In the summer of 1999, reports of odors were noted around vented structures along the Potomac Interceptor (PI), which conveys wastewater from Washington Dulles International Airport in Chantilly, Virginia along the Potomac River to the Potomac Pumping Station in Washington, DC. The extremely hot and dry summer of 1999 caused wastewater flows in the PI to be lower than normal. The low wastewater flows and warmer temperatures throughout the collection systems which discharge to the PI system resulted in reduced flow velocities, increased detention times, increased solids deposition, increased biological activity and increased odor potential. Odor was noticed by people living and recreating along the Potomac River and Potomac Interceptor right-of-way, and was reported to the District of Columbia Water and Sewer Authority (DCWASA) from the National Park Service.

DCWASA immediately committed to investigating the perceived odors along the PI. DCWASA and contributing jurisdictions to the PI and the Metropolitan Washington Council of Governments (MWCOG) to accelerate an an existing plan for assessing the hydraulic capacity and physical condition of the PI, including the evaluation of potential odors.

The odor remedy study was conducted in two phases to first collect and analyze field data, and then to complete a feasibility study to evaluate odor control technologies. During the data collection phase, sewer exhaust from more than 50 venting structures along the PI were sampled to obtain air flow velocity, percent oxygen, and hydrogen sulfide data. Wastewater samples were also collected at several locations to determine wastewater temperature, pH, dissolved oxygen levels, 5-day biochemical oxygen demand, and total and dissolved sulfides. Collected data were analyzed to identify trends throughout the interceptor system relating to odor potential. Conclusions were drawn to indicate areas with a high odor potential and the related causes.

A feasibility study was completed after the field data were collected and analyzed. Four categories of odor treatment technologies were examined in the feasibility study including passive odor controls, active odor controls, chemical odor controls, and biological odor controls. Passive odor controls evaluated included activated carbon filters, biological media filters, odor-neutralizing agents, and sealing vents to eliminate the exhausting of sewer air. Active odor control systems examined include forced-air carbon and biofilter systems, wet scrubbers and mist systems, and thermal oxidation. Chemical and biological controls evaluated included oxygen injection, nitrate addition, addition of oxidizers (hydrogen peroxide, potassium permanganate, ozone, iron salts), caustic "slugging" with sodium hydroxide, and the addition of biological enzymes or bacteria to control the sewer slime layer. Each category of odor control technologies were evaluated based on the technical feasibility of implementation, safety, effectiveness, and cost.

Because some sections of the PI parallel the Chesapeake & Ohio (C&O) Canal National Historic Park, implementation concerns included access issues, permitting, availability of utilities, and other site-specific considerations. Ultimately, it was determined that a combination of passive and active (forced-air) carbon systems would provide the most beneficial, low-impact and cost-effective long-term control of odors for the Potomac Interceptor sewer.

Please click on the link below to view a copy of the initial odor study report (you will need Adobe Acrobat Reader to view this file). The recommendations in the report have been further developed as part of the Long-Term Odor Control Plan.

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