Vacuum Degassing in Water Treatment

Water that looks clean can still be full of trouble. Dissolved gases—oxygen, carbon dioxide, nitrogen, and traces of others—are invisible, but they drive corrosion, interfere with treatment processes, and create compliance headaches. In boiler feedwater, dissolved oxygen eats away at steel pipes and heat exchangers. In drinking water, excess carbon dioxide throws off pH and makes disinfection unpredictable. In industrial cooling loops, dissolved air causes pump cavitation and reduces heat transfer.

Vacuum degassing is one of the most effective ways to strip these gases out. It is not a new idea—power plants and boiler houses have used it for decades—but tighter regulations, higher energy costs, and more demanding water quality specifications are pushing it into new applications across municipal and industrial water treatment.

This article explains what vacuum degassing does, why it matters, and how to choose the right system for your water treatment process.

Why Dissolved Gases Are a Problem

The gases dissolved in water are not there by accident. They enter through contact with the atmosphere, through aeration processes, or as by-products of chemical reactions. Once dissolved, they stay in solution until something changes—temperature, pressure, or chemical equilibrium.

Oxygen is the most notorious. Even a few parts per million can cause severe corrosion in carbon steel piping and boilers. The damage is not always obvious at first; it starts as pitting, then accelerates into leaks and failures. In boiler systems, oxygen corrosion is a leading cause of unscheduled downtime.

Carbon dioxide dissolves to form carbonic acid, lowering pH and making water aggressive toward metals and concrete. In drinking water, it complicates pH adjustment and can increase the demand for alkaline chemicals. In high-purity industrial water, even trace CO₂ interferes with conductivity and resistivity measurements.

Nitrogen is mostly inert, but when it comes out of solution it forms bubbles that cause pump cavitation, reduce heat transfer efficiency, and create noisy, inefficient operation.

Other gases—methane, hydrogen sulfide, chlorine, and volatile organics—appear in specific applications. Each brings its own problems, from odor to toxicity to explosion risk.

The common thread is that all of them are unwanted. Vacuum degassing gives you a way to take them out.

How Vacuum Degassing Works

The principle is simple: gases dissolve better at higher pressure and come out of solution when pressure drops. In a vacuum degasser, water is exposed to a low-pressure environment—typically 0.02 to 0.2 bar absolute—where dissolved gases are no longer soluble. They leave the liquid phase and are carried away by the vacuum system.

The efficiency of the process depends on three things:

• Surface area. The more surface area the water presents to the vacuum, the faster gases escape. This is why degassers use packing, trays, or spray nozzles to break water into thin films or droplets.

• Residence time. The water must stay in the vacuum long enough for gases to diffuse out of the bulk liquid.

• Vacuum level. Lower absolute pressure means lower gas solubility. Deeper vacuum removes more gas, but it also costs more to produce.

In practice, most water treatment degassers achieve 90–99% removal of dissolved oxygen and CO₂. The exact performance depends on the system design and the incoming gas concentration.

Where Vacuum Degassing Is Used

Boiler feedwater treatment is the classic application. Removing dissolved oxygen and CO₂ prevents corrosion in boilers, feedwater heaters, and condensate return lines. Modern boiler systems often combine vacuum degassing with chemical oxygen scavengers for a two-layer defence.

Drinking water treatment uses vacuum degassing to remove excess CO₂, adjust pH, and improve disinfection efficiency. It is also used to strip volatile organic compounds (VOCs) and radon from groundwater.

Industrial cooling water benefits from degassing because dissolved air causes pump cavitation and reduces heat exchanger performance. Removing air stabilises flow and improves energy efficiency.

Wastewater treatment uses vacuum degassing to remove odorous gases like hydrogen sulfide, and to prepare water for membrane filtration or discharge.

High-purity water systems—pharmaceutical, semiconductor, and power plant—rely on vacuum degassing to achieve ultra-low dissolved gas levels.

Choosing the Right Vacuum System

The vacuum pump is the heart of any degassing system. Several technologies are available, each with trade-offs.

Liquid ring pumps are a common choice for water degassing because they tolerate moisture and liquid carryover. They are robust, simple, and reliable. However, they consume seal water and have moderate energy efficiency.

Dry screw pumps offer oil-free operation and high efficiency. They are increasingly specified for new installations where product purity and energy costs matter. They require good upstream separation to avoid liquid ingress, but modern designs handle water vapour well.

Roots blowers are often used as boosters in combination with other pumps to achieve deeper vacuum or higher capacity. They are not used alone for wet applications.

Steam ejectors are an option where steam is available and electricity is expensive. They have no moving parts, but they consume steam and generate wastewater.

The right choice depends on the vacuum level required, the gas load, the water temperature, and the operating cost priorities of the facility.

System Design Considerations

A vacuum degasser is more than just a pump. The system includes a degassing vessel, packing or trays, water distribution, vacuum piping, and controls. Each component affects performance.

Vessel design determines how well the water contacts the vacuum. Packed columns offer high surface area but can foul with suspended solids. Spray systems are simpler but less efficient. Tray designs fall in between.

Water temperature matters because gas solubility decreases as temperature rises. Warm water degasses more easily, but it also increases the vapour load on the vacuum pump.

Vacuum control is essential for stable operation. If the vacuum is too deep, water may flash to steam. If it is too shallow, degassing is incomplete. Automatic control valves and pressure transmitters maintain the setpoint.

Materials must resist corrosion from the gases being removed. Carbon dioxide and hydrogen sulfide are particularly aggressive. Stainless steel or coated components are often specified.

Maintenance and Troubleshooting

Vacuum degassing systems are relatively low-maintenance, but they are not maintenance-free.

Pump performance degrades over time. Seal wear, internal corrosion, and fouling reduce efficiency. Regular monitoring of vacuum level and pump current catches problems early.

Packing and trays can foul with scale, biological growth, or suspended solids. Cleaning or replacement is periodic.

Vacuum leaks are a common cause of poor performance. Air leaking into the system raises the absolute pressure and reduces degassing efficiency. Leak testing should be part of routine maintenance.

Water quality affects system life. Hard water causes scaling; corrosive water attacks materials. Pretreatment may be necessary.

Summary

Vacuum degassing is a proven, effective way to remove dissolved gases from water. It prevents corrosion, improves treatment performance, and helps meet water quality standards. The technology is well-established in boiler feedwater treatment and is finding new applications in drinking water, industrial cooling, and wastewater.

Selecting the right vacuum pump—liquid ring, dry screw, or a hybrid system—depends on the specific application, the required vacuum level, and the facility’s operating priorities. A well-designed system will deliver years of reliable service, protecting equipment and ensuring water quality.

Technical FAQ

Q: What is the typical vacuum level for water degassing?
A: Most water degassers operate between 0.02 and 0.2 bar absolute. The exact level depends on the gases being removed and the desired removal efficiency.

Q: Can vacuum degassing remove all dissolved oxygen?
A: It can remove 90–99% of dissolved oxygen. For boiler feedwater, residual oxygen is usually handled by chemical scavengers to achieve the low ppb levels required.

Q: What is the difference between vacuum degassing and membrane degassing?
A: Vacuum degassing uses a low-pressure vessel to strip gases. Membrane degassing uses a hydrophobic membrane to separate gases from water. Membrane systems are compact but better suited to low flow rates and high-purity applications.

Q: How do I choose between a liquid ring and a dry screw pump for degassing?
A: Liquid ring pumps handle moisture well and are robust, but they consume seal water. Dry screw pumps are more energy-efficient and oil-free, but require good liquid separation. The choice depends on water quality, energy costs, and maintenance resources.

Q: What maintenance does a vacuum degasser need?
A: Regular checks of vacuum level, pump current, and seal condition. Packing or trays should be inspected for fouling. Leak testing should be done periodically. Water quality pretreatment may be needed to prevent scaling.