Critical Review pubs.acs.org/est
Intermittent Water Supply: Prevalence, Practice, and Microbial Water Quality Emily Kumpel†,‡ and Kara L. Nelson*,† †
Civil and Environmental Engineering, University of California, Berkeley, California 94720, United States The Aquaya Institute, Nairobi, Kenya
‡
S Supporting Information *
ABSTRACT: Intermittent water supplies (IWS), in which water is provided through pipes for only limited durations, serve at least 300 million people around the world. However, providing water intermittently can compromise water quality in the distribution system. In IWS systems, the pipes do not supply water for periods of time, supply periods are shortened, and pipes experience regular flow restarting and draining. These unique behaviors affect distribution system water quality in ways that are different than during normal operations in continuous water supplies (CWS). A better understanding of the influence of IWS on mechanisms causing contamination can help lead to incremental steps that protect water quality and minimize health risks. This review examines the status and nature of IWS practices throughout the world, the evidence of the effect of IWS on water quality, and how the typical contexts in which IWS systems often existlow-income countries with under-resourced utilities and inadequate sanitation infrastructurecan exacerbate mechanisms causing contamination. We then highlight knowledge gaps for further research to improve our understanding of water quality in IWS.
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INTRODUCTION Hundreds of millions of people in low- and middle-income countries receive water for drinking and domestic purposes through piped networks for only limited durations in a practice known as intermittent water supply (IWS). 1−4 While maintaining continuous positive pressure in piped distribution systems is an accepted standard for protecting water from external contamination, IWS systems are regularly without pressure. The number of people receiving water intermittently is likely to increase as climate change and rapid urbanization further stress resources for providing drinking water. The complex biological, physical, and chemical mechanisms that can degrade water quality during transmission from a source to consumer taps have been well studied in continuous water supplies (CWS).5−8 However, during IWS, pipes experience regular flow restarting, loss of pressure after shortened supply periods, and draining; these processes influence the mechanisms affecting water quality throughout the distribution system. Currently, international agencies and governments promote conversion to CWS or improving supply continuity, which while desirable, is a challenge for resource-constrained settings.9,10 A better understanding of the influence of IWS on mechanisms causing contamination can help lead to incremental steps that protect water quality and minimize health risks. Generalizing the nature of IWS is a challenge: the practice varies in different parts of the world, ranging from © 2015 American Chemical Society
piped systems that supply water for all but a few hours of every day to those that supply water for only a few hours per week. Supply durations can vary within a city and between different seasons. Despite its widespread practice, research on IWS and its effects on water quality is limited, as is quantification of the number of people served by IWS systems around the world. This paper reviews the status of IWS in the world, describes the nature of IWS, and reviews the evidence to date of its effect on water quality. We also discuss how the typical context in which IWS existslow-income countries with under-resourced utilities and inadequate sanitation infrastructurecan exacerbate mechanisms causing contamination. We end this review by identifying knowledge gaps and suggesting a research agenda for improving our understanding of water quality in IWS.
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THE NATURE OF INTERMITTENT WATER SUPPLY To our knowledge, no centralized urban piped water systems have been intentionally designed to provide intermittent supply; instead, a common scenario is that systems initially provided continuous supply but their ability to meet consumer demand throughout the system was limited by unsustainable changes, such as an increase in water loss, expansion of the Received: Revised: Accepted: Published: 542
August 16, 2015 December 13, 2015 December 15, 2015 December 15, 2015 DOI: 10.1021/acs.est.5b03973 Environ. Sci. Technol. 2016, 50, 542−553
Critical Review
Environmental Science & Technology
Figure 1. Average hours of water supply across all reporting utilities in a country for the latest year data were available using data from IBNET (www. ib-net.org)3 and country shape files (thematicmapping.org).
network, or a decrease in water resources (e.g., drought). Reasons for systems not providing water continuously are varied and have not been systematically studied.11 Totsuka et al.12 suggested that IWS is a method of rationing not only water quantity, but also of rationing economic, social, and technical inputs (e.g., power supply, chemicals for water treatment, meters). Strategies for designing and operating IWS systems have been proposed to overcome some of the inherent problems resulting from IWS, such as inequitable distribution and unreliable delivery timings, though no systems have been built specifically with these designs.12−17 Consumers who use water from IWS often develop coping strategies to overcome the inadequate volumes, poor quality, or untimely delivery of water from IWS systems, such as supplementing with alternative water sources or storing, transporting, pumping, or treating the water delivered through IWS systems.18−23 Because IWS often affects the quantity of water consumers have access to, there can be significant effects on health, livelihood, and well-being.24 However, these topics are not covered further in this review. Prevalence and Trends. Data on the prevalence of IWS are scattered, though there is evidence that average hours of supply can be low (Figure 1). According to one estimate from the World Health Organization (WHO), one-third of the population with piped water supply in Latin America and Africa and more than half in Asia receive water intermittently.4 Using data collected by the International Benchmarking Network (IBNET) through the World Bank Water and Sanitation Program, we estimate that at least 309 million people are supplied by utilities that provide water intermittently (Table 1).3 However, there are many more utilities and countries that do not report to this database that likely have IWS: for example, this database only includes data from 32 Indian utilities representing 64 million people. In a household survey in India, only 7% of respondents with piped water reported having 24 h of supply per day, with average hours per state ranging from 1 to 12 h (n = 19,112).25 It is therefore likely that most urban households with improved supply in India (representing 370 million people), many of whom have access to piped water, have intermittent supply.25,26 IWS encompasses a broad spectrum of practices with varying water delivery durations and patterns. South Asia has the lowest average hours of supply per day (7.2 h), followed by Europe and Central Asia (13.0 h), Sub-Saharan Africa (12.8 h), Latin America, North America, and the Caribbean (16.0 h), and then East Asia and the Pacific (16.7 h) (Table 1). However, there
Table 1. Number of Countries and Utilities Reporting Intermittent Supply out of the Total Number of Countries or Utilities Reporting Supply Durations (Countries Are Considered As Having IWS if They Have at Least One Utility with
