As part of the Clean Water Project, the Metropolitan District (MDC) of Hartford, CT is upgrading its combined sewer system by installing a 2+ mile-long storage tunnel to prevent raw sewage from overflowing into the Connecticut River. Each ventilation shaft and droptube along the tunnel will include odor control systems to manage foul air emissions during normal and storm conditions. 

The design required accurate prediction of foul air exhaust rates under two critical scenarios: 

• Buoyant Condition: Colder ambient air induces upward flow of warm foul air from the tunnel. 

• Storm Condition: Inflow of stormwater displaces foul air, forcing it out through ventilation shafts. 

Sizing odor control systems correctly depended on understanding these transient airflow patterns. 

Verdantas (formerly Alden Research Laboratories) developed two time-dependent Computational Fluid Dynamics (CFD) models using historical temperature and storm data for each droptube and ventilation shaft location. The simulations provided flow rate versus time for foul air exhausting or ambient air drawn in under both conditions: 

• Buoyancy Model: Simulated time-lapse effect of warm air rising and being displaced by colder ambient air. 

• Storm Model: Simulated extreme storm inflow displacing foul air and altering tunnel airflow patterns. 

The analysis predicted foul air exhaust rates at each vent for proper odor control system sizing, accounted for transient conditions over multiple hours during storm events, and ensured compliance with odor mitigation standards to minimize environmental impact. Time-dependent CFD modeling provided accurate exhaust rate profiles for each vent shaft, while the storm model offered additional insight into internal water flow patterns during extreme events. These results enabled precise sizing of odor control systems, ensuring effective odor mitigation and regulatory compliance.