Wastewater infrastructure nationwide is experiencing the effects of age, population growth, and severe weather. For most municipalities, underground collection systems are a patchwork of intricately connected sanitary sewers, storm sewers, and combined sewers; new sections intersect with old, and the functionality of different pieces of the system can vary widely. Even for cities that implement substantial maintenance programs and have extensive knowledge of their particular system, it can often be difficult to pinpoint the source of an issue and to identify which issues should be at the top of the priority list.
Effective asset management is dependent on having the most up to date, accurate information about a sewer system, and it is also facilitates community support for any necessary municipal wastewater infrastructure projects. One of the most comprehensive techniques for obtaining this information is to work with a consultant to conduct a Sewer System Evaluation Survey (SSES), particularly one that integrates the use of Geographic Information System (GIS) software.
Identifying system needs
Wastewater collection and treatment assets represent a significant investment for communities of any size, and one of the major challenges for municipal officials is protecting that investment and maintaining the value of the existing infrastructure. Effective maintenance leads to more stable user rates and fewer SSOs, CSOs, and structural failures. However, large-scale maintenance programs are costly, and cities operate within the confines of limited budgets and time. Implementing an SSES program can help cities understand exactly where to designate their resources.
An SSES uses a variety of different methods to provide a snapshot of a municipality’s sewer system, which provides a basis for identifying and prioritizing areas that need repair or rehabilitation. For most, addressing every structural flaw or removing all sources of infiltration/inflow (I/I) isn’t feasible due to the time, labor, and cost required to do so, but using a characteristic overview will make it possible to locate major weaknesses in the system. Once problem areas have been identified, a systematic plan for addressing the most pressing needs can be developed.
Taking a closer look
One challenge of maintaining wastewater infrastructure is that the vast majority of it is underground, which can easily contribute to an “out of sight, out of mind” mentality when there are other, more visible issues for municipal officials to attend to. The techniques used in an SSES allow for an up-close look that would be impossible otherwise.
Flow isolation can identify whether groundwater (infiltration) is getting into the sewer system. It is possible to quantify this excess flow by plugging, or isolating, a sewer line. This is executed during a time of ordinarily low flow, such as late at night or very early in the morning – when most of the flow observed can be contributed to infiltration. These sections of sewer will then be noted and marked for further investigation.
Closed Circuit Television (CCTV) inspection is performed with a remotely operated camera, typically mounted on a robotic device that drives up and down sewer pipes. It is highly useful for assessing conditions underground by locating structural and O&M related defects such as obstructions like roots, debris, or grease. CCTV inspection provides municipalities with a very detailed understanding of their buried infrastructure and serves as a primary aid to rehabilitation in most SSES programs.
Manhole inspections are a common part of the SSES process that utilize either surface inspection or confined space entry to inspect manholes for things like infiltration or structural defects. While a condition assessment of the manhole is being completed, it is also possible to use modern GPS units to spatially locate manholes with subfoot accuracies. This is an important step for GIS model development.
Building inspections are included as part of an SSES to identify sources of inflow. Potential sources include, roof downspouts, driveway drains, or sump pumps that might be connected to a sewer system. These items can contribute significant flow during intense rainfall events, overloading a treatment plant’s capacity as well as increasing the cost of treatment.
Smoke testing is a method in which nontoxic smoke is injected into separated sanitary manholes. In a correctly functioning system, the smoke will come up through nearby separated sanitary manhole covers. , However, if there are illicit connections to the sanitary sewer, such as catch basins or roof drains, smoke will billow out of the structure. This is a quick and efficient method for visually identifying cross connections.
Rainfall simulations (or Dye testing/Dye Flooding) involve injecting dyed water into a storm sewer or combined sewer to emulate what happens during a rainfall event. This technique is very helpful in confirming connections found during smoke testing or building inspections and also for finding cross connections between storm drains and a separated sanitary sewer.
GIS is a powerful software that maintains a database of both geographic and attribute information. Consultants can build a model of a municipality’s collection system using a combination of record drawings and field visits to help a city organize and understand its complex system. . By creating a digital model of the sewer system, each asset can be given a unique ID and updated in real time with accurate information. Sometimes, an aerial flyover is used and manipulated to place the existing infrastructure digitally into the correct geographic location. Using the GIS model as a geographic database to store the findings of the SSES, it’s possible to understand exactly where problems in the system are located.
Working with a consultant to conduct an SSES is an investment in risk management. Using a combination of the various inspection methods, municipalities can more capably and accurately develop preventive maintenance programs; put their information systems, such as GIS, to good use; and address system overflows. Most immediately, though, knowing the structural condition of your underground assets simply makes it easier to avoid emergencies, prioritize rehabilitation projects, and plan for the future.