Mathematical modelling of sulfide oxidation in microaerobic bioreactors

In most wastewater streams, sulfur is present in the form of sulfate. During anaerobic wastewater treatment, biochemical reduction of sulfate to sulfide takes place by the action of sulfate reducing bacteria. High concentration of sulfide in anaerobic reactors and the resulting hydrogen sulfide in biogas can cause major technological problems, such as inhibition of anaerobic processes, accumulation of inert materials (metal sulfides) in the sludge, corrosion (of tanks, piping, engines and boilers) and emission of sulfur dioxide from the combustion of biogas. Because of these complications, hydrogen sulfide has to be removed from biogas prior to its further use.

Using microaeration, i.e. controlled dosing of small amount of air into the anaerobic digester, very efficient desulfurization with minimum cost can be achieved and process can be fully automated. The main constraint for the successful on-line control of air dosing, however, is that the amount of air dosed into the headspace of the reactor must be high enough to oxidize all H₂S present in biogas, but should not cause excessive dilution of biogas by nitrogen.

The overall goal of this PhD research project is to control H₂S emissions from the anaerobic treatment of wastewater with a high sulfate content using microaerobic conditions in UASB reactors. This goal will be reached through performing short-term and long-term lab-scale experiments, combined with mathematical modeling and simulation. The project will involve three main research lines:

1. The identification of reaction mechanisms for sulfide oxidation under microaerobic conditions
2. Development of a mathematical model (based on existing ones) describing microaeration and its application in simulation studies.
3. The design of adequate control strategies for biogas desulfurization through on-line controlled microaeration.