Anaerobic digestion (AD) is a well-established process for waste and wastewater treatment, reconciling bioremediation to biogas production. Methane (one of the main components of biogas) is a renewable biofuel that can be used for the production of heat and electricity. It is desirable to have high methane yields in order to maximize the potential of AD processes for energy recovery. However, methanogenesis is normally the rate-limiting step of the whole AD process and is quite sensitive to inhibition by several compounds.
Inhibition of methanogenesis by sulphide has been reported. Sulphide can be formed from sulphate (often present in wastewaters, especially the ones resulting from industrial and mining activities) by sulphate-reducing bacteria (SRB). Heavy metals can also affect the anaerobic digestion since, unlike other toxic compounds, they are not biodegradable and they can accumulate to toxic concentrations. One of the most common methods for metal detoxification is the use of sulphide to precipitate the heavy metals. In this work we intend to study the effect of sulphide on methanogenic sludges and also how pH influence the metal toxicity and metal detoxification process using sulphide.
Aim and techniques
Differences in pH will affect the equilibrium between the different sulphide species (H2S, HS-, H+ and S2-) as well of the metal ions. These changes will influence the toxicity of sulphide and its ability to bind to metals and precipitate, which will have effects in biogas production and in the microbial community.
The main objective of this thesis is to study sulphide inhibition levels for methanogenic consortia, as well as to explore the viability of adding metals to control sulphide inhibition. Assays will be performed in batch mode, in order to test different process variables (e.g. sulphide concentration, metal species and concentration, pH). H2/CO2, acetate and propionate/butyrate will be used as carbon and energy source. Biogas production will be measured overtime and compared with control assays. Substrate consumption and sulphide/metal concentration will also be measured.
The project is designed for a MSc thesis (6 months) for students with interest in Microbial Physiology. Please fill free to contact for more information.
Lara Paulo (PhD)
Diana Sousa (Post-Doc)