxidative characteristics, suggesting that it is a suitable material for water and wastewater treatment. Recent advanced understanding of S0-respiring microorganisms and their metabolism further stimulated the development of S0-based technologies. As such, S0-based biotechnologies have emerged as cost-effective and attractive alternatives to conventional biological methods for water and wastewater treatment. For instance, S0-driven autotrophic denitrification substantially lower the operational cost for nitrogen removal from water and wastewater, compared to the conventional process with exogenous carbon source supplementation. The introduction of S0 can also avoid secondary pollution commonly caused by overdose of organic carbon. S0 reduction processes cost-effectively mineralize organic matter with low sludge production. Biological sulfide production using S0 as electron acceptor is also an attractive technology for metal-laden wastewater treatment, e.g. acid mine drainage. This paper outlines an overview of the fundamentals, characteristics and advances of the S0-based biotechnologies and highlights the functional S0-related microorganisms. In particular, the mechanisms of microorganisms accessing insoluble S0 and feasibility to improve S0 bio-utilization efficiency are critically discussed. Additionally, the research knowledge gaps, current process limitations, and required further developments are identified and discussed.