Carlsbad, California, USA
#00211
Case study of
#00196 Large-scale desalination sized as drought insurance
Implementer
Poseidon Water with San Diego County Water Authority
Timeline
Since Dec 1, 2015
Location
Description
The Carlsbad plant (December 2015) is the largest SWRO facility in the Western Hemisphere at 50 MGD, built for about $1 billion under a 30-year take-or-pay purchase agreement with San Diego County Water Authority. It delivers rainfall-independent supply but is the county's costliest source at ~$3,500 per acre-foot in 2025. The rigid take-or-pay contract produced $54M paid for unneeded water in the wet 2023–24 year and ~$35M owed for water never made during plant outages. Marine regulations forced an intake retrofit of ~$159M roughly two decades into the asset's life. Potable reuse projects in the same class cost roughly half as much per acre-foot.
Metrics
5Funding
Lessons learned
- Take-or-pay contracts bill ratepayers in wet years and even for water never made during outages: rigidity cuts both ways — negotiate flexibility triggers before signing.
- Environmental compliance is a moving target: marine intake rules forced a ~$159M retrofit; budget for regulatory change over a 30-year asset life.
- Where a wastewater stream exists, potable reuse of the same class costs roughly half per acre-foot: run that comparison before contracting a large desalination plant.
Documented Jul 24, 2026