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.