The El Paso Water Utilities (EPWU) operates four major wastewater treatment facilities. In anticipation of significant growth within the service area, the Northwest Treatment Plant was upgraded and expanded in the mid 1990s. New facilities included a new headworks, blower building,
aeration basins, DAFT thickeners, lime stabilization system, and two packed bed odor control systems. The scrubbers are countercurrent absorption towers operating with sodium hydroxide (caustic soda) and sodium hypochlorite (bleach) chemical solution. Since startup of the expanded facility
in 1994, the Plant has experienced relatively high odor levels and frequent occurrences of objectionable off-site odors, occurring primarily in winter, when cool weather promotes inversions in the Rio Grande valley. The initial investigations traced high odor levels to inadequate aeration
and septic conditions developing within the thickened sludge holding tank. The resulting hydrogen sulfide load, as well as amine and ammonia based odors from lime stabilization were believed to be overloading the solids handling odor scrubber. Other suspected odor sources included the aeration
basin influent channel, which handles raw influent wastewater directly from the grit tanks. Several minor modifications and operational changes implemented as a result of the initial evaluation produced significant improvements in ambient odor levels. However, off site odors would still,
occasionally, reach unacceptable levels. Thus, the odor control system evaluation work was expanded to include the following: Reduced sulfur analysis of scrubber inlet and outlet gases Odor panel
evaluation of key odor sources utilizing Dynamic Dilution Olfactometry Atmospheric dispersion analysis Qualitative assessment of the outlet air from each scrubber. The results of
the additional testing and air dispersion modeling produced odor isopleths for the surrounding area under a number of operating conditions. Key findings from the evaluation indicated that under very strong odor conditions resulting from septic sludge in the dewatering process, odor breakthrough
can occur from the solids handling scrubber. Thus, a second stage absorber will be required. The olfactometry testing and atmospheric modeling revealed that selected locations in the aeration process produce relatively high odor unit values and are significant sources, primarily due to their
proximity to Interstate 10 highway. The need for additional odor treatment for these areas was suggested by the modeling.
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