Development and Application of Biological H2S Scrubbers for Treatment of Digester Gas

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ABSTRACT
As part of the California Energy Group (Former California Energy Commission) Public Interest Energy Research (PIER) Program, biological hydrogen sulfide (H2S) biogas cleaning was evaluated. The biological H2S scrubber uses bacteria to oxidize the sulfide to produce both elementary sulfur and sulfuric acid. This system was tested at the Inland Empire Utilities Agency’s Regional Plant 1 (IEUA RP-1) in Ontario, California.

The biological H2S scrubber demonstrated good performance throughout the testing phase. Based on the field measurements, The system required only a few days for startup and acclamation. The scrubber, under normal operating conditions, maintained treated biogas H2S levels below the target level of 40 ppmv.

INTRODUCTION
Biogas from anaerobic digestion of wastewater sludge or animal waste (i.e., manure) typically contains 500 to 3,000 ppmv of hydrogen sulfide (H2S), depending on the solids composition. Removing H2S is needed to reduce air pollution, protect power generation equipment, and increase safety of the operations.

The most commonly used methods for H2S control in wastewater treatment plants is sulfide abatement through adding ferric chloride or pickle liquor to the wastewater or directly to the digesters. The iron precipitates out the sulfur and prevents the production of H2S in the digesters. In Southern California, to meet air quality requirements (40 ppmv H2S level), biogas is further polished using the iron sponge process.

Biological H2S removal is attractive because of low energy and chemical usage requirements, easy and automated operation, long life expectancy of system elements, potential for elemental sulfur recovery, and no solid waste stream requiring discharge or disposal. In the biological process, H2S from the gas stream is removed by facultative bacteria of the genus thiobacillus; sulfur and sulfuric acid is produced from this biological H2S oxidation reaction. The key to obtaining an efficient reaction is to provide an ideal habitat for the growth of sulfide-oxidizing bacteria, to the exclusion of competing microbes, which normally predominate in aerobic treatment processes. The bacteria are capable of oxidizing H2S at low pH levels. To grow, they need oxygen/air, nutrients, and humidity. Efficient removal of H2S requires media with enough surface area to maintain a large population of sulfide-oxidizing microbes.

Two commercial biological systems, THIOPAQ® (PAQUES) and Biopuric® (Biothane Corporation), were designed for removing H2S from the sour gas. The THIOPAQ® process uses caustic to adsorb the H2S under alkaline conditions (pH 8 to 9) and aeration, in the presence of thiobacillus bacteria, to oxidize the H2S to elemental sulfur. The Biopuric® process is a combination of a conventional chemical scrubber and a biological tricking filter. Both systems
have been reported to achieve over 90 percent removal, but both are two-stage processes and require chemicals and significant capital investment.

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