A detailed odor study and remedy plan was conducted for the Potomac Interceptor (PI) sanitary sewer system with a long-term goal of mitigating odor while maintaining the integrity of the concrete pipes. The odor study consisted of field data collection and odor assessments to define the scope of the odor along the PI system. Several options were considered for the abatement of odors including passive controls, active (forced-air) treatment systems, and chemical additives. Passive odor controls are being used as a temporary interim measure to reduce odors in public areas during the development and implementation of the long-term odor abatement program. A combination of passive and active controls was selected for long-term control of odors exhausting from several areas along the PI system. A field pilot study was then conducted to measure the effects of forced-air ventilation on airflows in the PI system. The environmental permitting component of the project is mainly complete, as described more below. Detailed final design is 95% complete pending architectural detail approvals and construction is anticipated to begin in 2010. Click on the links below for more information about the Environmental Assessment, public outreach, and the project timeline.

The long-term odor abatement program plan includes a combination of passive and active (forced-air) carbon treatment units for the control and removal of odorous compounds in the exhausting air from the Potomac Interceptor (PI). The goal of the long-term odor abatement program is to utilize a combination of active blower units, sealed vents and intake-only vents with passive carbon filters to effectively control and mitigate odor problems along a significant portion of the PI system. The picture thumbnails below show diagrams of the active blower units, sealed vents and intake-only vents with passive carbon filters for treating exhaust. Click on the thumbnails to view a larger image.

Factors evaluated for the design of the active blower units included diurnal wastewater flow patterns and corresponding flow depths, wastewater velocities in various segments of the sewer based on the hydraulic grades, "sewer drag" or wastewater flow friction effects on sewer airspace, and average concentrations of liquid-phase and vapor-phase hydrogen sulfide and other odorous compounds. The primary component of the design was to determine the airflow velocities and the volume of air movement in different parts of the sewer system under ideal (i.e., steady-state) conditions. This analysis required the calculation of wastewater velocities for many pipe segments where the pipe slope and/or pipe diameter changes. Areas of the PI where high exhaust rates were noted, public complaints were received, or where significant corrosion was noted provided a basis in determining the preliminary locations of the blower units.

The field pilot test was conducted using a 5,000 cubic feet per minute (cfm) blower unit at three different areas along the PI to supplement the air withdrawal calculations developed for the project. The vacuum blower was operated for a minimum period of 24-hours at each location. Differential air pressure data between the sewer atmosphere and the ambient atmosphere was logged before and during the pilot test at several upstream and downstream locations. Field pressure data were than used to account for pipe size and pipe slope changes, as well as the effect of air flows from service connections in the areas evaluated. Overall, the pilot testing yielded results which provided a relative comparison to the air volumes calculated assuming steady-state conditions. Please click on the thumbnails below for larger pictures taken during the pilot test.

Based on the evaluation of the factors previously described and results from the blower pilot test, the preliminary design indicates that seven blower units should be placed at various locations along the PI system. The total number of blower units includes a provision for the implementation of one active blower unit in the furthest downstream section of the sewer at a future date, pending anticipated changes in wastewater flows to the sewer segment in that area. The thumbnail below shows the planned locations for blower units along the PI main trunk and the anticipated areas of sewer air influence.

As indicated in the profile view (thumbnail below), the 12-mile sewer segment along the C&O Canal National Historic Park and the Clara Barton Memorial Parkway will include the design and implementation of four active blower treatment units in conjunction with multiple sealed vents, and several designated air intake-only vents. The four active blower units will withdraw and treat odorous compounds in the sewer air at a combined rate of approximately 35,000 cfm. The thumbnail below provides a schematic illustration that indicates how a series of active blower treatment units can effectively control ventilation within this segment of the PI system. This design is expected to provide complete odor control along this entire sewer segment while maintaining the adequate transfer of exhausting sewer air and intaking fresh air, which is crucial in prolonging the effective life of the concrete sewer pipes.

 
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