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Water Environment Research Article

Iron Sponge Reduces Toxic Emissions

60% of its original capacity after regeneration. Media can be regenerateed two to three times during its life of about 3 years.

Several wastewater treatment plants have successfully used the iron sponge process, including the city of Santa Cruz, Calif., the Union (Calif.) Sanitary District Wastewater Treatment Plant, and the Central Marin (Calif.) Sanitation Agency.

The Central Marin Sanitation Agency operates a 1300-L/s (30-mgd) peak wet weather facility in San Rafael. The plant uses two iron sponge scrubbing units in series. The vessels have been in operation for 8 years, and the spent media has been replaced three times. The cost of each replacement was approximately $15,000. One of the iron sponge units in operation is a primary unit; the second unit is a standby-polishing unit. The digester gas H2S concentrations are reduced from approximately 400 ppm to 35 ppm.

To regenerate the media, the vessel is taken out of service, filled with water again. On the second fililng, approximately 23 kg (50 lb) of sodium bicarbonate is added to kep the pH at approximately 8. Air bubbled through the media during regeneration. Close monitoring of temperature, pH, and air flow rate is required. The duration of regeneration of one vessel is 72 hours, and the regenerated media has 60% of its initial capacity.

-Wendell Kido, Sacramento (Calif.) Regional Wastewater Treatment Plant; Ravi Krishnaiah and Tanya Yurovsky, Brown and Caldwell, Pleasant Hill, Calif.; and Perry Schafer, Brown and Caldwell, Sacramento (adapted from a paper presented at the California Water Environment Association April 1994 conference)

To control emissions from a cogeneration facility that will use digester gas for fuel, the Sacramento Regional Wastewater Treatment Plant in Elk Grove, Calif., is incorporating iron sponge technology to reduce hydrogen sulfide (H2S) prior to combuston. Reducing digester gas H2S concentrations at this stage will result in lower sulfur oxide emissions in the combustion exhaust.

The Sacramento Metropolitan Utility District is constructing the cogeneration facility and will purchase 90% of the treatment plant's digester gas. The remaining gas from the treatment plant will be burned in waste gas burners onsite. New air quality regulations require that all new or modified combustion devices incorporate technology to reduce H2S concentrations.

Iron sponge technology will be used to meet the 3-hour maximum concentration of 50 ppm H2S established by the Sacramento Metropolitan Air Quality Management District as best available control technology. The treatment plant's present annual average gas flow is 2550 m3/h (90,000 ft3/h), and the design rate is 4220 m3/h (149,000 ft3/h), which is associated with the current project involving five new digesters (see "Solids Train Strategy," p. 56). The plant's average flow is projected to increase from 6000 L/s (137 mgd) to 11,350 L/s (259 mgd) by 2010.

The iron sponge technology will be installed as an add-on process to function within the treatment plant's existing iron chloride system. Iron chloride is normally added to plant influent, which increases the concentration of iron in the digesters. Sulfide in the digesters is chemically precipitated with the iron. The addition of iron chloride to the plant headworks reduces average H2S concentrations in the digester gas to 165 ppm. The H2S concentration before iron chloride addition was about 1500 ppm.

By using the iron sponge technology, a dry-scrubber process, plant operators hope reduce the H2S concentrations to below 50 ppm in the digester gas. The system consists of stainless steel cylindrical vessels with removable covers. Each unit will contain the iron sponge media consisting of wood chips impregnated with iron oxide. The media will be supported by a fiberglass grating (see Figure).

During normal operational conditions, the digester gas from all on-line digesters will be treated in primary units (see Table). The standby units will be used only as needed during media regeneration.

In the iron sponge, gas flows through the dry media in a low pressure vessel. Sulfur removal is increased by uniformly distributing the iron oxide across the wood chips. The wood chips increase the bed porosity and reduce the pressure drop across the bed. The H2S in the gas stram reacts with the iron oxide to produce iron sulfide and water.

The spent media can be regenerated by filling the vessel with water and passing air or oxygen through the bed and converting the iron sulfide back to iron oxide and elemental sulfur. The media can only be regenerated a few times since it gets coated with elemental sulfur, which blocks the media and increases the pressure drop across the bed. The spent media regains 50% to

Iron Sponge Design Criteria at the Sacramento
Regional Wastewater Treatment Plant
Parameter Unit
Number of Vessels 9
    Primary 6
    Standby 3
Diameter of vessel 3.7 m (12 ft)
Sidewall depth 1.8 m (6 ft)
Media depth 1.5 m (5 ft)
Media surface area per vessel 10.5 m2 (113 ft2)
Media life at average gas flow 365 days
Media volume in each vessel 3.7 m (12 ft)

Proudly reproduced by Marcab Company, Inc., manufacturers of digester gas and air purifiers.

This article appeared in the May 1995 issue of Water Environment & Technology magazine. We thank the Editor for permission to reprint it. Marcab Company, Inc., worked closely with Wendell Kido, Ravi Krishnaiah, Tanya Yurovsky and Perry Schafer to design and build this purifier system according to their specific requirements. We are grateful for the opportunity to work with the people at Sacramento Regional Wastewater Treatment Plant and the folks at Brown and Caldwell.

Marcab Company, Inc., designs and builds custom digester gas and air purifiers to remove hydrogen sulfide. Please contact us at our office in San Marcos or your local representative to discuss your hydrogen sulfide removal requirements.


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