Locating illicit connections in storm water sewers using fiber-optic distributed temperature sensing

doi:10.1016/j.watres.2009.08.020

Water Research

Volume 43, Issue 20, December 2009, Pages 5187-5197

https://doi.org/10.1016/j.watres.2009.08.020 Get rights and content

Abstract

A newly developed technique using distributed temperature sensing (DTS) has been developed to find illicit household sewage connections to storm water systems in the Netherlands. DTS allows for the accurate measurement of temperature along a fiber-optic cable, with high spatial (2 m) and temporal (30 s) resolution. We inserted a fiber-optic cable of 1300 m in two storm water drains. At certain locations, significant temperature differences with an intermittent character were measured, indicating inflow of water that was not storm water. In all cases, we found that foul water from households or companies entered the storm water system through an illicit sewage connection. The method of using temperature differences for illicit connection detection in storm water networks is discussed. The technique of using fiber-optic cables for distributed temperature sensing is explained in detail. The DTS method is a reliable, inexpensive and practically feasible method to detect illicit connections to storm water systems, which does not require access to private property.

Introduction

A separate sewer system consists of two parallel sewer pipe networks. One network is the sanitary sewer system, which transports household or industrial wastewater to a sewage treatment plant. The second network is the storm water system, which delivers clean rain or storm water to the surface water system without treatment. Ever since the introduction of separate sewer systems, system managers have been faced with illicit connections. These illicit connections are generally unintended hook-ups that either connect foul water outlets from residential or industrial premises to the storm water system or storm water outlets to the foul water system. For detecting storm water outlets that are connected to the sanitary sewer system, a reliable, inexpensive, and practically feasible detection method exists, namely the release of smoke in the sewer. Clearly, illicit connections that link foul water outlets to the storm water system are much more problematic, as these result in the release of untreated sewage in the surface water system. Yet, no straightforward method exists for detection of foul water outlets connected to storm water systems. All currently applied searching techniques come with disadvantages that reduce reliability, increase costs or make the method difficult to implement. Illicit connections contribute significantly to the pollution of receiving water bodies and their detection can be a demanding task.

A newly developed searching technique for illicit connection detection in storm water systems uses distributed temperature sensing (DTS). Introducing standard fiber-optic cables in storm water sewers allows monitoring of in-sewer temperatures with high spatial and temporal resolutions. Using the differences in temperature characteristics of storm water sewers and foul water discharges, it is possible to determine the time and location of most illicit foul water connections.

The objective of this paper is to introduce the DTS method for the purpose of illicit connection detection in storm water sewers. It aims at examining the possibilities the method offers as well as assessing the quality of the method in terms of reliability, implementation costs and practical feasibility. In the remainder of this paper we will first introduce illicit connections on occurrence, effects on receiving waters and current detection techniques. Next, the concept of using differences in temperature characteristics for the detection of illicit connections is discussed. The technique of using fiber-optic cables for distributed temperature sensing is explained. The DTS-technique has been tested in two municipalities in the Netherlands under different circumstances. The results of these experiments are presented and discussed.

Section snippets

Definition of an illicit connection

Separate sewer systems have been widely introduced since the early 1970s to circumvent the drawbacks of combined sewer systems. Since then, it has become clear that separate sewer systems also come with disadvantages. A major drawback is the occurrence of illicit connections. For storm water systems the term ‘illicit connection’ has many meanings in regulations, literature and practice. The strictest definitions consider an illicit connection a connection with a discharge that is not entirely

Temperature differences and variations

For a reliable, inexpensive and practically feasible method to detect illicit connections, changes in water temperature over time and space can be considered. Temperature monitoring for the detection of extraneous flows has previously been used in other in-sewer applications (e.g. Wirahadikusumah et al., 1998). Monitoring in-sewer temperatures allows searching for anomalous temperatures and temperature variations that point to a disruption of usual conditions inside the storm water system. For

Korendijk

Fig. 3 presents the results of DTS-monitoring in the Korendijk catchment area. The horizontal axis represents the length along the cable from x = 0 m at the HALO instrument to x = 1264 m at the end of the cable; the vertical axis represents a time-span of 21 h: Thursday October 9th 18h00 through Friday October 10th 15h00 2008. The figure consists of 632 × 2520 pixels that each represent a measured temperature value (1264 m divided by the 2 m spatial resolution gives 632 columns and 21 h divided by the 30 s

Predominant in-sewer temperatures and temperature variations

For both the Korendijk and the Groningen cases, mean in-sewer temperatures are presented in Fig. 5. The data constitute in-sewer temperatures for three consecutive dry-weather days at two locations along the studied sewer sections for which no anomalies in temperature readings have been recorded (Korendijk x = 500 m; Groningen x = 880 m). For these April results, Korendijk in-sewer air temperatures show a diurnal variation with values ranging between approximately 9 ^°C and 10 ^°C. This diurnal cycle can

Conclusions

Distributed temperature sensing with fiber-optic cables is a powerful tool to search for illicit connections in storm water systems. Its near-continuous temperature monitoring in both time and space allows recording any discharge of water with temperatures or temperature variations that differ from ‘normal’ storm water system temperature characteristics. Normal characteristics for this paper's case-studies constitute temperature values between 5 ^°C and 20 ^°C showing only small daily and larger

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Locating illicit connections in storm water sewers using fiber-optic distributed temperature sensing

Abstract

Introduction

Section snippets

Definition of an illicit connection

Temperature differences and variations

Korendijk

Predominant in-sewer temperatures and temperature variations

Conclusions

Science of the Total Environment

Water Science and Technology

Water Research

Automation in Construction

Water usage in the Netherlands

H2O

Probabilistic modeling of sewer deterioration using inspection data

Water Science and Technology

Simulation of the wastewater temperature in sewers with TEMPEST

Water Science and Technology