A piped water supply and distribution system is intermittent when water continuity is for less than 24 hours a day or not on all days of the week. [1] [2] During this continuity defining factors are water pressure and equity. [3] [4] At least 45 countries have intermittent water supply (IWS) systems. [5] It is contrasted with a continuous or "24/7" water supply, the service standard. [6] [7] No system is intentionally designed to be intermittent, but they may become that way because of system overexpansion, leakage and other factors. [8] [9] As of 2022, there was no feasible method for modelling IWS, including no computer-aided tools. [1] Contamination issues can be associated with an intermittent water distribution system. [10] Global public health impact includes millions of cases of infections and diarrhea, and 1560 deaths annually. [11]
A continuous supply is not practical in all situations. [3] In the short term, an IWS may have some benefits. [12] These may include addressing demand with a limited supply in a more economical manner. [13] An intermittent supply may be temporary (e.g., when water reserves are low) or permanent (e.g., where the piped system cannot sustain a continuous supply). [6] Associated factors resulting from an intermittent supply include water extraction by users at the same time, resulting in low pressure and a possible higher peak demand. [14]
About 41 percent of urban water supply systems around the world are intermittent; a critical misutilization considering the fact that these systems are designed to run continuously (i.e., 24 hours a day). [15] [16] About 1.3 billion people have a piped supply that is intermittent, including large populations in Africa, Asia, and Latin America and the Caribbean. [1] [14] This does not include those who do not get piped water at all, about 2.7 billion people. [1] Countries with intermittent supply in some areas and continuous supply in others include India [17] and South Africa. [18] In India, various cities are at various stages of constructing 24/7 supply systems, such as Chandigarh, [19] Delhi, [20] Shimla, [21] and Coimbatore. [22] In Cambodia, Phnom Penh increased coverage from 25% to 85% and duration from 10 to 24 hours a day between 1993 and 2004. [23]
Installation of storage and pumps at residences may offset the intermittency of the water supply. [6] Roof tanks are a common feature in countries where the water supply is intermittent. [24] In Jordan, most houses have one or more ground or roof tanks. An intermittent supply can be supplemented with other non-piped sources such as packaged drinking and cooking water bought from local shops or delivered to the house. [25]
The primary cause of intermittent water supply is the extension of distribution systems beyond their hydraulic capacity to provide service to more customers.