Steep Slopes, Odors, and Corrosion Flow Together

Steep Slopes, Odors and Corrosion Flow Together Richard J. Pope, P.E., BCEE Vice President - Hazen and Sawyer For over 100+ years sewer designs have maximized gravity in pipes/conduits to convey wastewater to the treatment facility. Unfortunately, land contours and obstructions require that pumps be used to lift and/or convey the wastewater vertically through force mains to reach a point where the wastewater can again run by gravity. This process is repeated as many times as it takes until reaching the treatment facility. The point where the force main ends and the wastewater flow turns back to gravity is commonly at a high point in relationship to the surrounding landscape to again take advantage of gravity. Sometimes the slope of this force main discharge is steep enough to create a negative air pressure in the pipe headspace, as the drag from the downward movement of the wastewater sucks air in. In many cases this force main discharge connects with another downstream sewer/interceptor line that has a flatter slope. The pipe intersection of a steep slope with a flatter slope, is known to create a hydraulic jump as the faster moving wastewater from the steeper slope pipe tries to overrun the slower moving wastewater. This causes a 'jump like' action in the flatter slope pipe. These features associated with force mains have been recognized and understood since the initial designs of collection systems. But it has taken decades for the engineering community to observe and understand the odor release and corrosion potential of these features. For too long our sewers have suffered from being out of sight and out of mind, and generally ignored. The recent heightened focus on wastewater facility and collection system asset condition and management, helped to open our collective industry eyes on the potentially devastating impact that these features can impart. Regarding these physical layout features, it is not clear that engineers have always anticipated the ramifications of their designs and how the physical layout of the sewer pipe network can exacerbate both odor formation and release. Odor release encompasses stripping from the wastewater into the headspace of the sewer where it travels through laterals and house connections, as well as escaping the confines of the sewer headspace into the ambient atmosphere where it can impact communities, properties and homes. This paper focuses on a typical physical sewer layout that has been designed by engineers for years and is common to many collection systems: the fast-moving wastewater flow of a steep slope pipe and the intersection where it meets a flatter slope pipe. These physical layout features occur quite often, particularly at the discharge end of a force main where it breaks to a gravity line. Our discussion is not intended to direct engineers to avoid these situations, that is likely impossible given local topography issues, but to recognize these transition zones for what they are, understand how the wastewater and airspace within the sewer will react and protect against their potential impacts. A few of the key issues with the steep slope and transition zone where steep and flat slopes intersect include: Steep slope sections of pipe create a negative air pressure along that line as the wastewater drags the headspace air with it as it accelerates downward. That vacuum can create a positive impact as it draws outside air into the system to replace the air it is dragging downstream. It can create a negative impact if the vacuum is strong enough to drain the water out of lateral and house traps that are in place to keep sewer gases out of upstream lines/homes. Hydraulic jumps occur at the transition zone, which means an area of high turbulence. Turbulence is directly related to stripping odors from the wastewater. The intersection of fast-moving water meeting slow moving water also creates a positive air pressure shock wave that radiates out and away in both directions from the transition zone. These are real problems facing many communities across the country. Four different case histories will chronicle the impact that these physical layout features, steep slope and intersecting steep and flat slope sewer pipes, had on the surrounding communities and the steps that were taken to address it. The four case histories represent: California creating nuisance odor problems for a downtown community New York intersecting steep and flat slope sewer odor release was followed and aggravated by an inverted siphon Kentucky a simple solution to odors released from intersecting steep and flat sewers saved a lot of money, capital and operating Kentucky house traps connected to a steep slope sewer were emptied, and odors were found in the homes. A background will be presented along with the approach taken and the solutions implemented for each of these case histories. In the bigger picture, practical examples like these should remind engineers that their designs need to consider the potential odor and infrastructure corrosion conditions that they may cause and include ways to address them.