Learning from Hanoi to imagine the future of water distribution

In many countries, water distribution systems consist of large, highly pressurized pipe networks that require an excessive amount of energy and that are vulnerable to large-scale contamination. To imagine the future of water distribution, we can learn from Hanoi, Vietnam, where water is distributed at low pressures and most buildings are equipped with a basement tank, a rooftop tank, and separate water treatment processes, resulting in a system that consumes less energy and that is more resilient.

are generally self-sufficient for several days when the water supply is interrupted. As an example, in October 2019, one water treatment plant in Hanoi was contaminated, which in turn contaminated a portion of the water network, but most buildings were not disrupted because they had isolated themselves from the network in a timely manner and they had stored sufficient water for several days. Moreover, because buildings are equipped with basement tanks, water pressure in the system can be lower. In Hanoi, low-pressure zones have pressures around 10 meters (the minimum acceptable for fire safety), thus requiring less energy than the golden standard-as an example, in the US, average water pressure tends to be~30 meters (roughly to supply a five-story building). More generally, beyond the context of Hanoi, water pressure management is well studied and has been recognized to help reduce leakage and energy consumption 3 .
In addition, most buildings in Hanoi have another smaller water tank placed on roofs. Water from the basement tank is first pumped to the roof tank-the process is usually automatic but it could be programmed to only run when electricity demand is low. For a typical house, the basement tank has a volume of 3-5 cubic meters while the rooftop tank contains about 1-2 cubic meters of water; see a picture with typical rooftop tanks, Fig. 2. Because the tank is on the roof, gravity alone is used to supply water throughout a building, and water can therefore be supplied during power outages (until the rooftop tank is empty).
Overall, in Hanoi, water distribution consumes about 100 Wh m −3 for treatment and distribution (data from the Hanoi Water Limited Company) and about 130 Wh m −3 to pump water from the basement tank to the roof tank (for a four-story building; see Methods), for a total of 230 Wh m −3 (about two-thirds the US average).
Naturally, the Hanoi model is not without flaws. Large pressure differentials in water distribution systems can degrade water pipes more quickly as is common in IWS systems. Moreover, as a result of allowing lower water pressures or even shutting off the water distribution systems to part of the city (as it used to be), contaminated groundwater may enter the pipe network and contaminate the entire system. To partly tackle water quality issues, in Hanoi, many buildings are further equipped with separate treatment processes, located after the rooftop water tank (i.e., after the SWS system) to treat potable water on demand (often in the form of water filters). This strategy further ensures that water quality is controlled by the user. In fact, once it passes the water meter, water is owned by the property owner. As a result, the owner is responsible for the quality of water once it reaches the SWS system; and, if not maintained well, water quality can be affected in SWS systems due to longer water stagnation times, higher temperatures, and loss of disinfectant residuals 4 . One consequence of transferring some of the responsibility to building owners is that some financial burden is placed onto building owners themselves.
Overall, Hanoi has achieved a hybrid system that is both centralized and decentralized. The distribution of clear, lowpressurized water is centralized, similar to the golden standard, except that the process tends to consume less energy. Once water enters a building, it is generally stored in a basement tank, then pumped to a smaller rooftop tank, and finally treated one last time into potable water. In contrast to the collect, treat, distribute, consume model, Hanoi follows a collect, treat, distribute, store, treat, consume model. In the end, the Hanoi model is both more resilient while potentially requiring less energy to operate.
Ultimately, the legacy from the end of the nineteenth century of keeping enormous pipe networks pressurized with potable water is simply neither sustainable nor resilient. Water distribution as a whole should be revisited and better integrated with other infrastructure systems [5][6][7] . Any effort to transform the golden standard will require substantial learning and a dramatic sociotechnical transition, and while the Hanoi model is not perfect, it offers valuable insights and ideas to set us on the path toward the future of water distribution.

METHODS
Equation (1) is used to estimate the energy consumption E pump to pump water from a basement to a rooftop tank.
where ρ is the water density (i.e., 1000 kg m −3 ); g is the gravity acceleration (i.e., 9.81 m s −2 ); V is the pumped water volume; H is the geodetic lift; h f is  the total friction loss (i.e., for indoor pipe network, friction losses are estimated to make up 30% of the geodetic lift); η pump is the pump's efficiency (for indoor systems η pump is~30-60% depending on pump's age 8 ); η motor is the pump motor's efficiency (for small motors, with rated power under 2.2 kW, η motor is~75%). For a typical four-story residential house (H = 14 m), assuming a pump efficiency of 50%, the energy consumption to pump 1 m 3 of water from the basement to the roof is E pump ¼ 1;000 9:81 1 1:3 14 0:5 0:75 3600 ¼ 132:25 Wh, thus 130 Wh.

Reporting summary
Further information on research design is available in the Nature Research Reporting Summary linked to this article.

DATA AVAILABILITY
No datasets were generated or analyzed during the current study. One data point was collected from the Hanoi Water Limited Company (http://hawacom.vn/home/), as described in the following figshare metadata record 9 : https://doi.org/10.6084/m9. figshare.13160015.