An economic valuation model for alternative pressure management schemes in water distribution networks

Highlights

• According to beneficiaries, the benefits arising from implementing PM are classified into direct and indirect categories.

• A field-based economic evaluation framework proposed for achieving benefits by changing the FO-PRV to TM-PRV and FM-PRV.

• ALC activities, carbon emissions, property damages, and energy reduction are introduced to the economic evaluation framework.

• The computed Benefit-Cost Ratios indicated that FM-PRV was the most efficient pressure reduction.

Abstract

Pressure management (PM) is commonly used in water distribution networks (WDNs) to provide a wide range of benefits. This study presents an economic evaluation framework to support the decision-making process relating to alternative PM schemes. The methodology allows for the assessment the principal direct and indirect benefits and costs associated with using pressure-reducing valves (PRVs). The methodology is applied to a district metered area in a WDN in Mashhad, Iran, by changing the existing fixed–outlet (FO-PRV) to time-based (TM-PRV) and flow-based modulation (FM-PRV). The results indicated that FM-PRV is the most beneficial scheme for the studied case. The importance of estimating direct and indirect benefits is highlighted. The presented methodology is essential to persuade water utility decision-makers to recognize the economic feasibility and significant benefits of implementing PM schemes and justify the associated investment.

Introduction

Water pressure is an essential factor in operating water distribution networks (WDNs). Most of WDNs are designed to safely deliver water to all consumers at some acceptable level of pressure. To ensure safe and reliable water delivery, a minimum service pressure should be provided at all points in the network throughout the day (Hamilton and McKenzie, 2014; AWWA 2016). In striving to attain this minimum level of service, most WDNs tend to operate at pressures significantly higher than required and often experience excess pressure, especially during off-peak periods. The existence of excessive pressure can significantly exacerbate the flow rate from active leaks. Moreover, high pressures during low-demand periods may cause more frequent pipe bursts and leaks (Thornton and Lambert 2007; AWWA 2016; Ghorbanian et al., 2015). Therefore, controlling pressure in WDNs is a concern and challenging task for water utilities worldwide.

Pressure management (PM) is often defined as the practice of managing system pressure to strive for ensuring an optimum level of service to consumers while reducing unnecessary excess pressures and eliminating transient and large pressure fluctuations (Thornton and Lambert 2005; GIZ 2011; Martínez-Codina et al., 2015). The most common and effectively PM approaches in WDNs are 1) pump controls, especially using variable-frequency drives, 2) implementation of controlled pressure management areas, 3) automatic valves for pressure regulation, and 4) using microturbines and pump as turbine systems (Vicente et al., 2015; Samora et al., 2016; Venturini et al., 2017; Sinagra et al., 2017). In practice, the use of pressure-reducing valves (PRVs) is the most common approach to reduce excessive pressure, which of increasingly being used by water utilities (Fanner et al., 2007; Vicente et al., 2015; AWWA 2016).

A wide range of benefits can be achieved by PM in WDNs. The three of the most common benefits perused by water utilities worldwide are reductions in water leakage, pipe burst frequency, and water consumption (Thornton and Lambert 2005; Lambert and Fantozzi 2010; Vicente et al., 2015). First, the PM is a cost-effective tool for leakage control, affecting all leakage components (i.e., reported leakages or bursts, unreported leakages, and background leakages) (EU 2015; AWWA 2016).

Furthermore, it has been widely recognized that PM yields a decrease in the frequency of pipe breaks (Lambert 2001; Thornton and Lambert 2005, 2007; Girard and Stewart 2007; Lambert et al. 2013; Ghorbanian et al., 2015; Martínez-Codina et al., 2016).

Lastly, few studies have investigated the influence of PM on customer water consumption (Fantozzi and Lambert 2007; Lambert and Taylor 2010; Garmendia et al., 2018). Three primary types of modulation controlling PRVs are: 1) fixed outlet pressure, 2) time-based pressure, and 3) flow-based pressure (GIZ 2011). The most common practice used by water utilities throughout the world is the fixed outlet (FO) PRV control (Hamilton and McKenzie, 2014; Vicente et al., 2015). Over the last two decades, more sophisticated hydraulic and electronic controls by new devices and control algorithms are emerging in practice; these are often referred to as advanced pressure control (APC) methods. The most-reported methods of APC involve using time-based (TM), flow-based (FM), and remote node-based (RNM) modulation (GIZ 2011; Vicente et al., 2015).

The economic valuation and the suitability of PM in a particular system can be assessed by undertaking an economic analysis that accurately predicts the potential benefits and costs associated with PM (Malm et al., 2015). The economic analysis is central to developing a decision support tool, which can be applied to determine the cost-effectiveness and ultimately justify the investment decisions for implementing PM schemes (Girard and Stewart 2007; Mutikanga et al. 2011). The lack of decision support is one of the main reasons why PM schemes have not had wide application in most developing countries (Mutikanga et al. 2012). Economic analysis can also help to establish the economic level of pressure reduction and the associated leakage level, after which further reduction in leakage provide no additional economic benefit (Pearson and Trow 2005; Fanner et al., 2007; Thornton, Sturm, and Kunkel 2008; Moslehi et al., 2020).

Several studies are associated with the economics of leakage reduction activities, including active leakage control, district metering and network rehabilitation and renewal (Ahopelto and Vahala, (2020); Haider et al., 2019; Malm et al., 2020; Islam and Babel, 2013; Nafi and Brans, 2019; Moslehi et al., 2020). Few studies focused on cost-benefit analysis (CBA) of PM to compute the potential direct and indirect benefits of implementing PM schemes. Girad and Stwart (2007) examined the benefits of pressure and leakage management (PLM) strategies in the Gold Coast, Australia. However, only the financial benefits of leakage reduction and corresponding savings due to the reduction in pipe breaks were estimated. Awad, Kapelan, and Savić (2008) presented a methodology based on valuation models for evaluating several principal benefits associated with the PRV-based PM in WDNs. They found that the reduction in burst frequency is the most significant benefit (more significant than the leakage reduction benefit). Moreover, the obtained value of the net benefit from pressure-dependent consumption was found to be negative, which means that the revenue losses for water utility due to reducing water consumption outweighs the benefit obtained from reducing pressure-dependent consumption.

Gomes et al. (2011) proposed a methodology that uses a head-driven network simulation model, along with pressure-leakage and pressure-consumption relationships, to evaluate the benefits achieved by PM in WDNs. The results of the developed methodology for two District Metered Areas (DMAs) showed that PM could lead to significant economic savings in terms of water loss reduction. Gonelas and Kanakoudis (2015) analysed the economic benefits and revenue losses caused due to the reduction in operating pressure of the network in Kozani city, Greece. The reductions in water leakage and revenue losses due to reduced pressure-dependent water consumption were estimated. Moreover, the direct and indirect potential benefits of reducing new breaks frequency were estimated for all PM interventions. Creaco and Walski, 2017 presented an economic analysis of pressure control solutions, including PRVs and real-time control (RTC) valves. The benefits of pressure control due to the leakage reduction and the attenuation of pipe bursts were calculated by decreasing variable O&M costs and pipe burst costs.

Therefore, the literature review on economic analysis of the implementation PM schemes reveals that the majority of previous studies relied on the conventional analysis of three direct benefits (reduced water leakage, burst frequency, and water consumption). However, there was no obvious inclusion of other direct and indirect benefits, such as reduced ALC activities, carbon emissions, property damages, and energy reduction, to carry out a comprehensive economic evaluation.

The main objective of this research is to propose an economic evaluation framework and field data-based methodology for assessing the economic feasibility of PM schemes in WDNs. This framework uses valuation models to assess direct and indirect benefits and costs associated with PRV-based PM schemes. Moreover, new valuation models such as reduced ALC activities, carbon emissions, property damages, and energy reduction are introduced and added to the economic evaluation framework. The proposed framework is applied to a large DMA in Mashhad (Iran), by means of changing the fixed pressure valve to both TM-PRV and FM-PRV at the inlet of the DMA. The results demonstrate that other direct and indirect PM-related benefits, in addition to the conventional ones may have a significant influence on the economic feasibility of PM schemes.