Overburdened treatment plants, increased treatment costs, surcharged lines and environmental pollution are the results of
infiltration. For years, efforts to reduce or correct infiltration and its associated problems have concentrated on rehabilitating
the mainlines, although manholes and other underground structures commonly account for 30 to 50 percent of a system's infiltration.
- Leaking manhole rings
- Cycle loading from groundwater
- Infiltration through loose mortar
- Leaks around stub outs
- Subsidence from traffic loading
- Infiltration through precast joints
- Cracks caused by shifting & expanding soil
- Drainage from Micro-Biological corrosion
- Hydrogen Sulfide formation
|This infiltration typically occurs at manhole rings, through loose mortar and precast joints and around mainline and stub out connections.
With the implementation of more frequent sophisticated flow-monitoring programs, there has been an increased awareness for the need to rehabilitate each
component of the sewer system. Graph 1 illustrates the average amounts of quantifiable I/I occurring in the various components of a sewer collection network,
as well as the typical cost associated with rehabilitating each of these components. As one can easily see, manhole rehabilitation offers the most cost
effective means of eliminating upwards of 30% of the total quantifiable I/I.
Using as little as 10% of the average rehabilitation budget, manhole rehabilitation can typically eliminate from 30% to 50% of your total I&I.
Subsidence from traffic loading, shifting and expanding soils, temperature variation and cyclic ground water loading seriously
weaken manholes and other sewer system structures. Over time, ground water will find its way through fatigue cracks and
weakened joints, leading to further deterioration of the structure. Fallen bricks and mortar can block the sewer flow, and
may eventually lead to collapse, seriously endangering the surface and surrounding environment.
Sulfide-rich effluent's, a warm, humid environment and long retention times create the perfect conditions for microbiologically
induced corrosion (MIC). MIC, a result of an acid-producing bacteria known as Thiobacillus, is the principal cause of corrosion
in a municipal sewer system. These microorganisms metabolize elemental sulfur oxidized from H2S sewer gas and produce sulfuric
acid as a waste product which then attacks the substrate. This sulfuric acid can quickly destroy ordinary concrete-based
materials in a municipal sewer system.