Gas Injection into Liquids: Fundamentals, Technologies and Applications

Gas and water are a difficult combination – and precisely because of that, they form a discipline of their own. Gases dissolve only in limited quantities, escape easily, and consume energy in the process. Anyone who needs to deliver oxygen or CO₂ into water or wastewater with precision has to think physics, process engineering and chemistry in a single breath.

1. Introduction

Without gas injection, water treatment would barely function: activated sludge needs dissolved oxygen, alkaline wastewaters are neutralised with CO₂, and drinking-water treatment regulates pH and carbonate equilibrium via carbon dioxide. The underlying challenge is always the same: transferring a gas into the liquid phase as completely as possible and with as little energy as possible, before it escapes unused.

2. The Physics: How Do Gases Dissolve in Water?

The basis is Henry's law. It states that the dissolved concentration c of a gas is proportional to its partial pressure p in the gas phase:

c = k_H · p

Here, k_H is the Henry constant in mol/(L·atm). Real values at 20 °C:

• Oxygen (O₂): k_H ≈ 1.4 · 10⁻³ mol/(L·atm). In air-saturated water (partial pressure of O₂ ≈ 0.21 atm) this gives a saturation concentration of about 9.1 mg/L – the classic O₂ saturation value at 20 °C.
• Carbon dioxide (CO₂): k_H ≈ 3.6 · 10⁻² mol/(L·atm). CO₂ is therefore roughly 25 times more soluble than O₂. Under pure CO₂ (1 atm), around 1.6 g/L dissolves at 20 °C.

Two dependencies shape practice: as temperature rises, solubility falls (warm water holds less gas, which is why O₂ saturation drops from ~11 mg/L at 10 °C to ~9 mg/L at 20 °C). As pressure rises, solubility increases linearly – the basis of every pressure-aeration system. The key parameter is always the partial pressure, not the total pressure: pure oxygen at 1 atm delivers five times the O₂ concentration of air.

3. Oxygen Injection

The standard processes differ in bubble formation and energy carrier:

• Fine-bubble aeration: membrane or ceramic discs produce bubbles < 2 mm. SOTE (Standard Oxygen Transfer Efficiency in clean water) typically 30–40 %, in clean basins up to ~45 %. SAE (Standard Aeration Efficiency) is around 1.8–2.5 kg O₂/kWh.
• Membrane diffusers (PTFE/EPDM): a variant of fine-bubble technology with a self-cleaning surface, SOTE 30–38 %, lower pressure loss, SAE 2.0–3.0 kg O₂/kWh.
• Pressure aeration: water is aerated with air/O₂ under pressure. Higher solubility yield, but SAE drops to 0.8–1.5 kg O₂/kWh.
• Pressure saturator: water is saturated with O₂ at 4–6 bar (up to 40–60 mg/L) and discharged into the basin – relevant for high-load stages, SAE usually 0.5–1.2 kg O₂/kWh, with very high transfer efficiency.

Rule of thumb: large bubble surface + long rise time + deep installation = high SOTE, but only fine bubbles + low pressure loss deliver good SAE.

4. CO₂ Injection

CO₂ is used predominantly for the neutralisation of alkaline wastewaters. The chemistry: CO₂ dissolves as carbonic acid and dissociates:

CO₂ + H₂O ⇌ H₂CO₃ ⇌ H⁺ + HCO₃⁻ (pK_a1 ≈ 6.35)

The free H⁺ neutralises OH⁻, while excess CO₂ is bound as bicarbonate HCO₃⁻. Compared with mineral acids (hydrochloric acid, sulphuric acid), this offers three advantages: CO₂ is self-buffering – overdosing will hardly push the pH below ~4–5. It introduces no foreign ions such as chloride or sulphate, which would increase salinity. And it is safer to handle than concentrated mineral acid. The drawback: CO₂ is weaker than strong mineral acids, so at very high pH values (> 11) it requires larger quantities and better mixing.

5. Technologies Compared

• Venturi nozzles: gas is drawn in through the vacuum created by a pipe constriction, providing intensive shear mixing; SOTE 15–25 %, energy supplied via pump pressure (Δp 0.5–1.5 bar), compact, well suited to inline gas injection.
• Bubble columns: simple, large-volume, bubbles rise freely, SOTE 20–35 %, low pressure loss, low maintenance – the workhorses of wastewater biology.
• Membrane contactors: gas diffuses bubble-free through a hydrophobic membrane directly into the liquid. No phase mixing, closed system, close to 100 % utilisation with proper counter-current flow, but higher capital costs and sensitive to fouling.
• Pressure saturators: saturate water to a high level under pressure and dose it precisely – high transfer efficiency, low SAE, sensible wherever high concentrations are required.

6. Outlook

The trend is moving towards bubble-free membrane contact: no bubbles, no aerosol, no sources of odour or pathogens, full utilisation of the gas – increasingly used for CO₂ and ozone. In parallel, digital process control is on the rise: online probes (DO, pH, CO₂ partial pressure) combined with model-based control dose gas according to actual demand rather than a fixed schedule. Energy and gas savings of 15–30 % are realistic, and the investment typically pays back through the aeration electricity bill – which is, in any case, the single largest cost line on every wastewater treatment plant.