Delivering water to city dwellers can become far more efficient, according to Rice University researchers who said it should involve a healthy level of recycled wastewater.

“All the technologies needed to treat wastewater to drinking water quality are available,” said Qilin Li, an environmental engineer at Rice University and corresponding author of the study that appeared in Nature Sustainability. “The issue is that today, they’re still pretty expensive. So a very important part of the paper is to look at how cheap the technology needs to become in order for the whole thing to make sense financially and energy-wise.”

Using Houston as a model, researchers at Rice’s Brown School of Engineering developed a plan to recycle wastewater to make it drinkable once again and reduce the need for surface water (from rivers, reservoirs, or wells) by 28 per cent.

While the cost of energy needed for future advanced purification systems would be significant, the researchers said the savings realized by supplementing fresh water shipped from a distance with the direct potable reuse of municipal wastewater would more than make up for the expense.

“Another way to improve potable water would be to cut its travel time,” Li said. “Water delivered by a system with many distribution points would pick up fewer chemical and biological contaminants en route.” Houston, Li noted, already has well-distributed wastewater treatment. Making that water drinkable would facilitate shorter travel times to homes.

The model used by the researchers of the study showed there will always be a trade-off between the acquisition of potable water, the energy required to treat it, the cost of transporting it without affecting its quality, and attempts to find a reasonable balance between those factors. The study evaluated these conflicting objectives and exhaustively examined all possibilities to find systems that strike a balance.

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“Ultimately, we want to know what our next-generation water supply system should look like,” Li said. “How does the scale of the system affect distribution? Should it be one gigantic, centralized water source or several smaller distributed sources. In that case, how many sources should there be, how big of an area should each supply and where should they be located? These are all questions we are studying. A lot of people have talked about this, but very little quantitative work has been done to show the numbers.”

Li admitted Houston may not represent major municipal infrastructure systems because the city’s wastewater system is already highly distributed, but its water supply system is not. The challenge of having a highly centralized water supply was demonstrated by a dramatic 96-inch water main break in February 2020 that cut off much of the city’s supply.

“That was an extraordinary example, but there are many small leaks that go undetected underground that potentially allow contaminants into homes,” Li said.

The study only looked at direct potable reuse, which was a more economic option for established cities according to the mode. But Li said the best option for a new development—that is, building a distribution system for the first time—may be to have separate delivery of potable and non-potable water.

“That would be prohibitive cost-wise in a place like Houston, but it would be cheaper for a new community, where wastewater effluent can be minimally treated, not quite drinkable but sufficient for irrigation or flushing toilets,” Li said. “Though maybe it would be to Houston’s advantage to use detention ponds that already exist throughout the city to store stormwater and treat it for non-potable use.”

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