The Environmental Impact of Desalination: Intake, Brine, Energy and How Permits Address Each
By Sean Alford, Permitting Plus
Desalination, making fresh water from salt water, has three environmental impacts that matter: what the intake does to the sea or the aquifer, what the brine does where it is discharged, and what the electricity does to the carbon footprint. Everything else is detail. A desalination facility that handles those three well is one of the more benign industrial facilities on a coast. One that does not is a permit problem, a reputation problem, and sometimes an ecological one.
This article is written for people who have to get desalination approved: owners, developers, and the engineers and lawyers working for them. It explains each of the three, what the science says about it, what the regulator asks, and what a permit application has to show. It is not an argument for or against desalination. Where freshwater is scarce, desalination is often the least damaging supply available, and desalination technology has improved on every one of the three impacts in the last twenty years, and the case rests on how the facility is designed rather than on whether it exists.
Permitting Plus helps companies plan, permit, and set up desalination systems in the United States and worldwide. The environmental review is where most of our permitting work on these projects happens. Our desalination page explains the full process.
Table of Contents
The three impacts, and the questions behind them
| Impact | What happens | What the regulator asks | What the application has to show |
|---|---|---|---|
| Intake | Marine life is drawn into or trapped against the inlet; groundwater levels and salinity change | How much, which species, what alternatives were considered | Inlet design, screens or subsurface option, entrainment and impingement studies, aquifer model |
| Brine | Dense, salty water sinks and raises salinity near the outfall; chemicals ride along | How far the plume spreads, how salty at the edge, what lives there | Dispersion modeling, salinity limits at the mixing zone, toxicity testing, monitoring plan |
| Energy | Electricity use adds emissions to the carbon footprint | How much per cubic meter, from what source | Energy recovery, specific energy figures, renewable or waste heat plans |
There are lesser impacts as well: land take, construction disturbance, noise, chemical storage, and the visual effect of a coastal facility. They are handled by the ordinary industrial permits and are rarely what decides a desalination project. The three above are.
The intake: what desalination takes from the sea
A seawater desalination facility draws in roughly two cubic meters of seawater for every cubic meter of freshwater it makes, because reverse osmosis recovers 40 to 50 percent of the feed. That draw of water from the sea is the first environmental question.
Impingement and entrainment
Open ocean inlets pull water through a pipe and a screen. Larger organisms, fish and invertebrates, can be trapped against the screen, which is called impingement. Smaller ones, eggs, larvae, and plankton, pass through the screen and into the facility, which is called entrainment, and they do not survive. The scale depends on the volume drawn, the velocity at the screen, the location, and the season.
The mitigations are known. Low inlet velocities, fine screens, and siting away from spawning grounds reduce impingement. Subsurface inlets, beach wells and infiltration galleries that draw seawater through the seabed or the beach sand, eliminate both problems because the sediment does the screening. They also deliver cleaner water that needs less pretreatment. Their limitation is capacity and geology: not every coast allows them, and very large plants may need more than the seabed can supply.
Groundwater wells
Brackish desalination plants draw from wells, and the question becomes an aquifer question. Pumping lowers water levels, can raise salinity as saltier water moves in, and can affect neighboring wells and any springs or streams the aquifer feeds. The application has to show a groundwater model, monitoring, and production limits. We cover that side in our article on brackish water desalination.
What the permit asks about the inlet
In the United States, seawater inlets at coastal plants are reviewed under the Clean Water Act and state coastal programs, with entrainment and impingement studies, an alternatives analysis showing why a subsurface inlet was or was not chosen, and mitigation where impacts remain. Several coastal states, California most prominently, require subsurface inlets unless they are shown to be infeasible. Outside the United States, the marine survey and the alternatives analysis are part of the environmental impact assessment. Either way, the decision should be made with the survey in hand, because an open inlet in a biologically rich bay can be the item that stops the project.
Brine: the discharge that decides most permits
Every desalination facility produces brine, a concentrated salt water at roughly one and a half to two times ocean salinity, carrying the residual chemicals from pretreatment and cleaning. Where it goes, and what happens when it gets there, is the environmental impact that decides most desalination permits.
What brine does in the sea
The concentrate is denser than seawater. Released without care, it sinks and spreads along the seabed as a layer of elevated salinity, and organisms that live on or near the bottom, seagrass, corals, and benthic invertebrates, are the ones affected. Elevated salinity stresses them; lower dissolved oxygen in the dense layer stresses them further; and residual chemicals, antiscalants, coagulants, and the cleaning agents used on membranes, add to it. Studies of poorly designed outfalls have found measurable effects on seagrass and benthic communities in the marine environment within tens to hundreds of meters of the outfall.
The same studies show the other side. Well-designed outfalls with diffusers that mix the brine quickly into the water column, sited in areas with good currents and away from sensitive habitat, show effects confined to a small mixing zone and returning to background within it. The difference is design, not desalination.
How the concentrate is controlled
Diffusers. A length of pipe with angled nozzles that jet the concentrate upward into the water column so it mixes with seawater before it can sink. Diffuser design is the single largest lever on the size of the affected area.
Dilution before discharge. Blending the concentrate with cooling water from a power plant or with treated wastewater lowers the salinity before the water reaches the sea. It was common at plants sited next to power stations and is less available as those stations close.
Siting. Releasing into well-flushed water with strong currents, away from seagrass, coral, and nursery habitat.
Limits and monitoring. Permits set a salinity limit at the edge of a defined mixing zone, often a fixed increment above natural background at a fixed distance, plus limits on temperature, pH, residual chlorine, and other chemicals, and a monitoring program to prove compliance.
Reducing the volume. Higher recovery, concentration, or full recovery of the salt and water means less or nothing to release. On our project in Indonesia, the reject brine from a seawater desalination facility, tens of thousands of cubic meters a day, is planned to be turned into salt and chlor-alkali chemicals with no liquid returned to the sea. That is the far end of the spectrum, and it exists because the regulatory direction in Indonesia and the marine setting of the outfall pushed the project there.
Inland concentrate
Brackish plants inland have no sea to return water to. Their concentrate goes to deep injection wells, evaporation ponds, sewers where the volume allows, or concentration toward zero liquid discharge, which is a whole treatment process of its own. Each has its own questions for the regulator: groundwater protection for wells and ponds, land take and wildlife for ponds, salt load for sewers. We compare the routes in our guide to brine disposal methods.
What the permit asks about the concentrate
Dispersion modeling that predicts the salinity plume under the range of conditions the site sees. Whole effluent toxicity testing on the actual concentrate, in many jurisdictions. A salinity limit at the mixing zone boundary and the diffuser design that meets it. Limits on residual chemicals. A baseline survey of the receiving environment before construction, and a monitoring program after. And, increasingly, an analysis of alternatives to release: higher recovery, blending, or recovery of the concentrate. In the United States this sits under the National Pollutant Discharge Elimination System permit; outside the United States it is the core of the environmental impact assessment. The concentrate section is where most desalination applications are won or lost, and it is where we spend most of our time.
Energy and the carbon footprint
Desalination uses more electricity per cubic meter than any conventional fresh water supply, and it is the reason for most of the criticism desalination receives. Seawater reverse osmosis at a modern desalination facility with energy recovery devices uses roughly 3 to 4 kilowatt-hours per cubic meter; brackish water far less; thermal desalination far more. That electricity has a carbon footprint, the emissions, set by the grid it comes from, and regulators and communities now ask about it directly.
What has changed
The electricity used by seawater desalination systems has fallen by more than half since the early plants, mainly through energy recovery devices that capture the pressure in the brine and return it to the feed water, and through better membrane technology. Modern plants run much closer to the theoretical minimum energy for the separation than early plants did, so further large gains are harder to find. What remains is the source of the electricity.
Renewable desalination
Reverse osmosis desalination runs on electricity, so the technology can run on renewable electricity, and many new desalination systems are being planned that way. Solar photovoltaic power in sunny regions, wind where it is reliable, and grid supply with a rising renewable share all reduce the carbon footprint per cubic meter. The qualification is that reverse osmosis prefers to run continuously and solar does not, so storage or grid backup is usually part of the design. On the concentrate side, where concentration and evaporation are thermal, solar heat and recovered waste heat matter more, and on our project in Indonesia both are part of the base case because evaporating the full reject stream on purchased fuel alone was judged unlikely to be economical.
What the permit asks about energy
Specific energy in kilowatt-hours per cubic meter, and the energy recovery design that achieves it. The electricity supply and its emissions. Renewable, solar, or waste heat plans. In some jurisdictions a greenhouse gas emissions assessment is a formal part of the review; in most, it is a question the reviewer asks and the application should answer before it is asked. On our project in Indonesia, the energy source and carbon footprint were among the items prepared for the regulator.
Desalination systems and the environment: a process view
It helps to see desalination as a set of systems, each with its own question, rather than as one thing.
The inlet system draws water from the sea or the aquifer. Its question is what it takes with the water.
The pretreatment system removes solids and organics and doses chemicals. Its question is where the backwash and the spent chemicals go; they are a wastewater stream in their own right and belong in the water balance.
The reverse osmosis system separates fresh water from salt water. Its question is power, and where that power comes from.
The post-treatment system stabilizes and disinfects the freshwater. Its question is chemical storage and handling on site.
The concentrate handling system returns the brine to the sea, sends it to a well or pond, or recovers it. Its question is the salinity plume, or the land and groundwater, or the energy of recovery.
Seen this way, the review is a review of five water systems working as one process, and the application should describe each: what goes in, what comes out, and where the outputs go. Desalination systems that are documented this way, system by system and process step by process step, pass review faster, because the reviewer can follow the process from sea to tap and see that nothing has been left out. Systems that are described as a single package, with the pretreatment waste and the cleaning waste unmentioned, invite the questions that cause delay.
Freshwater, the ocean, and the goal of the review
The goal of an environmental review is not to stop desalination. It is to make sure that producing freshwater from the sea or the aquifer does not cost more, ecologically, than it returns. That framing matters for how an application is written.
A good application states its own goal and shows how the design meets it: a water supply of a given volume, drawn and returned with impacts confined to a defined area, on an energy budget with a stated carbon footprint, monitored for the life of the plant. Each element of that goal maps to a permit condition, and each is about water: how much is drawn, how much is made, how much is returned, and in what state. An application that presents the facility as an engineering object and leaves the goal to the reviewer to infer gets a harder review.
The same goal-first approach applies to the marine environment specifically. The marine baseline survey establishes what lives near the site; the goal is that the monitoring after construction shows the same marine community within the mixing zone boundary and beyond. Where the goal is written down, the monitoring has something to be measured against, and the environment has a defined protection rather than a general assurance.
The case for sustainable desalination
It is worth stating plainly, because the impacts above are real and the alternatives are often worse.
Where fresh water is scarce, the alternatives to desalination water are over-pumping aquifers until they collapse or turn saline, damming and diverting rivers with their own ecological cost, importing water over long distances with its own power cost, or going without, which for an industrial site means shutting down and for a community means hardship. Against those, a well-sited reverse osmosis facility with a subsurface intake, a properly designed diffuser or a recovery facility for its concentrate, and renewable power is modest and manageable.
The case therefore turns on design choices. Desalination is not environmentally good or bad in itself. Its inlet, its concentrate handling, and the electricity it runs on make it one or the other, and all three are decided before construction, in the design and the permit application. That is why we insist the concentrate route and the inlet alternatives are settled with the facility concept, not after.
Sustainable desalination: what the phrase should mean
Sustainable desalination is now a stated goal of most water utilities and many industrial owners. The phrase gets used loosely. In our work it means five specific things, each of which can be checked.
A subsurface inlet where geology allows, or an open inlet with low velocity, fine screens, and siting away from sensitive habitat, with the alternatives analysis on paper.
A concentrate plan that keeps salinity at the mixing zone boundary within the permitted increment, proven by modeling before and monitoring after, or that removes the discharge altogether through recovery.
Energy recovery at the facility and a credible plan for renewable, solar, or low-carbon supply.
A water balance that accounts for every stream, including the ones desalination proposals usually leave out: pretreatment backwash, membrane cleaning waste, and site stormwater.
A monitoring program that reports, over its life, what the facility actually does to the sea or the aquifer, published or available to the regulator, so the claim can be tested.
A desalination facility that meets those five is sustainable in the sense that matters, and its claim can be defended to a regulator, a lender, and a community. A facility that uses the word without them is not.
Impacts by desalination technology
Not all desalination has the same footprint, and the water systems inside each technology differ.
| Technology | Intake impact | Brine impact | Energy impact |
|---|---|---|---|
| Seawater reverse osmosis, open inlet | Impingement and entrainment; mitigated by screens and siting | Brine at 1.5 to 2 times seawater; diffuser required | 3 to 4 kilowatt-hours per cubic meter |
| Seawater reverse osmosis, subsurface inlet | Minimal; sediment screens the water | Same concentrate, cleaner, fewer chemicals | Same, slightly less pretreatment energy |
| Brackish water reverse osmosis | Aquifer drawdown and salinity change | Small concentrate volume, inland disposal | Low, often under 1.5 kilowatt-hours |
| Thermal desalination | Large seawater draw for cooling | Warm, salty discharge; thermal plume | Very high unless waste heat is free |
| Reverse osmosis with concentrate recovery | As above | Little or no liquid discharge; solid salt to market or landfill | Higher, for concentration |
Conventional thermal desalination technology, multi-stage flash and multi-effect distillation, carries the largest environmental footprint per cubic meter: a much larger seawater draw, a warm and salty outflow, and several times the electricity and heat. It persists where steam is free, next to power stations, and it is being displaced by reverse osmosis technology almost everywhere else. Thermal desalination plants of this kind are still being built in the Gulf, usually paired with power stations, and theirs is the footprint that older studies of desalination and the environment describe. Conventional open-inlet reverse osmosis is the middle case and the most common. Subsurface inlets and concentrate recovery are the direction the industry and the regulators are moving.
How the environmental review works
The environmental review is where the three impacts on water, sea, and air are examined together, and it follows a recognizable path in most countries.
Screening. The agency decides how much review the desalination project needs, based on its size, its location, and how it draws and returns water. Large coastal desalination facilities get the full assessment; small packages with subsurface inlets and small outflows may get a lighter review.
Baseline. Surveys of the marine or groundwater environment before anything is built: what lives there, what the currents and salinity are, what the aquifer looks like. The baseline is what future monitoring is compared with, and a project without one has no way to prove it did no harm.
Assessment. Modeling of the salinity plume, the entrainment estimate, the energy and emissions, and the construction impacts, against the baseline, with alternatives considered and mitigation proposed.
Consultation. Public and agency comment, which for coastal projects can be extensive. Fishing communities, tourism operators, and community groups have standing, and the salinity plume is what they ask about.
Decision and conditions. Approval with conditions: the salinity limit, the monitoring program, the inlet velocity, the reporting.
Operation and monitoring. The desalination plant reports against its conditions for its life, and the permit is renewed on that record.
In the United States, this runs through the National Environmental Policy Act where federal permits or funding are involved, state environmental review laws, the Clean Water Act discharge permit, coastal zone consistency, and state drinking water approval for municipal plants. In Indonesia and much of Asia it runs through the environmental impact assessment, the AMDAL for larger projects, or the lighter environmental management and monitoring plan for smaller ones, followed by the environmental approval and the sectoral permits. We map the pathway for the specific plant and sequence it alongside engineering, because the review can take as long as the design and construction combined.
What we flag before the regulator does
These are the items we prepare before a desalination or brine recovery file goes to the regulator. Several apply to every desalination project.
The environmental permitting pathway itself, mapped for the specific plant rather than borrowed from a reference elsewhere.
The scope of any zero discharge or low-impact claim, defined stream by stream, so that pretreatment backwash, cleaning waste, and site effluent are inside it or shown separately.
The energy source, its emissions, and the carbon footprint, with renewable or solar options considered.
The land footprint of any pond or salt field, which needs its own review of seepage, land use, and ecology.
Local content and community benefit, framed as commitments.
A before-and-after baseline, so the reviewer can see the change at a glance.
Finding those before the filing is the difference between a review and a rejection, and it is the part of the work that an equipment proposal on its own does not cover.
Desalination and water systems that already exist
A new desalination plant rarely arrives on an empty coast. It joins water systems that are already there: a municipal supply, an industrial water treatment plant, a wastewater treatment works, sometimes a power station with its own cooling water systems. The environmental review looks at the new plant in that context, and the best applications use it.
Sharing an outfall with an existing wastewater or cooling water system dilutes the concentrate before it reaches the sea and can remove the need for a new marine pipeline. Sharing an inlet with a power station’s cooling water systems, where one still operates, avoids a second draw on the sea. Sending the concentrate to an adjacent industrial process that can use the salt turns the discharge into a supply. And pairing desalination with wastewater reuse in the same water systems plan cuts the desalination capacity needed and the impacts with it.
Each of these is a design decision with a payoff for the environment, and each is easier to make at the concept stage than after the plant is drawn. We look for them on every desalination project, because desalination is a process that touches every other water process on the site, and because the source of the least impact is usually a connection to something that already exists rather than a new piece of equipment.
Frequently asked questions
Is desalination bad for the environment?
It has real impacts, at the inlet, in the concentrate, and in the power use, and each can be managed by sustainable design. A well-sited desalination plant with a subsurface inlet, a good diffuser or concentrate recovery, and low-carbon power has a modest footprint. The alternatives in a water-scarce region are often worse for the environment.
What does desalination brine do to the ocean?
Released badly, it forms a dense salty layer on the seabed that stresses seagrass, corals, and bottom-dwelling life near the outfall. Released through a well-designed diffuser in well-flushed water, its effects are confined to a small mixing zone. Recovered as salt and chemicals, it does not reach the sea at all.
How much energy does desalination use?
Roughly 3 to 4 kilowatt-hours per cubic meter for modern seawater reverse osmosis with energy recovery, far less for brackish water, with emissions that follow the grid that supplies it, and far more for thermal desalination. The carbon footprint depends on the electricity source.
Can desalination be sustainable?
Yes, desalination is sustainable in the checkable sense: subsurface or low-impact inlet, brine within permitted limits or recovered, energy recovery and low-carbon or solar supply, a complete water balance with monitoring for the life of the facility, and monitoring that proves it. The word means little without those five.
What environmental permits does a desalination plant need?
An inlet approval with entrainment and impingement studies, a discharge permit with salinity limits and monitoring, an environmental review scaled to the project, and the ordinary construction, land, and, for drinking water, health approvals. The outfall permit is usually the one that decides the schedule.
In short
Desalination’s environmental impact comes down to the inlet, the concentrate, and the energy. Each is understood, each can be controlled by design, and each is examined in the environmental review, and each can be controlled by technology choice. The desalination plants that struggle are the ones that chose an open inlet without looking at the alternatives, assumed the outfall, and left the power question for later. The plants that pass are the ones that settled all three with the concept, which is what sustainable desalination means in practice.
If you have a desalination project heading into environmental review, or a review that has stalled, send us the concept and the site and we will tell you what the regulator will ask and what the file is missing. Call (470) 714-8564 or request a consultation. Our desalination page explains how we run the whole project, and our guide to desalination plant costs covers what the review work costs in studies and time.
Talk to us about your desalination project
Send us your water analysis, the volume you need, and the site, and we will tell you what system fits, what it takes to permit, and how long it will take.