Desalination in India: Where It Fits, and What It Really Costs
For a coastal city watching its reservoirs fall, the sea is a maddening thing to look at — an ocean of water you cannot drink, next to a city that is running out. Desalination is the technology that closes that gap, and India, with a long coastline and several acutely water-stressed coastal cities, has been building it at scale. It works. It is also widely misunderstood as "free water from the sea," which it emphatically is not.
Why coastal India turned to the sea
Chennai is the clearest case. After repeated crises where the city's reservoirs ran dry and water arrived by train, Tamil Nadu invested in large seawater desalination plants to give the city a supply that does not depend on the monsoon. The logic is simple: a desalination plant produces the same output in a drought as in a good year, because its source never runs low. For a coastal megacity with no reliable inland alternative, that drought-proof quality is the whole point.
The scale is real. Chennai's Nemmeli plant produces on the order of 150 million litres a day; a much larger plant at Perur, in the range of 400 MLD, is being built to become one of the biggest in the region. These are not pilots. They are backbone infrastructure.
How it works
Almost all modern seawater desalination uses seawater reverse osmosis (SWRO) rather than the older thermal, boil-and-condense methods. The principle is the same RO used everywhere — push water through a membrane that rejects dissolved salts — but seawater makes it far harder, because seawater is roughly 35,000 mg/L of dissolved solids, seventy times a typical borewell. Overcoming that much osmotic pressure needs very high pressure, and therefore a lot of energy.
A SWRO plant is, in sequence: an intake from the sea; heavy pre-treatment (seawater is biologically alive and fouls membranes aggressively); high-pressure pumping through SWRO membranes; and an energy-recovery device that reclaims pressure from the reject stream to cut the power bill. The output splits into fresh permeate, which is remineralised for taste and stability, and a concentrated brine that goes back to the sea.
What it really costs
Two costs define desalination, and neither is the membrane.
The first is energy. SWRO typically consumes on the order of 3.5–4 kWh per cubic metre of water produced, even with energy recovery — dramatically more than treating freshwater. Energy is the single largest operating cost, which means desalinated water's price is chained to the price of electricity. A modern plant can bring the lifecycle cost down to roughly ₹35 per thousand litres, but that figure rests on efficient energy recovery and large scale; it is not cheap water, it is affordable-at-scale water.
The second is capital. These are large, complex plants — a 150 MLD facility runs into hundreds of crores — which is why they are built as long-term public projects, often on PPP structures that spread the cost and the operating risk over fifteen years or more.
The brine problem
The reject nobody puts on the brochure is brine. A desalination plant does not make salt disappear; it separates the sea into fresher water and saltier water, and that hyper-saline brine — often twice the salinity of seawater — has to go somewhere. Discharged carelessly, it sinks and spreads along the seabed near the outfall, stressing marine life in a zone around the discharge. Doing it responsibly means diffuser outfalls that dilute the brine rapidly, and siting studies that most people never hear about. Brine management is to desalination what reject and sludge are to every other separation process: the quiet, unavoidable second half of the job.
When desalination is the right answer — and when it isn't
Desalination earns its place on a specific set of conditions: you are on the coast (brine has to go somewhere, and pumping seawater far inland is uneconomic), you serve a large, steady demand, and you have no cheaper, reliable freshwater alternative. Under those conditions it is unmatched — a supply that does not care about the monsoon.
Away from those conditions, it is usually the wrong tool. For an inland city or an industrial plant, the cheapest "new" water is almost never the sea; it is the water you already have and are throwing away. Treating and reusing wastewater typically costs less energy per litre than desalinating seawater, because treated effluent starts at a far lower salinity than the ocean. For most industry, in most locations, a serious look at reuse and an internal water balance will beat desalination on cost every time. Desalination is the answer when there is genuinely no fresher source left — which, for a coastal city in drought, is exactly the situation, and for an inland factory, almost never is.
If you are weighing desalination, reuse, or a mix for a coastal facility and want help running the numbers on energy, recovery and brine, that is a worthwhile conversation to have with us — [email protected] or +91-98100 00233.