Cheap solar makes the energy for reuse and desalination cheap. Energy was never the hard part. Permitting, brine, and construction time are. Here is the honest engineering reality.
Many hope that falling solar power costs will make manufacturing new water from salty sources a cheap and abundant reality. While lower energy costs are a welcome development, from an engineering perspective, energy was never the primary barrier to large-scale desalination and reuse. The dream of millions of acre-feet of new water is slowed by a far more complex and time-consuming reality.
The true binding constraints are multi-year permitting processes, the immense challenge of brine disposal, and the sheer time it takes to build these massive infrastructure projects. This page offers an honest assessment of the different types of manufactured water, their realistic potential, and the engineering challenges that define their slow pace. This is not a story of what is impossible, but a clear-eyed look at what is realistically achievable in the near term.
There are four real ways to add or free water with energy. None of them is fast, and for each the wall is not the power bill.
Takes highly treated municipal wastewater and purifies it to drinking water standards to recharge groundwater basins. [6, 7]
Treats salty groundwater found in inland basins to make it usable for drinking or agriculture.
Treats saline agricultural runoff to recover fresh water and reduce salt loading in rivers or soils. [4]
Removes salt from ocean water through reverse osmosis, typically exchanging the new supply for reduced draws on imported sources.
For any inland desalination project, the single biggest challenge is answering the question: where does the salt go? Unlike coastal plants that can, with extensive permitting, discharge brine to the vast ocean, inland facilities have no easy options. The concentrated brine, or reject water, cannot simply be dumped. Disposal methods are limited and expensive. Deep-well injection is only possible in specific geological formations and carries risks of contaminating groundwater. [18] Evaporation ponds require huge amounts of land in sunny, arid climates and still leave behind a solid salt waste that needs disposal. [10]
These disposal methods can account for up to half the total cost of an inland desalination project. [10] Technologies for Zero-Liquid Discharge (ZLD) exist but are prohibitively expensive for large-scale water supply. This is why inland brine disposal is considered a show-stopper. Coastal plants trade this problem for another. They face a wall of regulatory hurdles related to building and operating ocean intakes and outfalls, with a permitting process that can take a decade or more to navigate due to concerns over marine life. [24, 27]
The vision of 1.5 to 3.0 million acre-feet of manufactured water per year is a worthy long-term goal, but it is not a near-term solution. Achieving that scale would require building the equivalent of 10 to 20 Orange County Groundwater Replenishment Systems. [6, 7] Given that each of these megaprojects takes a decade or more to bring online, a more honest engineering forecast places this target at least 15 to 20 years in the future, and likely longer.
A credible, near-term number for new manufactured water by 2035 is far more modest. We can realistically expect an increase on the order of 250,000 to 350,000 acre-feet per year. This growth will be dominated by large-scale potable reuse projects that are already in advanced planning, like the Metropolitan Water District's Pure Water Southern California, which aims to produce nearly 155,000 acre-feet annually. [11, 16] These projects are the most feasible because they are expanding on existing wastewater infrastructure. This is a multi-decade build-out, not a quick fix for drought.