How to Degas Water: Methods for Removing Dissolved Gases

Degassing water means stripping out dissolved gases like oxygen, nitrogen, and carbon dioxide, and there are half a dozen reliable ways to do it. The most common methods are boiling, vacuum treatment, sparging with an inert gas, hollow-fiber membrane contactors, ultrasonic treatment, and chemical scavenging. Which one you should use depends entirely on how pure the water needs to be and the scale you’re working at, because these methods range from a kitchen stove to industrial membrane systems that can push dissolved oxygen below one part per billion.

Boiling Works, but Not as Well as You’d Think

Boiling is the first thing most people try, and it does remove dissolved gases. But it has a ceiling that surprises a lot of users. At a rolling boil under normal atmospheric pressure, water vapor bubbles form, and dissolved oxygen hitches a ride into those bubbles and escapes at the surface. The problem is that vigorous boiling also creates a violently turbulent surface, which lets atmospheric oxygen dissolve right back in. This back-reaction makes boiling at normal pressure both inconsistent and only partially effective.

Studies comparing common degassing techniques have found that boiling under reduced pressure is a different story. When you pull even a modest vacuum while boiling, the oxygen that escapes from the liquid is swept away rather than hovering at the surface waiting to redissolve. That makes the process far more effective and reproducible.¹ If you only have access to a stovetop, boiling will still lower dissolved oxygen meaningfully, but don’t expect it to get you to the low levels that laboratory or industrial work demands. And once boiled water cools back down in an open container, gases start redissolving within minutes.

Sparging with an Inert Gas

Sparging, sometimes called purging, means bubbling an inert gas like nitrogen or helium through the water. The inert gas displaces dissolved oxygen and other gases by changing the equilibrium at the bubble surface. The key factor is bubble size: smaller bubbles have a larger total surface area relative to their volume, so they exchange gas with the surrounding water much more efficiently. Using a sintered glass frit or porous sparger to produce fine bubbles is critical. Coarse bubbles from an open tube will work eventually, but the process takes much longer and the result is less thorough.¹

Sparging is standard practice in analytical chemistry labs, particularly for preparing mobile phases in liquid chromatography and media for pharmaceutical dissolution testing. One wrinkle that catches people off guard is that sparging with nitrogen removes dissolved oxygen but can leave total dissolved gas levels high if the nitrogen itself remains dissolved. For dissolution testing, where the water is heated to body temperature, even slight supersaturation can cause outgassing and bubble formation that disrupts the test. Measuring total dissolved gas pressure rather than just oxygen concentration is necessary to confirm proper deaeration.²

Vacuum Degassing

Applying a vacuum to water lowers the partial pressure of every gas above the liquid, which shifts the equilibrium so that dissolved gases come out of solution. You can do this by placing water in a sealed flask connected to a vacuum pump, or in continuous-flow industrial systems where water passes through a chamber held under negative pressure. The approach works for all dissolved gases simultaneously, not just oxygen.

At larger scales, vacuum degasifiers are used to strip problem gases from wastewater. In systems treating anaerobically processed wastewater, multi-stage vacuum degasifiers fitted with nozzles have achieved roughly 94% methane removal and 88% hydrogen sulfide removal. Nozzle-fitted designs outperformed media-packed columns in side-by-side comparisons, and the energy balance was favorable enough that the recovered methane could generate surplus energy of 0.05 to 0.07 kWh per cubic meter of water treated.³

Hollow-Fiber Membrane Contactors

For applications that demand extremely low dissolved gas levels, membrane contactors are the gold standard. These devices use bundles of hollow fibers made from hydrophobic polymers like polypropylene. Water flows on one side of the fiber wall and a vacuum or sweep gas is applied on the other. Dissolved gas molecules diffuse through the liquid, pass through the membrane’s pores, and are carried away. Because the membrane keeps the liquid and gas phases separated, there’s none of the back-mixing that plagues boiling or open vacuum systems.

Highly efficient hollow-fiber modules can reduce dissolved oxygen to less than one part per billion.⁴ That level of purity is essential in semiconductor manufacturing, where even trace oxygen in ultrapure rinse water can oxidize chip surfaces. In those facilities, compact membrane modules originally designed as blood oxygenators have been adapted for water degasification, sometimes with protective coatings on the fibers to resist residual oxidizing agents in the feed water.⁵

The physics of these systems is straightforward: because common gases like oxygen and nitrogen have low solubility in water, the bottleneck to mass transfer is almost entirely on the liquid side of the membrane. That means performance depends heavily on the ratio of liquid flow rate to available membrane surface area. For oxygen removal under vacuum using fibers with a 140-micrometer internal diameter, an efficiency of 98% has been modeled when the flow-to-area ratio is kept low enough.⁶ Membrane contactors are also effective for removing dissolved ammonia, with removal efficiencies of 99% or better demonstrated using polypropylene hollow fibers with an acid extractant on the permeate side.⁷

Ultrasonic Degassing

Applying ultrasound to water creates rapidly oscillating pressure waves that form and collapse microscopic cavitation bubbles. Dissolved gas molecules migrate into these bubbles, which then rise to the surface and release the gas into the atmosphere. The method is fast and works without any consumables, which makes it appealing for small-volume batch processing.

Research on ultrasonic degassing across a wide frequency range found that frequencies between roughly 200 kHz and 1 MHz were most effective at driving degassing at atmospheric pressure. But the technique has a hard floor: cavitation stops occurring once dissolved oxygen drops below about 2 milligrams per liter, because there aren’t enough gas nuclei left in the water to sustain bubble formation.⁸ That lower limit means ultrasound is useful for taking water from fully saturated down to moderately degassed, but it cannot reach the sub-ppm levels that vacuum or membrane systems deliver. For that reason, ultrasonic degassing is sometimes used as a pre-treatment step before a more thorough method finishes the job.

Chemical Oxygen Scavengers

In boiler systems and other closed industrial loops, dissolved oxygen is sometimes removed chemically rather than physically. Oxygen scavengers are compounds added to the water that react with dissolved oxygen and convert it to harmless products. Hydrazine, for example, reacts with oxygen to produce nitrogen gas and water, leaving no solid residue behind.⁹ Sodium sulfite and catalyzed sodium sulfite are other common choices, reacting with oxygen to form sodium sulfate.

Chemical scavengers are effective but come with caveats. Hydrazine is toxic and a suspected carcinogen, so handling it requires serious safety precautions, and it is not used in any application where the water contacts food, beverages, or living organisms. Sulfite-based scavengers add dissolved solids to the water, which can be unacceptable for high-purity processes. Chemical methods also only target oxygen specifically; they do not remove dissolved nitrogen, carbon dioxide, or other gases. This limits their use to situations where oxygen is the sole concern, as it typically is in corrosion prevention for boiler feedwater and steam systems.

Freeze-Pump-Thaw for Research Labs

Researchers who need thoroughly degassed liquids in small volumes often use a technique called freeze-pump-thaw. The liquid sample is frozen solid, typically by immersing the flask in liquid nitrogen. A vacuum is then applied while the sample is still frozen, which removes the gas that has been expelled from the solid matrix and any gas in the headspace. The sample is then allowed to thaw, and the cycle repeats, usually three to five times, until essentially all dissolved gas has been extracted.¹⁰

Freeze-pump-thaw is extremely effective, capable of producing water with vanishingly low gas content. It is also slow, labor-intensive, and only practical for volumes of a few hundred milliliters at most. That makes it a niche technique for surface science, emulsion research, and other experiments where even trace dissolved gas changes the outcome. Nobody is using it to prepare industrial quantities of water.

Why Dissolved Gases Cause Problems in the Lab

The reason analytical chemists care so much about degassing goes beyond theoretical tidiness. In high-performance liquid chromatography, dissolved oxygen forms light-absorbing complexes with many common solvents, and the effect is especially pronounced at ultraviolet wavelengths below 260 nanometers. As oxygen concentration shifts during a run, the UV detector drifts, introducing noise and baseline instability. Dissolved oxygen also quenches fluorescence in both solvents and dissolved compounds, which means a fluorescence detector’s sensitivity literally depends on how well the mobile phase has been deoxygenated.¹¹ If you’ve ever seen an HPLC baseline wander for no apparent reason, poorly degassed solvents are a common culprit.

In pharmaceutical dissolution testing, dissolved gases pose a different but equally disruptive problem. Dissolution tests run at 37°C, and if the test medium was deaerated at room temperature but still contains enough total dissolved gas to be supersaturated at that higher temperature, tiny bubbles form on the surface of the dissolving tablet. Those bubbles change the effective surface area exposed to the medium and throw off the results. Proper deaeration therefore requires checking total dissolved gas pressure, not just oxygen, before starting the test.²

Corrosion Prevention in Piping and Boilers

Dissolved oxygen is one of the most aggressive drivers of corrosion in metal piping. In copper fire sprinkler systems, electrochemical testing has shown that the corrosion rate at higher dissolved oxygen concentrations can be roughly ten times the rate at lower concentrations. Air pockets that form at the ends of sprinkler pipes serve as an ongoing oxygen source, accelerating localized pitting corrosion from the inside out.¹²

This is why power plants and large heating systems go to elaborate lengths to deaerate their boiler feedwater. Thermal deaerators, which spray water into a steam-filled chamber, are standard in power generation. The steam heats the water to drive off dissolved gases and simultaneously blankets the water surface to prevent re-absorption. Chemical scavengers then mop up whatever trace oxygen remains. The costs of these systems are easily justified: a single pitting failure in a high-pressure boiler tube can force an expensive, unplanned shutdown.

Protecting Fish from Gas Bubble Disease

Dissolved gas supersaturation is not just a laboratory nuisance; in natural and managed waterways, it can be lethal to fish. Gas bubble disease occurs when the total dissolved gas pressure in water exceeds atmospheric pressure. Gases come out of solution inside the fish’s tissues and blood vessels, forming bubbles that obstruct circulation and damage organs.¹³ The condition is a well-known problem downstream of large dams, where water plunging over spillways entrains air and forces gas into solution at elevated pressure.

In Atlantic salmon exposed to supersaturated water, histological examination revealed bubble-like structures forming in the thin-walled blood vessels of the fins, along with widespread fibrin-dominated lesions. Within a week, inflammatory cells infiltrated the damaged tissue, and the skin overlying the lesions began to break down.¹⁴ In aquaculture, degassing incoming water with packed-column aerators or vacuum systems is a routine precaution. Some facilities use activated carbon beds, which provide nucleation sites that help supersaturated gases escape before the water reaches the fish.

Degassing an Entire Lake

The most dramatic degassing project in history has nothing to do with a laboratory or a factory. Lake Nyos in Cameroon sits in a volcanic crater, and carbon dioxide seeps continuously into its deep water from magmatic sources below. In 1986, a sudden overturn of the lake released a massive cloud of CO₂ that suffocated roughly 1,700 people and thousands of livestock in surrounding villages. The disaster prompted an engineering effort to degas the lake before it could happen again.

The solution was conceptually simple: install vertical pipes extending from the CO₂-rich deep water to the surface. Once primed, the dissolved gas comes out of solution as the water rises and the pressure drops, creating a self-sustaining fountain. The rising two-phase mixture of water and gas accelerates as more bubbles form, turning the latent energy of the dissolved CO₂ into the kinetic energy that keeps the flow going. Engineers have described it as a controlled version of the very limnic eruption they are trying to prevent.¹⁵ After more than three decades of continuous operation, the challenge has shifted from emergency drawdown to balancing the rate of CO₂ discharge through the pipes against the rate of natural recharge from below, so that the lake never rebuilds a dangerous gas reservoir.

Degassing in Microfluidic Devices

At the opposite end of the scale spectrum, microfluidic chips used in biomedical diagnostics and drug screening face their own bubble problems. Channels in these devices are often narrower than a human hair, and even a microscopic air bubble can block flow, disrupt mixing, or ruin an optical measurement. Conventional degassing methods are too bulky or too slow for chips that are meant to be disposable and cheap.

One approach uses a gas-permeable membrane layer built directly into the chip. A reusable substrate with vacuum channels sits beneath a disposable chip superstrate, and when the two are pressed together, vacuum pulls trapped bubbles through the membrane and out of the fluid path. The assembly can be separated and the disposable layer replaced between runs.¹⁶ Similar designs use a hydrophobic membrane sandwiched between the fluid layer and a microsupport array, creating an active bubble trap that prevents bubbles from ever reaching the working channels.¹⁷ These integrated solutions reflect a broader trend: as devices shrink, degassing has to be designed into the hardware rather than handled as a separate preparation step.

Picking the Right Method

If you’re working in a home kitchen and want to reduce fizzing or air content for a recipe, a simple boil followed by cooling under a lid will get you most of the way there. For a chemistry lab preparing HPLC solvents, helium sparging or inline vacuum degassing is standard and effective. If you need water with dissolved oxygen in the low parts-per-billion range for semiconductor fabrication or microelectronics rinsing, hollow-fiber membrane contactors under vacuum are the only practical choice. Ultrasonic methods fill a middle ground: quick and consumable-free but unable to reach the lowest concentrations. Chemical scavengers are powerful but limited to closed industrial systems where toxicity and dissolved solids can be managed.

One practical mistake that cuts across all methods is failing to protect the degassed water from re-absorbing gas afterward. Water re-equilibrates with the atmosphere surprisingly fast, especially in thin layers or turbulent flow. Degassed water should be stored under inert gas or vacuum, used immediately, or kept in sealed containers with minimal headspace. All the effort you put into removing dissolved gas is wasted if the water sits in an open beaker for ten minutes before you use it.