Date of Award
Doctor of Philosophy
Civil, Construction, and Environmental Engineering
The presence of sulfur compounds (e.g. protein, sulfate, thiosulfate, sulfite, etc.) in
the feed stream generates highly corrosive and odorous hydrogen sulfide during anaerobic
digestion. The high sulfide level in the biogas stream is not only poisonous to many novel
metal catalysts employed in thermo-catalytic processes but also reduces the quality of
methane to produce renewable energy. This study used an innovative, low-maintenance,
low-cost biological sulfide removal technology to remove sulfides simultaneously from both
gas and liquid phase. ORP (Oxidation-Reduction-Potential) was used as the controlling
parameter to precisely regulate air injection to the sulfide oxidizing unit (SOU). The microaeration
technique provided just enough oxygen to partially oxidize sulfides to elemental
sulfur without inhibiting methanogenesis. The SOU was equipped with a diffuser at the
bottom for the dispersion of sulfide-laden biogas and injected air throughout the column.
The SOU can be operated as a standalone unit or coupled with an anaerobic digester to
simultaneously remove sulfide from the biogas and effluent.
The integrated system was capable of reducing hydrogen sulfide in biogas from 2,450
to less than 2 ppmV with minimal sulfate production at the highest available sulfide loading
rate of 0.24 kg/m3-day. More than 98% of sulfide removed was recovered as elemental
sulfur. However, the standalone SOU was able to operate at high hydrogen sulfide loading
of 1.46 kg/m3-day at inlet sulfide concentration of 3000 ppmV and reduce the off-gas
hydrogen sulfide concentrations to less than 10 ppmV. The experiment also revealed that the
ORP controlled aeration was sensitive enough to prevent oxygen overdosing (dampening
effect) during unexpected surges of aeration. Using generalized linear regression, a model predicting output H2S concentration based on input H2S concentrations, SOU medium
heights, and biogas flow rates, was derived. With 95% confidence, output H2S concentration
was affected by changes in liquid heights the most, followed by changes in flow rates.
Feasibility studies for H2S removal from biogas by micro-aeration were conducted at
the Ames Water Pollution Control Facility (AWPCF) by using different types of liquid media
available at the plant, i.e. plant effluent, mixed liquor, and digester supernatant. From the
experiment at AWPCF, it was found that operating pHs were affected by the amount of
alkalinity in the liquid media and that the removal efficiencies were affected by the operating
pH. Among all the liquid media tested, digester supernatant showed the greatest potential
with more than 99% H2S removal at an operating pH of 7.0 and volumetric biogas flow rate
of 21.6 m3/m3-hr. By increasing trace metal contents and temperature of the medium, the
hydrogen sulfide removal rate was greatly improved. The operating cost of the full-scale
system was estimated to be approximately $2/kg-S-removed. In addition, it was also
revealed that abiotic sulfide oxidation accounted for 95% of overall sulfide oxidation.
This technology is expected to widen the use of biogas as a renewable fuel since the
maintenance requirements of biogas handling equipment, the methane purification costs, and
the emissions of SOx will dramatically be reduced. Importantly, the technology does not
require inoculation of special bacteria, addition of nutrients and trace elements, or chemicals
for pH control.
Duangmanee, Thanapong, "Micro-aeration for hydrogen sulfide removal from biogas" (2009). Graduate Theses and Dissertations. 10748.