A coastal city can turn to the ocean, recover used water, or use less. The difficult part is knowing how much each can deliver, at what cost, and with what consequences.
ILLUSTRATIVE CITY Port Meridian · 300,000 residents, plus port, tourism and industry. A severe dry-year service gap. No real procurement.
Every conclusion has a way
to change our mind.
Desalination makes more sense when verified conservation and eligible reuse cannot cover a persistent shortage, and a workable coast, power supply and financing support the remaining capacity.
Our starting point is to test the 20 / 60 / 20 portfolio. The split is an assumption. The case for measuring the alternatives is much stronger.
Keep the service.
Use less water.
At half the sustained yield, the same program costs $1.00 per m³ saved. Savings must be measured against a credible baseline.
Recover the water
already in the system.
Its energy advantage can shrink with more treatment or a difficult delivery route. A safe, approved treatment chain is essential.
A large source.
A demanding conversion.
The source is less exposed to rainfall. Production still depends on electricity, seawater quality and functioning infrastructure.
All contributions and scenario costs above are illustrative. Conservation is a reduction in demand; reuse and desalination add supply. The energy boundary is incremental treatment plus the stated delivery allowance. Common downstream distribution is excluded.
“Observed” means reported by the source. Click any node to inspect its limits.
Hold the full service target at 100,000 m³/day. Change electricity price, utilization and grid emissions. Lower utilization requires a bigger plant to deliver the same annual volume.
Delivers 36.50 million m³/year in this annual model. Storage and redundancy are additional.
Availability check. Reuse is an equal-volume cost comparator here. Port Meridian has only 60,000 m³/day of assumed reuse yield; the cheaper column cannot supply the whole target.
Volume stays fixed: V = 100,000 × 365. Capacity = 100,000 ÷ utilization. Electricity = V × energy intensity. Changing utilization changes capacity and fixed costs; annual electricity remains constant.
Cost: K × (capital recovery factor + 0.02) ÷ V + 0.15 + energy × electricity price. The factor is 0.057830 at 4% real and 30 years. K scales linearly from $350m SWRO or $200m reuse at 100,000 m³/day nameplate.
Energy: 3.5 + 0.3 = 3.8 kWh/m³ for SWRO; 0.84 + 0.16 = 1.0 for reuse. First term: plant. Second: delivery. Reuse starts at available secondary effluent. Costs are assumed 2026 USD, annualized; not tariffs, quotes or total sewage-service costs.
Exclusions: new secondary treatment, major trunk upgrades, unusual site works, land, storage, contingency beyond the capital allowance, finance during construction, taxes and subsidies. Carbon is electricity-use CO₂e only. No lifecycle or marginal-grid inference.
Python calculation source · Browser calculation source · Formula & assumption ledger
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Sources retrieved 2 September 2026. A targeted synthesis, not a systematic review. Historic cost dates and geography are retained. Some PDF links open at the relevant page; viewer support varies.
Measured demand. Sustained savings. Eligible dry-weather effluent. Treatment requirements. A delivery route. Marine baseline and mixing. Dependable power. Comparable local costs.
