Question 5.29: Environmental engineers who design geothermal power plants (...
Environmental engineers who design geothermal power plants (47), reverse-osmosis water plants (60), wastewater-treatment facilities (61), septage receiving facilities (62), or sanitary landfill sites (63) must address H_{2}S/odor control during the design process rather than a retrofit measure in response to pressure from nearby residents. H_{2}S/odor problems of 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)
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_{2}S/odor 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 nonodorous 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 development 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 population 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. Singlestage, 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 second stage is an acidic wash using H_{2}SO_{4}. Automatic dosage systems are a necessity in 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 characterized 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.
The cost for operating the chemical scrubber can be divided into chemical cost and energy cost. Energy will always contribute most to the total cost of operation. For the two-stage scrubber (see Fig. 22), the energy cost will be approximately two-thirds of the total operational cost. Although some simpler types are available, chemical scrubbers are generally applicable only at large treatment plants, where biological methods of control are not feasible.
