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

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? What ventilation rate is appropriate? The results of sewer system hydraulic modeling, sewer system ventilation modeling, CFD modeling and full-scale testing demonstrate that it is possible to centrally ventilate and treat odors from collection systems up to 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 introduced in a step-feed fashion and overall pressure drop can be minimized. The results show that with sufficient ventilation, negative pressure (vacuum) conditions can be consistently maintained throughout a regional collection system to prevent the release of odorous air. A ventilation rate of 5,000 CFM corresponding to 12.0 air changes per hour was associated with stable vacuum conditions for nearly all manholes. A higher ventilation rate of 7,000 CFM corresponding to 16.8 air changes per hour was adopted for design to maintain vacuum conditions of at least 0.20' of water column and to allow more uniform flow regulation among inlet manholes. In some ways, the steeply sloped and ventilated collection system operates as a giant linear stripping column. The collected odorous air may then be treated using conventional vapor-phase treatment technologies. Design of the facilities is underway and features 13 air-inlet assemblies, jumper ventilation lines, a central vapor-phase carbon adsorption system with dispersion stack.