Graden avlegges ved Universitetet i Sørøst-Norge (USN), Fakultet for teknologi, naturvitenskap og maritime fag.
The controlled stabilization of organic matter is one of the solutions for carbon footprint reduction.
One of the methods for organic matter stabilization is biogas production by anaerobic digestion.
Biogas is mainly a mixture of methane and carbon dioxide, but it also contains small amounts of other gases like hydrogen sulfide. Hydrogen sulfide causes corrosion and is a major occupational hazard in agriculture, aquaculture, biogas processing, industry and sewage processing.
Thus, its removal is necessary.
We studied a biological method for hydrogen sulfide removal using nitrate as electron acceptor in a granular sludge bed. The method is an autotrophic denitrification process producing low amount of sludge. Proposed concept overcome the complexity of physicochemical methods and the granular sludge can accumulate elemental sulfur that can be harvested for sulfur recovery. The experiments were performed in a high rate reactor, demonstrating the efficiency of the approach.
Experimental work was focused on the temperature impact on the process, important for cold-climate countries like Norway. Effects of the ratio between feed nitrate and hydrogen sulfide on sulfur accumulation were also investigated.
The process is much more sensitive to nitrate and hydrogen sulfide ratio changes than temperature. The feed ratio between nitrate and hydrogen sulfide that maximize elemental sulfur accumulation was experimentally determined.
Higher feed ratios caused the oxidation of earlier accumulated elemental sulfur as sulfate.
Hydrogen sulfide removal decreased with temperature, but was still efficient at the lowest investigated level (10 °C).
This implies that autotrophic sulfide removal can be a feasible option in cold climates. Elemental sulfur accumulation decreased more than sulfide removal with temperature, explained by a temperature induced changes in microbial communities.