May 2011, Vol. 23, No.5

Problem Solvers

Monitoring sewers with distributed-temperature measurements

Problem: Failing to differentiate sanitary and stormwater pipes leads to untreated wastewater discharge.
Solution: Fiber-optic distributed-temperature sensing enables identification of illegal hookups.

Separated sanitary and stormwater sewers have been the preferred solution for urban development since the 1970s. Advantages of separation include overflow prevention, reduced volume for treatment, and more-consistent flows to treatment plants.

But these positive effects are jeopardized when separation is degraded. For example, even a small number of users with sanitary sewers hooked up to storm sewers can significantly load surface water systems with untreated wastewater. If 1% of households in the United States and Europe are incorrectly connected to stormwater sewers, rather than to sanitary sewers, this creates an annual load similar to overflows in a combined collection system.

Detecting improper sewer connections often is hindered by unknown problems and needed access to private property. But using a method that distinguishes temperature differences between sanitary and stormwater flows shows promise for avoiding these problems.

Illegal or incorrect hookups often are unintentional, occurring when a plumber or property owner does not realize that separate lines exist or connects pipes to the wrong system. For example, in the Netherlands, pipe-color conventions are a problem, because in two-thirds of municipalities red/brown pipe indicates sanitary and grey pipe indicates stormwater; but in the remaining areas, this color-coding scheme is reversed. There also are cases in which small municipalities have merged, and these opposing conventions are used within the same district.

Separation also is degraded when groundwater infiltrates into sanitary sewers. This can occur where water tables are high and collection lines have cracked due to age or damage. Infiltration increases the volume and variability of flow that wastewater treatment plants must handle. Infiltration also poses a problem for combined systems, since it increases flows that must be treated.


Existing monitoring techniques

Several techniques exist to detect incorrect hookups and infiltration, but each has limitations. A smoke machine can be placed in the sewer to identify smoke-escape locations. But this technique requires access to property and only works to detect stormwater pipes without the S-bends, or traps, found on sanitary sewers.

Camera inspections show hookups to a main line but not which pipes and devices are connected to the other ends of the inlets. A camera also is unlikely to observe an occasional or intermittent discharge and has difficulty identifying subtle defects that could be infiltration sources. Cameras can look for plant-root intrusion and resulting cracks in pipes, but the information reported depends on current flow conditions when the inspection was conducted, which may not be representative flow information.

Flushing dyes or milk, a numbered rubber marble, or a floating track-and-trace chip down a toilet is another option. However, this requires a storm drain that is not submerged, access to property, a nearby manhole, and — at least — minimal water flows in drains under dry circumstances.

The sonic-measurement techniques require a sound source with two different frequencies in the sanitary and stormwater drain. A sensor listens at the S-bends of the water devices to the dominant signal of the two. Measurements identify which sewer connection is present, but access to property is required, and fixtures and appliances may have to be examined individually.


Fiber-optic system tested in the Netherlands

Sanitary sewer systems were being illegally connected to the storm sewer in the municipality of Korendijk, located south of Rotterdam in the Netherlands. When considering all the measurement techniques available, the team investigating connections decided to use a fiber-optic distributed-temperature-sensing (DTS) system to detect connections and infiltration. The team was led by Olivier Hoes, a researcher at Delft University of Technology (The Netherlands) and at SelkerMetrics (Portland, Ore.), a measurement and analysis company specializing in DTS.

With the DTS system, when a fiber-optic cable is attached to specialized equipment, it reveals temperature along its entire length. A laser sends light down the glass fiber, and a small amount of light interacts with the glass and is scattered. The scattered light can reach a sensor at the end of the fiber, and the frequencies and time measurements reveal temperature at each location.

In the municipality, 1250 m (4100 ft) of fiber-optic cable was placed in the storm sewer and was able to monitor temperature every meter at intervals of 30 seconds with a precision of about 0.1°C.

The data collected revealed six locations where water temperature was occasionally warmer than 16°C, which indicated a sanitary sewer connection. These locations were identified, inspected, and verified to be sanitary connections. The municipality moved the identified connections from the storm sewer to the sanitary sewer.

The DTS system identified occasional and intermittent releases along long sections of pipe during an extended period without having to inspect multiple properties.


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