Policy Analysis pubs.acs.org/est
Water Loss Control Using Pressure Management: Life-cycle Energy and Air Emission Effects Jennifer R. Stokes,§,* Arpad Horvath,† and Reinhard Sturm‡ §
NSF Engineering Research Center ReNUWIt, Department of Civil and Environmental and Engineering, , University of California, , Berkeley, CA 94720, United States † Department of Civil and Environmental Engineering, University of California, Berkeley, United States ‡ Water Systems Optimization, San Francisco, California, United States S Supporting Information *
ABSTRACT: Pressure management is one cost-effective and efficient strategy for controlling water distribution losses. This paper evaluates the life-cycle energy use and emissions for pressure management zones in Philadelphia, Pennsylvania, and Halifax, Nova Scotia. It compares water savings using fixedoutlet and flow-modulated pressure control to performance without pressure control, considering the embedded electricity and chemical consumption in the lost water, manufacture of pipe and fittings to repair breaks caused by excess pressure, and pressure management. The resulting energy and emissions savings are significant. The Philadelphia and Halifax utilities both avoid approximately 130 million liters in water losses annually using flow-modulated pressure management. The conserved energy was 780 GJ and 1900 GJ while avoided greenhouse gas emissions were 50 Mg and 170 Mg a year by Philadelphia and Halifax, respectively. The life-cycle financial and environmental performance of pressure management systems compares favorably to the traditional demand management strategy of installing low-flow toilets. The energy savings may also translate to cost-effective greenhouse gas emission reductions depending on the energy mix used, an important advantage in areas where water and energy are constrained and/or expensive and greenhouse gas emissions are regulated as in California, for example.
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INTRODUCTION North American water utilities, concerned about droughts, shortages, and climate change, are focusing increasingly on water conservation. Traditional conservation efforts emphasized consumer demand management rather than controlling losses from utility infrastructure.1 Concurrently, water distribution pipelines in the United States are deteriorating. The U.S. Environmental Protection Agency (EPA) estimates that rehabilitating drinking water infrastructure will cost $334.8 billion over 20 years, 60% due to transmission and distribution networks.2 Though half of all water providers are privately owned, public systems supply over 90% of the water delivered.3 Budget crises at all levels of government can cause pessimism about the potential for infrastructure rehabilitation. Deteriorating infrastructure is exacerbating supply side water losses. The American Society of Civil Engineers estimates that 7 billion liters of treated drinking water are lost from distribution systems daily.4 Similar issues exist globally; in the developing world, leaks can exceed 50%.5 Minimizing distribution losses improves energy efficiency.6 Less water must be extracted, treated, and transported to meet demand. Smaller throughput limits treatment plant capacity, pump size, and pipe diameters. Sometimes leaked water drains into sewers, resulting in additional treatment requiring more chemicals, energy, and money without beneficial use.7 If the water or wastewater infrastructure is nearing capacity, expansion, replacement, or new construction may be needed prematurely. © 2013 American Chemical Society
A 2002 paper on utility water loss control (WLC) programs and energy savings evaluated losses and energy consumption in a simplified pipe system under different leak conditions and found that the economic value of the lost water, including embedded energy, was more significant than the costs of additional energy consumption from operating a leaking distribution system.8 A more recent empirical study evaluated several water conservation pilot programs considering their embedded energy, including commercial conservation, low-flow toilets (LFT), customer auditing, managed landscapes, recycled water, and others.9 One pilot program evaluated comprehensive WLC programs, including benchmarking existing water losses, designing economically optimized strategies, implementing leak detection programs, and estimating savings from potential pressure management strategies in three Southern California utilities. These supply side WLC programs offered greater water and energy savings at a lower cost compared to the other alternatives. Demand management alternatives with lower capital costs exist (e.g., irrigation restrictions, water rate adjustments) but, unlike Special Issue: Design Options for More Sustainable Urban Water Environment Received: Revised: Accepted: Published: 10771
February 7, 2013 July 17, 2013 July 19, 2013 July 19, 2013 dx.doi.org/10.1021/es4006256 | Environ. Sci. Technol. 2013, 47, 10771−10780
Environmental Science & Technology
Policy Analysis
Studies have evaluated the optimal design and economic benefits of PMZs. Two studies presented hydraulic simulations to optimally place PRVs within a distribution system to control pressure and leaks. One considered the optimal opening adjustments.13 Another implemented a multiobjective optimization of PRVs, balancing leakage reduction with installation costs in an Italian water system.14 Other studies captured the economic benefits of implementing pressure management after optimizing PRV type, location, and settings. One study reported reduced losses with increasing pressure control: FO (7.7%), TM (8.1%), and FM (8.5%).15 They monetized the benefits using the difference between the water’s production cost and retail price. The benefits of time- and flow-modulated PRVs were 3% and 9% higher than fixed-outlet PRVs, respectively. The same authors analyzed rehabilitation and replacement needs while maintaining minimum pressure requirements and minimizing total cost (i.e., pipe maintenance, flowmeter, and PRV costs) and maximizing economic benefits, finding pressure management economically beneficial.16 In addition to saving energy and chemicals while avoiding or delaying system expansion, pressure management can decrease leaks and pipe breaks. Among the four WLC strategies, pressure management uniquely changes the value of unavoidable real losses (e.g., background losses, water lost before breaks are detected) in both directions. When all else is equal, higher pressure increases and lower pressure decreases leak rates for reported, unreported, and background leaks. The relationship between excessive and/or transient pressure and break frequency is an evolving research area. Break frequencies are affected by many other factors, including pipe materials, traffic load, pipe depth, pipe age, ground conductivity, ground movement, water, and ambient ground temperature, and installation quality. However, a study of 112 PMZs worldwide revealed a strong pressure and break connection.17,18 Researchers are seeking predictive relationships between system pressure and breaks using theoretical principles and empirical data.18−20 A recent paper recommended using historical maintenance data to identify the pressure-dependent breaks, excluding seasonal effects, and predict future break rates when pressure is reduced.21 The SI discusses these techniques applied to one case study. Three common critiques of pressure management concern impeded firefighting capacity, water quality degradation, and revenue loss. International implementations and North American pilot programs have shown that fire flow and water quality standards can be met within a PMZ without significantly affecting revenue.6,10,22
supply side WLC, these often reduce consumer satisfaction and utility revenues, affecting their long-term cost effectiveness. Through WLC, infrastructure, water, and energy concerns can be addressed concurrently by encouraging utilities to strategically repair and replace aging infrastructure when the life-cycle costs of intervention are lower than the value of water lost, optimize operations to limit losses before identifying leaks and implementing repairs, and to reduce inevitable background losses (e.g., seeping pipe joints not detectable with acoustical surveys). WLC generally falls into four strategies: pressure management, proactive leak detection, infrastructure management (i.e., rehabilitating or replacing infrastructure), and improved leak response (e.g., reducing response time and improving repair quality).10 Some countries proactively limit distribution leakage, notably England, Germany, Australia, and Singapore.10,11 In North America, WLC tends to be reactive. Utilities commonly fix pipe leaks when water bubbles to the surface and is reported by a customer, leading to long leak-run times before discovery, slower response times after discovery, increased losses, and higher costs. Current WLC practice is discussed further in the Supporting Information (SI; table and figure numbers preceded by “SI” are found there). This paper presents a life-cycle assessment (LCA) which quantified the energy and environmental benefits of implementing pressure management in two case study utilities by comparing leak volumes, embedded energy, chemicals use, and equipment needs for operating the system with no pressure control (NC), fixed-outlet (FO) control, and flow-modulated (FM) control.
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BACKGROUND ON PRESSURE MANAGEMENT Regulations restrict the lower limits of utilities’ operational pressures to avoid backflow events, intrusion, and insufficient fire flow. No mandated upper pressure limit exists, so utilities pay less attention to high pressure. The capacity for WLC from pressure management is potentially significant, especially in pumped systems. A recent study determined that pressure is the most important contributor to distribution system leakage, followed by infrastructure age.12 The pressure and leak relationship is discussed in the SI. A common first step in pressure management is creating district metered areas (DMA) by closing existing or installing additional valves to hydraulically isolate an area. Water flows are metered and demand monitored so that changes in real losses (i.e., volume of physical leaks of treated, pumped water) can be observed. (DMAs are further discussed in the SI.) A DMA can be converted to a pressure management zone (PMZ) by installing a pressure-reducing valve (PRV) at the inlet. Other means of limiting pressure, such as controlling pumping with a variablefrequency drive, are not explicitly evaluated herein but are addressed in the Discussion section. A fixed-outlet PRV lowers pressure entering the PMZ to a set point. Fixed-outlet PRVs are commonly used to maintain reasonable operating pressures in varied topography. In fixedoutlet applications, pipes tend to be overpressurized at lowdemand times.6 More automated and customizable control is available with PRVs modulated by time or flow. A timemodulated (TM) PRV has an internal timer set to change the outlet pressure using historical demand curves. Flow-modulated PRVs adjust pressure and flow within a preset range, increasing pressure when demand is high and reducing pressure when demand falls.
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LIFE-CYCLE ASSESSMENT OF PRESSURE MANAGEMENT Life-cycle assessment is a systematic, comprehensive method for quantifying the cradle-to-grave, or cradle-to-cradle, resource inputs to and outputs from a product, process, or system, including raw material extraction and processing, manufacturing or construction, transportation, storage, use, maintenance, and end-of-life treatment. Inputs typically include materials, energy, water, and land. Outputs include air, water, and land emissions, byproducts, and waste. Two established LCA methodologies are widely used, process-based LCA and economic input−output analysis-based LCA (EIO-LCA), which have been discussed in prior publications.23−29 The authors’ prior work on LCA of water systems discusses the development and use of the Water-Energy Sustainability Tool 10772
dx.doi.org/10.1021/es4006256 | Environ. Sci. Technol. 2013, 47, 10771−10780
Environmental Science & Technology
Policy Analysis
American PMZs that have monitored and documented their performance for an extended period, providing a unique opportunity to assess the life-cycle effects of pressure management. Unlike prior water LCAs, this study focuses on a small design change within a larger network. Pressure management requires a concrete chamber, one or more valves, a flowmeter, and a small amount of piping, electrical, and control equipment, marginal additions given the scale and costs of even small distribution networks. Similar design questions are frequently considered by water professionals without considering the environmental effects. Access to a tool like WESTWeb enables utilities to include life-cycle environmental implications in their decisions, big or small. This paper demonstrates how LCA can inform a real-world decision by describing how to frame the analysis and introducing a freely available tool that can be used for similar questions.
(WEST) to establish the environmental footprint of utilities, depending on water source and treatment technology.30−34 Subsequently, a simplified, streamlined version of WEST has been made available online.34 WESTWeb (http://west.berkeley. edu) focuses on water and wastewater utilities’ operations found to contribute most to the environmental effects in prior studies, namely energy use, chemical consumption, and production of pipeline materials, concrete, and certain process equipment. It uses a hybrid approach, combining aspects of both process-based LCA and EIO-LCA, similarly to WEST. In addition to the authors’ work, LCA has been used to analyze other urban water issues. Complete urban water systems, including water treatment and provision and wastewater processing35−38 and water treatment and distribution systems,39−49 have been assessed. Studies have focused on distribution systems, proposing predictive maintenance strategies50,51 or analyzing material selection.43 Others focused on desalination36,41,52−54 or disinfection processes.55 Water reuse systems were also analyzed.36,41,52,56 One study analyzed conservation programs.57
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CASE STUDIES Pressure management was evaluated using LCA for existing PMZs at the PWD and the HRWC. Table 1 summarizes utility and PMZ characteristics. Details were compiled for Philadelphia from references,10,58,59 for Halifax from references10,22,60−62 and utility personnel. Table 2 summarizes operational data for the three pressure control conditions. For both utilities, each pressure scenario
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OBJECTIVES OF STUDY Prior work identified extensive economic benefits of pressure management, but no study has attempted to quantify the lifecycle environmental effects. In this study, LCA was used to quantify the embedded energy, greenhouse gas (GHG) emissions, and other environmental impacts avoided by reducing water leaks and breaks with pressure management. The avoided pumping energy, chemical consumption, and system maintenance were compared to the effects of manufacturing piping, fittings, valves, concrete, control, and electrical equipment, and other materials needed to implement a PMZ. The boundaries used for the LCA are shown in Figure 1.
Table 1. Case Study Utility and PMZ Characteristicsa characteristicb
units
System population served customer connections mains length annual volume processed average zone pressure head, average average zone pressure head range average pipe agec marginal cost of productiond PMZ customer connections mains length pipe composition
pipe diameter average pipe age payback period using marginal coste date of pilot pressure testing
Figure 1. Boundaries of pressure management LCA.
PWD
HRWC
# #
1 500 000 >500 000
35 000
