Grad student’s research could save water — and money — around the world

For most people, the wait for hot water at the showerhead is one of life’s mysterious little inconveniences. For doctoral student Toritseju Omaghomi, the delay between turning on the tap and hopping beneath the spray is a sign of a much bigger issue. The speed of hot water delivery depends largely on the sizes of pipes in a building—and most of those pipes, she says, are far too big for their job.  

The problem is essentially one of keeping up with the times. Building engineers determine how big pipes should be after estimating the peak water demand for a building. Thanks to high-efficiency fixtures such as low-flush toilets, low-flow fixtures and increased awareness of water conservation, most buildings use less water today than they did decades ago. But the way engineers estimate water demand, Omaghomi says, hasn’t substantially changed since first developed by a researcher named Roy Hunter in the 1940s. 

“What Hunter did was very clever and convenient,” she explains, but Hunter’s observations came from hotels during morning wake up-calls, when many people used water simultaneously. “Think halftime at the Super Bowl: everybody’s running to the restroom, similar to a business hotel in the morning,” Omaghomi says. “That does not happen in residential buildings, so peak demand really drops down.” 

Working from Hunter’s method, engineers often overestimate peak water demand, which leads them to install larger pipes for meeting that perceived need. And oversized pipes cause more problems than just a long wait for hot water. Builders spend more on plumbing than necessary, while consumers waste energy heating water that eventually cools off in the pipes and goes down the drain at the next hot water use. Big pipes even pose health hazards when they hold large volumes of warm, stagnant water—the perfect breeding ground for harmful opportunistic bacteria that cause Legionnaires’ disease and other infections.

Through her research, Omaghomi hopes to modernize the estimation of peak water demand. She bases her work not on a handful of antiquated hotel observations, but on a recent water-use survey of over 1,000 single-family homes from across the United States. This massive data set provided a representative sample of diverse water use patterns for analysis. Unlike that of hotel guests or football fans, the peak water use of people at home proved much harder to predict. “At 2 a.m., some home used the most water, and you’re wondering what’s happening there!” she laughs about the data. 

No matter the hour of the peak, Omaghomi’s next step was to determine the probability that a given fixture would be in use during that time. For example, if a building has only one showerhead, and it runs for an average of 15 minutes during the peak hour, its probability of use would be 0.25. After calculating this value for every type of fixture in a house, she randomly modeled thousands of water-use scenarios based on probable combinations of fixtures. From those results, she figured out the water supply needed to meet a house’s demand at least 99 percent of the time, the same reliability achieved using Hunter’s method.

Omaghomi’s presentation of “Water Distribution in Buildings Then and Now” at UC’s inaugural Three Minute Thesis (3MT®) competition took top honors. She admits that the 3MT required a change of pace from her previous presentations at scientific conferences, with less jargon and more concrete examples such as the wait for hot water. But Omaghomi says that attending the 3MT workshop held by the Graduate School prior to the competition improved her approach to communicating research. “For the first time my friends told me, ‘You’ve been presenting to us for a while, but we just now understood what you said!’” she jokes. 

Omaghomi believes her 3MT experience will help her share research with an even tougher crowd: building and plumbing engineers. Hunter’s method is a part of the Uniform Plumbing Code and International Plumbing Code used for construction projects around the world. Clear, accessible communication will be vital to making the case for an updated method.