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
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
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
concentrations in the digester gas to 165 ppm. The H2S
concentration before iron chloride addition was about
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