The implementation of water conservation measures may allow a city to extend its water supply and thereby delay the need for additional water sources. Water conservation measures are divided into two classifications: those that increase the supply (supply side) and those that reduce demand (demand side). Examples of each type are listed below:
- Supply Side Conservation Measures
- Increase production of existing sources (such as wells)
- Reduce water loss with leak detection and repair
- Demand Side Conservation Measures
- Conservation rate structure
- Secondary water for outdoor use (irrigation)
- Public education campaign
- Retrofit programs
Supply side conservation measures are closely associated with proper management of the water system. On the supply side these measures can include: testing and calibration of the supply source meters, tracking of mainline flushing and new construction line flushing, large customer meter test and repair programs, large meter right-sizing programs, calibration of SCADA instruments, increases in pipe repair or replacement, residential meter change-out programs, water audits and leak detection.
This article will focus on the leak detection efforts a city can employ to reduce the amount of water lost to leakage and therefore potentially reduce the need to increase the amount of supply needed through expansion of the water plant or addition of new wells. Both of these will be far more costly than implementing a proactive leakage control program to aid in reducing non-revenue water loss.
Organizations such as the EPA Office of Water, the American Water Works Association (Water Loss Control and Water Conservation committees) and the Alliance for Water Efficiency all promote water auditing and leak detection programs as effective supply side conservation measures. The effects of drought and climate change are making news every day as are the increases in large diameter water main breaks and the damage caused by them. A proactive leakage control program can show your customers that you are doing your part to effectively manage the water system.
A proactive leakage reduction program involves having the right tools in your toolbox. First and foremost is to establish the volume of water loss that is attributed to leakage. This begins with a top-down or paper water audit. First you must establish the true volume of water that is produced compared to the volume of water consumed by the customer. This will establish the beginning metered ratio. This figure will not identify if the losses are due to leakage, metering error, operational use of water or theft, it is just a starting point.
Using this figure as a percentage has been discouraged for many years, but it is still widely stated in numerous reports when describing water loss. The volume of water not initially accounted for is the most important figure. The cost to produce and pump the water to the customer is the second most important figure. Production costs vary considerably across water systems in the United States. Knowing the value of the water loss allows the establishment of the ?Economical Level of Leakage,? or that level where the return on the effort to find and fix the leakage costs more than the water lost.
Tools of the Trade
Leak detection is a craft that must be learned. Instruments used to locate leaks have become more sophisticated and sensitive in the past decade, but it takes constant use to become proficient at using them so ?dry holes? are not a common occurrence. The tool a city chooses depends on the type of survey they intend to complete. There are acoustic sounding survey tools, sound loggers, a two-channel leak noise correlator, multi-point cross correlating pods with three, six, eight or 12 pods, and free-swimming acoustic and tethered hydrophones.
Acoustic Sounding Survey
Acoustic sounding instruments are the most common leak detection tool utilized by leakage technicians. These instruments amplify the sound signals generated by a leak escaping from a pressurized pipe. These range from simple sounders to more sophisticated ones that have different filter ranges. The ranges can be applied depending on the pipe material. Some that can store different noise levels and display these so comparisons can be made to aid the operator in identifying if they are moving closer to or farther away from a leak sound. Many also include a ground microphone for use in pinpointing a leak.
Leak Correlation
While acoustic sounding instruments can find many leaks, especially hydrant and valve leaks, more challenging water main leaks require the use of a leak correlator to pinpoint the location of the leak sound. Leak correlation instruments come in different configurations and sensor combinations. They are normally equipped with two accelerometers. Newer models have three, which aid in establishing the true velocity of the sound. Accelerometers are surface-mounted sensors that measure the vibrations emitted from the leaks as they escape from the pipe. These leak signals are amplified and digitized. Then software, either built into the correlation unit or supplied for use on a computer, performs the cross correlation using Fast Fourier Transforms (FFT) analysis. FFT provides an advanced filtering of the signal and coherence checks of
the signals.
Some of the current correlation units can also deploy hydrophones instead of accelerometers. Hydrophones sense the vibration through the water column and pipe wall and are especially useful on non-ferrous pipe and large diameter transmission mains. Leak location on large diameter transmission mains and non-ferrous plastic pipe has always been problematic due to either, long distances between access points on large diameter transmission mains or the low frequency of the noise signals on non ferrous and PVC pipe. These newer technologies are overcoming the historical problems and assisting leakage technicians in evaluating those larger water mains before failures occur, which tend to cause more damage than just a small hole in the street. Advances in signal processing, filter selection and the ability to store the recorded signals offsite for further analysis are aiding in reducing the number of ?dry holes? and reducing the cost for a city to locate and repair main breaks.
Correlating Loggers
Another advancement in leak correlation in the past few years has been the implementation of correlating loggers. These instruments are different than regular two- or three-sensor correlators in that three, six, eight or 12 loggers can be deployed at one time across a large section of the distribution system. These instruments then cross-correlate between each other to locate leak noise between many loggers.
Advances in the software used to analyze leak signals have also improved greatly in the past few years due to the processing power available in current laptop and desktop computers. Older correlators had the leak signal processing built into the receiving unit but newer units rely on the processing power of computer processors. The ability to add combinations of different pipe material and varying filters to specific sections of recorded leak signals has greatly improved the accuracy of leak detection.
Conservation starts at home, but this statement would normally be applied to supply side conservation. If we look at home as the supplier of the water to the citizens of a city and the city is your home, then the statement fits. Efficient management of the water system from the source through the transmission and distribution system to the customer is the goal of every municipal employee. Having the right tools in the toolbox assists these employees in being as conservation minded as they want their customer to be. Each has a duty to do their part to utilize this limited resource as efficiently as possible.
Jim Fisher is Senior Project Manager for ADS Environmental Services. He has 38 years of experience in the water and wastewater industry and is currently working with the Pitometer Group of ADS to combine traditional water service methodology with modern instrumentation and customer focused solutions.
