Sewer Siphons And Odor Control: A Practical Approach

Siphons, sometimes called depressed sewers or sags, allow wastewater to flow through a pipe under low lying areas or obstructions such as rivers, utilities, or other obstructions, where flow by gravity at these locations is impossible. The flow through a siphon is driven by a difference in hydrostatic pressure without the need for pumping. A major drawback with siphons is foul odor created from the displaced air flow at upstream siphon structures. The evacuation of air occurs during the transition from gravity flow in the upstream wastewater main to pressure flow in the siphon.Siphons are commonly used in combination with an air jumper or an odor control design methodology, such as carbon filters or scrubbers. Air jumpers are air pipes used to prevent sewer headspace pressurization within the pipeline and capture odorous sewer gasses which help to maintain the sewer ventilation within the pressurized siphon.Air jumpers span the length of the siphon from box to box. Another method of odor control is the use of liquid phase or vapor phase treatment, which work to treat and control the escape of these gases and odors to the atmosphere.Although there have been a few research papers, guidelines, and “rules of thumb” published, there is not an abundance of industry information available on the design of siphons in combination with air jumpers. However, for air jumpers, there are practical concepts of fluid mechanics that need to be followed in order for the process to work. In other cases, liquid or vapor phase systems are used for odor control, or no odor control system is implemented at all. This paper discusses the various guidelines or “rules of thumb” and empirical data/criteria for air jumper design including fluid/air flow characteristics, minimum pipe size, velocities, and other assumptions, as well as the pros and cons of using air jumpers versus other odor control methods. Another focus of this paper is to provide analysis through case studies including the design and construction of siphons with air jumpers and odor control systems as well as the addition of air jumpers to existing siphons in the form of a retrofit replacement. Advantages and disadvantages of constructability, operations, and maintenance of the air jumper and odor control system designs will also be presented.