Desalination plant cost: capital, operating cost and energy per m³

“How much does a desalination plant cost?” has two answers: what it costs to build, and what each cubic metre of water costs to make. Over the life of a plant the second number matters more, and most of it is set by energy, by capital recovery and by how well the plant is run.

The figures below are indicative ranges for reverse osmosis plants. Real costs depend on site, size, feed water, energy price and financing, so treat them as a starting point for a feasibility study, not a quote.

The three numbers that matter

• Capital cost (CAPEX): the cost to design and build the plant, usually quoted per m³/day of capacity.
• Operating cost (OPEX): energy, chemicals, membranes, labour and maintenance, quoted per m³ of water produced.
• Levelized cost of water: capital recovery plus operating cost, per m³. This is the number that allows two projects, or two technologies, to be compared.

Capital cost

Indicative capital cost for reverse osmosis plants:

Plant type	Capital cost per m³/day of capacity
Large seawater RO (over 100,000 m³/day)	about US$1,000–2,000
Medium seawater RO (10,000–100,000 m³/day)	about US$1,500–2,500
Small seawater RO (under 10,000 m³/day)	US$2,500 and above
Brackish water RO	about US$300–1,000

Brackish plants cost far less because they run at much lower pressure, need less pretreatment and can often use wells instead of an open sea intake.

What drives capital cost:

• Intake and outfall. For seawater plants, the marine works can be a large share of the cost, and they are the most site-specific item.
• Pretreatment. Poor or variable feed water needs more steps, such as dissolved air flotation, media filtration or ultrafiltration.
• Pressure. Seawater RO needs high-pressure pumps, duplex steel pipework and energy recovery devices.
• Post-treatment and concentrate disposal, which depend on the use of the water and on discharge permits.
• Scale. Cost per m³/day falls as plants get bigger.

Energy per m³

Energy is usually the largest single operating cost.

Plant type	Typical specific energy (RO system)
Seawater RO with energy recovery	about 2.5–3.5 kWh/m³
Seawater RO, whole plant (intake to product pumping)	about 3–4.5 kWh/m³
Brackish water RO	about 0.5–1.5 kWh/m³

The theoretical minimum for seawater at 50% recovery is about 1 kWh/m³, so modern plants are already within a factor of three of it. Most of the gain since the 1990s came from energy recovery devices, which return the pressure in the concentrate to the feed, and from better membranes.

To turn energy into cost: energy cost per m³ = specific energy × electricity price. At 3.5 kWh/m³ and US$0.10 per kWh, energy costs US$0.35 per m³.

Operating cost

A typical split of operating cost for a seawater RO plant:

• Energy: often 40–60% of the operating cost.
• Membrane replacement: seawater membranes typically last 5–7 years, so about 15–20% of the elements are replaced each year.
• Chemicals: coagulant, antiscalant, acid, cleaning chemicals and post-treatment minerals.
• Labour, maintenance and spares.
• Concentrate disposal, monitoring and permits.

Indicative total operating cost is about US$0.40–0.90 per m³ for seawater RO and US$0.15–0.40 per m³ for brackish water RO.

Levelized cost of water

Adding capital recovery gives the full cost of water. Indicative ranges:

• Large seawater RO: about US$0.50–1.20 per m³. The largest projects in regions with low energy prices have reported prices below US$0.50.
• Small and medium seawater RO: about US$1.00–2.50 per m³.
• Brackish water RO: about US$0.30–0.80 per m³.

A worked example

A 50,000 m³/day seawater RO plant:

• Capital cost US$75 million (US$1,500 per m³/day).
• Financed over 25 years at 8%: a capital recovery factor of 0.094, so US$7.0 million a year.
• Running at 95% availability, it produces 50,000 × 365 × 0.95 = 17.3 million m³ a year.

Cost item	US$ per m³
Capital recovery (7.0 ÷ 17.3)	0.41
Energy (3.8 kWh/m³ at US$0.10)	0.38
Labour, maintenance and spares	0.10
Chemicals	0.05
Membrane replacement	0.04
Concentrate, monitoring and other	0.03
Total	about 1.01

Capital and energy make up almost 80% of the cost of water. Both are fixed at design, but energy keeps moving for the rest of the plant’s life.

Where operating costs creep up

Once the plant is built, most of the cost that can still be controlled is in how it runs:

• Fouling raises feed pressure. If fouling lifts feed pressure by 5%, the high-pressure pumps use roughly 5% more energy. In the example above, that is about US$250,000 a year.
• Late or ineffective cleaning. Fouling that builds up is harder to remove, and a clean that does not recover performance shortens membrane life. See how to measure CIP recovery.
• Running away from the design point. Recovery, flux and the energy recovery devices drift from their design values, and the energy cost rises.
• Early membrane replacement. Replacing elements because performance looks poor, when the cause is fouling or a bad reading, is an avoidable cost.

The first step is knowing the real condition of the membranes. Raw pressures and flows change with temperature and output, so they hide the trend. Normalized data and normalized differential pressure show it.

How to reduce the cost of desalinated water

• At the design stage: compare intake options, pretreatment and the number of passes on a life-cycle cost basis, not on capital cost alone. A feasibility study is the place to do this.
• In operation: track normalized performance, clean on a trigger rather than a calendar, and check that cleaning works. Plant optimisation reviews where the energy and chemical costs of an existing plant are going.
• With software: RO Insight™ normalizes permeate flow, salt passage and differential pressure for each stage automatically, and flags energy and fouling trends before they become costly.

For more on seawater and brackish plants, see UF & RO desalination.