ODOR CONTROL CONSIDERATIONS FOR BIOLOGICAL TREATMENT SYSTEMS

The Hampton Roads Sanitation District (HRSD) has designed and installed extensive odor control facilities at each of its nine treatment plants in southeastern Virginia. Most of these facilities include two-stage packed tower scrubbers for the removal of hydrogen sulfide, with caustic used in the first stage and caustic and sodium hypochlorite (or chlorine) used in the second stage.As part of the goal of no off-site odors, HRSD retained PBS&J to design odor control improvements to three of their treatment plants. At two of these plants, some or all of the odorous air to be scrubbed with packed tower scrubbers originates from biological treatment systems. As part of the design process, PBS&J performed detailed modeling of the proposed scrubber systems, using chemical engineering fundamentals, including mass transfer theory and stoichiometric chemical calculations. The results of these modeling efforts indicated that the conventional caustic first stage scrubber would not be cost effective for scrubbing the air from biological systems, as the carbon dioxide concentration was too high. Carbon dioxide is removed from air streams in the same method as hydrogen sulfide. Because the carbon dioxide concentration is two orders of magnitude higher than the hydrogen sulfide concentration, the caustic usage becomes excessive. As such, it was recommended that HRSD provide oxidants in both stages of a two-stage packed tower scrubber system.In addition, the air streams of concern contain moderate to high concentrations of organic sulfides. Previous experience has indicated that a neutral pH hypochlorite scrubber is more effective in removing organic sulfides than a basic pH hypochlorite scrubber. Therefore it was tentatively recommended to provide a neutral pH first stage and a basic pH second stage with sodium hypochlorite oxidant in both stages.However, it was not known how a effectively a neutral pH hypochlorite scrubber would remove the high levels of hydrogen sulfide expected. As less hydrogen sulfide will be ionized at a neutral pH, the oxidation of hydrogen sulfide must occur almost instantaneously to obtain a substantial removal efficiency. In addition, traditional controls have not been effective in controlling hypochlorite dose in a neutral pH scrubber, so an alternative method had to be developed.To answer the questions of concern, a pilot test was undertaken on one of the existing odor control scrubbers. The results of the pilot test indicated that a neutral pH hypochlorite scrubber was effective in removing hydrogen sulfide. In addition, a gas phase chlorine monitor was successfully tested for controlling hypochlorite dose.Based on the evaluations performed, the systems for the two treatment plants were designed / modified to allow for the use of a neutral pH hypochlorite first stage scrubber and a caustic bleach second stage scrubber. Each stage was designed to use a gas phase chlorine monitor to control hypochlorite dose.