Pipe Inspection System Challenges and Technologies

Manholes form a crucial part of wastewater collection systems. The end goal of collection system personnel is to treat all wastewater while eliminating any sewer overflows. All activities related to collection system maintenance and rehabilitation stems from data collected on the condition of the system. From data culled from field inspections and reports, asset ratings and projections are made. Projects can then be planned and work prioritized. From these plans of action; maintenance, cleaning, and rehabilitation work on the collection system is then performed.

One of the major causes of sewer overflows is the occurrence of infiltration and inflow (I/I). The term I/I is described as the occurrence of groundwater and storm water entering the sewer system. Aside from causing pollutants to enter waterways, harming the ecosystem and posing a public health risk to humans, each instance of I/I is also extremely costly. For example, if a treatment plant were to reduce treated flow by 100,000 gallons per day (gpd), that treatment plant would save approximately $50,000 every year[1]. The cleanup costs of overflow events is also substantial in terms of the cost to remove wastewater from streets and basements, litigation and insurance claims, as well as the substantial state/provincial and federal fines associated with these events.

Typical Problems in Manholes

The problems manholes face can be broken down into seven general types: leaking manhole rings and covers, traffic loads, ground water loads, infiltration, hydrogen sulfide corrosion, runoff and inflow, and alteration by third-party utilities Leaking manhole rings and covers: a damaged seal between the manhole cover and the frame will be a source of inflow into the collection system. Occasionally covers can become loose in fitting over time due to improper design, and the inspector must take note of whether the manhole cover fits the frame properly.[4] More likely, loose-fitting covers may be due to deterioration of the cover itself, as these components of the manhole are subject to cracking, and corrosion.

Traffic loads: manholes on busy streets, particularly those frequently traversed by heavy commercial vehicles, are particularly prone to defects from bearing traffic loads. The primary areas affected are the frame and cover section and the manholes chimney. As dynamic weight bears down on the manhole over time, the frame section of the manhole will shift and may sink below grade. This can affect how the cover sits on the frame and can create avenues for inflow from storm water runoff. The chimney can also be pressed into the cone or barrel section of the manhole and this can create problems such as joint displacement or more serious structural defects.

Ground water loads: this is an often overlooked challenge to manholes, but is becoming more of a priority as collection system managers attempt to tackle infiltration. Because water from the ground is constantly trying to enter the collections system, when a break does occur, the water pressure at the point of infiltration can be intense. Manholes are no different than pipes in this respect. Generally speaking, the deeper the manhole, the more susceptible it is to groundwater pressure.

Infiltration: an important issue for sure, there are several key places in the manhole where infiltration can occur. In brick manholes, any spot where the mortar has deteriorated or individual bricks have given way can be a prime spot for infiltration. On concrete manholes, the joints connecting segments of the cone and barrel are susceptible to infiltration, particularly if the structure has shifted and the joints have offset. Any type of connection from a pipe into the manhole, particularly break-in connections made after the manholes installation need to be checked for infiltration. In concrete manholes, cracks and fractures can allow infiltration.

Hydrogen sulfide corrosion: in concrete manholes and sewers, corrosion is the prime cause of structural deterioration in 3 out of 4 damaged assets.[5] Decomposing human waste gives off sulfuric acid, and when oxidized, becomes hydrogen sulfide (H2S). Theobacillus bacteria thrive on H2S and are made worse by hydraulic problems in the collection system, heat, and turbulence that can release festering pockets of H2S.[6] The result is corrosion on the surface of the concrete pipe or manhole.

Runoff and inflow: manholes must be checked as sources of runoff and inflow. Older manhole covers typically contain many pick-holes or vent-holes, which allows storm water from the street to easily seep into the collection system. Converting all manhole covers to a design with fewer holes or with no holes and bolted access can have a major effect on limiting runoff and inflow.

Alteration by third-party utilities: this is becoming more and more serious of a problem, particularly in older, crowded cities and due in part to the proliferation of high-speed Internet services. It is not uncommon to find other utility lines crossing with manholes and pipes in the collection system. If the utility line is a live gas pipe, this can be very dangerous. Also electrical lines that have penetrated the inside of a manhole can pose a unique risk of explosion as the cables deteriorate and their insulation erodes, giving off flammable gas and building up pressure inside the manhole. When an arc flash occurs, the built-up gases are ignited and an explosion can result[8]. Regardless of a fire hazard or explosion, each instance of alteration of the manhole after installation poses risks of becoming points of infiltration, while undermining the manholes structural integrity.