Air Dispersion Model as part of Odor Mitigation Optimization: DC Water - Potomac Interceptor

The Potomac Interceptor (PI) is one of the longest sewer systems that conveys flow from the Washington DC suburbs to the Blue Plains Advanced Wastewater Treatment Plant. It conveys approximately 20% of inflow to Blue Plains with an average flow of 55 mgd and peak flow of 150 mgd. This interceptor system starts from Dulles International Airport and flows cross-country until it nears the Potomac River. From there, it runs nearly parallel to the Potomac River and enters the Maryland/District boundary. The main trunk of the Potomac Interceptor is approximately 50-miles long. The Potomac Interceptor Service Area is so large that flow can take up to four days to reach Blue Plains. This long travel and time of sewage retention in the sewer system causes a strong rotten egg smell generated by H2S. It also causes a rotten cabbage smell generated by DMS, a Protein produced inside the sewer system. DC Water has built six odor control facilities-two in Virginia, two in Maryland, and one in DC-designed to remove foul air from the sewer system by passing it through activated carbon and permanganate reactors, releasing nearly odorless air to the atmosphere. These facilities are effective in removing air and creating negative pressure in the entire sewer system greatly reducing foul air releases in neighborhoods. However, the treated air dispersion at the odor control facilities is not ideally designed to direct air through their exhausts in a direction away from adjacent neighborhoods, parking lots, and restaurants. DC Water has developed several options to optimize air dispersion at the odor control facilities and further reduce the impact of odor in PI service areas. One of the options is to model the air movement and, based on the result, to determine the exhaust velocity by adjusting the diameter of the exhaust pipes at the odor control facilities. Another outcome would be to re-orient the exhaust angle in conjunction with the air dispersion model results to minimize impacts to adjacent neighborhoods. Air Dispersion Model DC Water performed an odor dispersion modeling analysis to determine odor impacts from the six odor control facilities along the PI to the surrounding communities. This analysis uses the EPA's latest standard for atmospheric dispersion modeling, AERMOD (v. 18081). Model Selection The AERMOD model (v. 18081) was used with options as recommended in the EPA's Guideline on Air Quality Models (EPA, 2005). The following supporting preprocessing programs for AERMOD were used:

* BPIP-Prime (v. 04274)

* AERMAP (v. 18081) This model is recommended for short-range (< 50 km) dispersion from the source. The model incorporates the Plume Rise Model Enhancement (PRIME) algorithm for modeling building downwash. AERMOD was run with the following options:

* Regulatory default options

* RURAL dispersion option