Keeping It Under Vacuum: Vapor-Phase Odor Control For Nearly One Mile of Large Diameter Interceptor Sewer

PROJECT OBJECTIVES AND OVERVIEW
Is it possible to keep an entire mile of 27-36' diameter interceptor sewer and 30 manholes under negative pressure (vacuum) conditions at all times to prevent the escape of odorous air? How does one induce the vacuum, ventilate, capture and treat the odorous air from the collection system? What ventilation rate is required to maintain the vacuum conditions throughout the collection system? What is the effective zone of influence achievable? What are the odorous compounds and their design loadings for subsequent treatment? Can we apply similar approaches elsewhere? These are the key questions Fairfax County and Dewberry sought to answer in response to ongoing odor complaints at a major linear recreational park along Holmes Run (a major stream) near Alexandria, Virginia. The Y-shaped (three-branch) regional collection system traverses through the linear park which is visited by thousands of residents each year, so an effective odor control solution was deemed essential. Unlike most vapor-phase odor control applications which involve a single point-source that requires treatment, this location involves nearly a mile of interceptor sewer and 30 manholes. The subject paper presents the results of a comprehensive evaluation and full-scale pilot testing on the regional collection system performed in the Spring of 2021.
METHODOLOGY
The project approach included the following evaluation elements:
Wastewater flow monitoring to characterize diurnal variations in both flow and depth of flow in the various sewer segments
Sewer model development to predict performance under minimum, average and maximum wastewater flow conditions
Ventilation model development to evaluate pressure drop versus induced air flow rates through the regional collection system under minimum, average and maximum wastewater flow conditions
Computation Fluid Dynamics model development to confirm the air-water interface through various sewer segments
Determination of the theoretical zone of influence where negative pressures can be maintained
Design and fabrication of manhole test assemblies / air inlet assemblies for up to 14 manholes
Designing and procuring a full-scale pilot unit to ventilate the regional sewer system to induce vacuum conditions throughout
Full-scale pilot testing and gas sampling (continuous and grab)
Ambient monitoring of differential pressure and air flows at multiple manhole locations (without forced-air ventilation)
Monitoring of differential pressure and air flows at multiple manhole locations with forced-air ventilation at overall rates of 4,000 cfm, 6,000 cfm and 8,000 cfm
Determining the recommended ventilation rate and the actual zone of influence achievable
Technical evaluations of vapor-phase treatment options
Economic evaluations of vapor-phase treatment options
Recommending the overall vapor-phase, odor control solution and ventilation system configuration
Development of the design criteria for the recommended solution Manhole air inlet assemblies were temporarily installed at 13 manhole locations distributed throughout the mile long study area. The inlet assemblies were each equipped with a butterfly valve to regulate air inlet flow and instruments to continuously monitor differential pressure and air velocity. Air was withdrawn from two manifolded, centrally-located, adjacent manholes along two converging branches of the Y-shaped (three-branch) regional collection system at varying collection system ventilation rates of 4,000 cfm, 6,000 cfm and 8,000 cfm. This configuration allowed for induced air-flow through all three branches. A specially-fabricated, diesel-driven ventilator unit rated for up to 8,000 cfm at 16' of water column was used to allow for overall flow regulation by varying the speed (RPM) of the unit. The manholes were also monitored under ambient (unforced ventilation) conditions to assess the natural behavior of the collection system.
FINDINGS
At overall system ventilation rates exceeding 5,000 cfm (12.0 air changes per hour), nearly the entire regional collection system in the study area was consistently maintained under negative pressures. A ventilation rate of 7,000 cfm (16.8 air changes per hour) was required to consistently maintain a minimum desired vacuum level of -0.20 inches of water column in nearly all the manholes comprising the study area. Jumper ventilation lines were recommended across selected locations to extend the achievable zone of influence out further from the central withdrawal point so that some additional manholes in the study area could also be maintained under vacuum conditions.
Vapor-phase testing demonstrated that as the overall ventilation rate increased, the concentration of hydrogen sulfide (H2S) in each of two withdrawal manholes steadily decreased. For example, ambient levels (unventilated) in one of the two manholes were as high as 21 ppm and averaged 7.1 ppm, but were reduced to an average of 2.1 ppm at an overall system ventilation rate of 8,000 cfm. The total pounds per hour of H2S withdrawn from the collection system (mass loading) increased with increasing ventilation rate and appeared to reach a near-maximum level at 8,000 cfm suggesting that further increases in the ventilation rate would be of minimal benefit.
Continuous monitoring of H2S at the furthest downstream manhole in the study area demonstrated ambient (unventilated) concentrations of 0.23 to 2.0 ppm. These levels were consistently reduced to zero ppm during forced air ventilation testing, suggesting that the vast majority of H2S was effectively stripped out upstream via ventilation in the steeply sloped, upstream sewer sections. Essentially, the steeply sloped sewers coupled with effective ventilation function as a giant linear stripping column. Other odorous compounds beyond hydrogen sulfide detected through supplemental grab sampling included carbonyl sulfide, methyl mercaptan, and dimethyl sulfide. Several vapor-phase treatment technologies including wet scrubbing, biofiltration, biotower, and carbon adsorption were evaluated via a weighted criteria analysis. A potassium hydroxide-impregnated, carbon adsorption system was ultimately recommended considering factors such as removal efficiency, operator safety, accessibility, maintainability, constructability, noise abatement potential, aesthetic shielding potential, and overall costs.
RELEVANCE
The results demonstrate that it is possible to centrally ventilate and treat odors from collection systems on the order of one mile in length provided that sufficient free area exists in the collection system for unrestricted air flow (above the water surface). Multiple air inlets along the collection system are favored over fewer inlets so that inlet air is fed in a step-feed fashion and overall pressure drop can be minimized. The results demonstrate that with sufficient ventilation, negative pressure (vacuum) conditions can be consistently maintained throughout a regional collection system to prevent the release of odorous air. The collected odorous air may then be treated using conventional vapor-phase treatment technologies.