Concepts

Tools of the Trade: Safeguarding Your Sewer SystemIt’s not uncommon to hear about a sewer backup in the news after a heavy rain. As our wastewater infrastructure ages and populations continue to grow, municipalities are increasingly faced with infrastructure shortcomings. There could be a number of reasons why sewage backs up into a basement or a manhole overflows, spilling raw sewage into a neighborhood stream. One of the most common is extraneous storm water and groundwater, which engineers refer to as infiltration and inflow (I/I).

Infiltration describes water that seeps into the sewers through physical defects in the

manholes, sewer mains, or laterals. It can occur during dry weather when the sewer is below the groundwater table, or as a result of a rainfall event. Inflow describes water that gets into the sewers through a direct connection with part of the storm water system, such as a catch basin, yard drain, or eaves trough. Inflow is usually absent in dry weather, but after a significant rainfall, it finds its way into the sewers and creates much higher flow than normal. Sometimes I/I combine and overwhelm a system. This may lead to water backing up into basements or being discharged into state waters, which is classified as a sanitary sewer overflow. This creates considerable

unexpected costs to the state and municipality, and a mess no one wants to deal with.

Locating excess water sources and estimating a particular rainfall’s effects can present a challenge. Solutions vary widely, from installing larger sewers and treatment facilities

to finding the source and eliminating it. Data gathered using various tools provides the information necessary to develop cost-effective, short- and long-term solutions specific to each community. These tools include flow monitoring, hydraulic modeling, field investigations, and the strategic use of geographic information system (GIS) data.

Flow Monitoring Installing flow monitors in strategic locations within the sewer system can help determine if there is a significant I/I problem and begin to identify areas that are contributing the most significant amount of I/I. Flow monitoring also provides the data necessary to calibrate a hydraulic model of the system.

Hydraulic Model Flow monitoring is a valuable method of acquiring sewer system flow data; however, it only provides information while the monitors are installed. A properly calibrated hydraulic model can project flows in any storm event and, consequently, test potential solutions for their effect on the sewer system. For example, would increasing the sewer’s size eliminate basement flooding or a sewer overflow?

Fishbeck, Thompson, Carr & Huber, Inc.

July 2012

onceptsC

Roof Drain Connec�on

UncappedCleanout

Connected Founda�on

Drain

Storm Cross-Connec�on

Drainage Intake

Inflow Source

Faulty Manhole Cover or Frame

Faulty Lateral

Connec�onCracked or Broken Pipe

Deteriorated Manhole

Broken House Lateral

Infiltra�on Source

Root Intrusion Into Lateral

SANITARYSEWERMAIN

STORMSEWER

Field Investigation When an area is suspected of having potentially high I/I, field investigation techniques can provide a positive means of identifying the specific sources. A number of techniques can be employed to identify various sources. These include smoke testing (filling a sewer with odorless, non-staining smoke and looking for where it escapes the system), dye testing (adding dye to water at a suspected source to see if it appears in the sewer), storm cross-connection testing (filling a storm sewer section with dye water to see if it appears in the sanitary sewer), or television inspection (inserting a camera in the sewer mains to document their condition).

GIS Data If a community has a GIS populated with system data, it can be utilized to isolate portions of the system that have characteristics pointing to potential I/I problems. These characteristics could include pipe age or material, or sections of sewer in close proximity to high groundwater conditions or water bodies. The GIS data can also serve as the backbone for developing a hydraulic model.

These tools can provide a community the information it needs to develop a holistic answer to its I/I problem. Merely identifying and fixing specific problems does not ensure the rate payers’ money is spent in the most cost-effective manner. Alternative solutions can be tested, and their effect on the collection system can be examined and compared to other solutions and their associated costs. Solutions can vary widely from system to system and are specific to the problem’s characteristics. Sometimes it might make sense to rehabilitate or replace sewer system components, such as pipes or manholes. Other instances might lead to constructing new facilities, such as retention basins or treatment facilities for overflows. These results should be compared to determine the best possible solution for each problem.

www.ftch.com/concepts

Funding I/I Studies and CorrectionsMany funding sources are available and vary from state to state. For example, Michigan has provided grant funding through the State Revolving Fund (SRF) program. S2 Grants, as they are called, pay for 90 percent of the cost of putting together an SRF project plan. These plans typically require all the steps identified here and can be quite economical. FTC&H has prepared several project plans through this program. For more information on how the SRF program works, click here…

For more information regarding I/I data gathering, please contact:

Rick Miller, P.E. at (513) 247-8562, rcmiller@ftch.com

Jeff Brown, P.E. at (517) 887-4016, jjbrown@ftch.com