Santa Monica got about 10% more drinking water out of its Arcadia treatment plant without building a new treatment train. The city’s $77.5 million Olympic Well Field Restoration and Arcadia Water Treatment Plant Expansion took the Excellence in Sustainability award in ENR’s 2026 West Best Projects, published Oct. 26, for what the team calls the world’s first municipal-scale reverse osmosis retrofit of an operating conventional RO system with flow-reversal technology.
The goal is local supply. With the work done, Santa Monica can draw up to 70% of its drinking water locally. Walsh Construction Co. II was general contractor and Brown and Caldwell led design. ROTEC supplied the flow-reversal RO technology, and Trojan Technologies provided the UV and advanced oxidation equipment. The project wrapped up in August 2025.
Groundwater treatment for an aquifer industry fouled
Two new production wells pull from groundwater contaminated by historic industrial use. A new advanced treatment facility hits that water with ultraviolet light and hydrogen peroxide, then runs it through granular activated carbon. Raw water treatment capacity went to 13 million gallons a day from 10.
How the reverse osmosis retrofit raised recovery
The more interesting work happened on the existing skids. Flow reversal periodically switches the direction feed water moves through the membrane pressure vessels and rotates the membrane stages. Scale doesn’t get a chance to form, and no single membrane sits in the most concentrated water for long. The city piloted two high-recovery approaches side by side during design before choosing this one.
Recovery rose to better than 90% from 80%. That’s worth nearly 1,200 acre-feet a year. The construction scope was narrow: added valves, pumps and piping, with 56 valves per RO train. Specific energy use for the RO process fell to 0.287 kWh per cubic meter from 0.314, a 10% drop, so the plant now makes more water at lower energy intensity.
Operations got cheaper too. Reworked antiscalant selection and dosing stretched membrane run times enough to drop one clean-in-place cycle per skid per year, worth about $150,000 annually. Sulfuric acid spending fell by roughly $100,000 more. Higher recovery also leaves less concentrate to discharge.
Why water retrofits win on embodied carbon
Because this was a retrofit, it didn’t add the material use, land disturbance and embodied carbon that a conventional capacity project would have. Utilities rarely make that argument out loud. Agencies chasing local supply usually reach for new plants, like Fort Lauderdale’s Prospect Lake Clean Water Center or Hawaii’s Kalaeloa seawater desalination facility. Where there’s no existing RO plant to squeeze, building new may be the only route.
Where there is one, the question is how much treated water is going down the drain as concentrate. Santa Monica’s answer was nearly 1,200 acre-feet a year, recovered with valves and piping instead of a new building.