Page 314 - Air Pollution Control Engineering

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Wet and Dry Scrubbing 293

Example 29

Environmental engineers who design geothermal power plants (47), reverse-osmosis water
plants (60), wastewater-treatment facilities (61), septage receiving facilities (62), or sani-
tary landfill sites (63) must address H S/odor control during the design process rather than
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a retrofit measure in response to pressure from nearby residents. H S/odor problems of
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waste-treatment or waste handling facilities can be solved by proper siting and application
of exist technologies, including chemical scrubber, filters, combustion, biological processes,
and so forth. As an environmental engineer in charge of a design project, please provide
discussions on the following:
1. Siting considerations
2. Commerically available chemical scrubbers (62)
Solution
1. Siting considerations that should be considered by an environmental engineer are in
the following discussion. It is very important to identify the main source of H S/odor
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at the facility and treat only the odorous gases. A simple approach to isolating the
odorous gases would be to enclose the component of the facility generating the odors.
The gases would be confined in a housing structure and thereby isolated from non-
odorous air. This would reduce the volume of air to be treated and thus the overall
cost. Designer must understand the dangers (toxic and explosive potential) of the
closed spaces to operating personnel.
During the site-selection process, consideration should be given to the impact that
offensive odors may have on nearby residents. Zoning ordinances and land devel-
opment patterns must be reviewed. An isolated area, if residentially zoned, may
develop in the near future and result in pressure being applied to retrofit a facility
without odor control. Care should be taken to locate the facility in a well-ventilated
area (e.g., an open space on a hilltop) and downwind from existing or projected pop-
ulation centers. Provisions for adding odor control systems in the future should be
considered.

2. The following presents a discussion on commercial chemical scrubbers. Sodium
hypochlorite has been used successfully as an oxidizing agent in commercial chemical
scrubbers to control odor at many waste-handling or waste-treatment facilities. Single-
stage, two-stage, or three-stage scrubbers have been used. In Fig. 22, a two-stage
scrubber is shown. The first stage is alkaline oxidation (NaOH+NaOCl), and the sec-
ond stage is an acidic wash using H SO . Automatic dosage systems are a necessity in
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preventing accidents when using the concentrated chemicals required for this system.
Another type of chemical scrubber used at treatment plants that receive septage
(shown in Fig. 23) generates sodium hypochorite by electrolysis of salt (NaCl).
Because this scrubber produces hypochlorite (concentration less than 2%) and no acidic
step is involved, there is less need for special care concerning the delivery, handling, and
dosing of dangerous chemical.
The results from total odor strength measurements of different chemical scrubbers
show odor reduction efficiencies between 95% and 98%. The air has been character-
ized as being “free from sewage odors, but it smells like chemicals.” It seems that a
chemical scrubber always gives this “scrubber odor.” However, if the scrubber is
incorrectly operated, this “scrubber odor” changes to a chlorine odor.