Odor control of combined sewer collection systems where transport and storage is principally by tunnels presents a challenge to municipalities because of the infrequent but often noxious emissions from venting structures. So long as flow levels are low to average, entrained air exhausted
from these large diameter “pipes” can be effectively dispersed with proper ventilation design. But with low flow rates, particularly during dry or near-drought conditions, near full strength wastewater can turn septic resulting in very malodorous releases. This is most evident
during initial, “first flush”, rain events where tunnel levels, flow rates and air volumes can change rapidly resulting in large odor-laden air displacement. During the early operating months following the start-up of its Nancy Creek Tunnel in 2006, the City of Atlanta experienced
complaints from residents and commuters near two vent points on the upper reaches of the 16-foot (4.9 m) diameter, 44,000 linear foot (13.4 km) hard rock “interceptor” especially immediately following heavy rain events before stormwater dilution naturally reduced odor intensity.
Using temporary solid media odor control systems, the City quickly responded to treat the releases. While continuing to “pilot” the solid media technology in the temporary systems, the odor constituents were sampled and analyzed along with monitoring air flow cycles from the tunnel's
vents which were identified as the offending exhaust points. Employing prior pilot history along with the on-site operating experience and with odor control system vendor assistance, life cycle costs were computed pitting solid media against bio-filters for a permanent solution to the problem.
Passive odor control systems were then designed and installed at both sites that were cost effective, low maintenance, required no additional site utilities, and were aesthetically acceptable.
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