Design and testing of a back-pressure regulating valve to reduce the water contamination risk in New Delhi

Abstract

PROBLEM: NEGATIVE SUPPLY PRESSURE Although 3.7 billion people worldwide have access to piped water on their premises, there are still significant quantity and quality concerns with their water [1]. Piped water systems in poorer regions of the world often operate intermittently. The Asian Development Bank’s study of 20 major cities in India found that they supplied water for an average of only 4.3 hours per day [2]. Whenever a water pipe is not positively pressured, contaminants can infiltrate through the holes and cracks in the pipe network and create significant quality concerns [3, 4]. As part of the MIT-Tata Center for Technology and Design the authors worked in collaboration with New Delhi’s water utility and other private water suppliers to define and address this problem. When such intermittently-pressurized systems are pressurized, consumers withdraw water as rapidly as possible. High flow rates increase frictional losses and decrease the system pressure. Lower system pressure means that water can no longer reach the rooftop storage tanks that many consumers use. To address this low-pressure, many consumers connect half- or one- horsepower booster pumps directly to their water supply pipe. When switched on, these booster pumps can create a suction pressure of up to eight psi. This suction pressure creates two harmful effects. First, where the water utility’s supply pressure is less than eight psi, it can induce negative pres- sure in the last and smallest pipe leading to the house as shown in Figure 1. Negative pressure can allow contaminants to infiltrate into this supply pipe, which water-supply engineers in New Delhi cite as the most frequent location for contamination to occur. Second, when one house uses a booster pump, the local pressure is reduced, which causes a reduction in the flow to neighbors’ houses. This in turn forces neighbors to install booster pumps and further exacerbates the booster pump problem.