Collection System Slopes Lead to Upset Neighbors

For years odor assessment and installed odor control technologies centered on wastewater treatment plants because that is where neighborhood complaints originated and focused. Collection systems odors were not very well understood since they suffered from being out of sight and out of mind, essentially underground mysteries, and were generally ignored. With the more recent emerging concern over asset management these underground assets have become a more integral part of the wastewater management process, and the condition of those assets has become a critical question that for so many years had gone unanswered. What was discovered was an underground network of neglected sewer pipes, manholes, lift stations, pump stations, diversion structures, surcharged pipes, inverted siphons, etc., that transported the wastewater of the communities through a network of pipes that supported the formation of nuisance odors that were also corrosive to the infrastructure. These systems may have been designed to endure for 50 to 75 or more years of service, but not all systems survive their design life cycle. Some are older, some are younger but their condition (level of corrosion) varies significantly, and is primarily based on the characteristics of the wastewater, the water age (Time it remains in the system between the home/source and the water resource recovery facility.), the materials of construction and the physical layout of the pipe network. Regarding that physical layout, 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. That odor release encompasses stripping from the wastewater into the headspace of the sewer, as well as escaping the confines of the sewer headspace into the ambient atmosphere where it can impact communities, properties and homes. This presentation focuses on a typical physical sewer layout that has been designed by engineers for years and is common to many collection systems: the intersection where a steep slope meets a much flatter slope. This physical layout condition 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 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 this physical layout, 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 problems for a downtown community - New York & the intersecting steep and flat slope sewer was followed and aggravated by an inverted siphon - Kentucky & a simple solution saved a lot of money - Kentucky & house traps 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, can case histories like these guide the engineers to design sewer layouts that anticipate potential odor and infrastructure corrosion conditions and include ways to address them.