Ozone Transfer Mechanisms and Solubility in Water Treatment

Ozone (O₃) is a powerful oxidant and disinfectant widely utilized in water treatment. For effective disinfection and oxidation, ozone gas, typically produced on-site by an ozone generator, must be efficiently dissolved into the water. Unlike many other gases, predicting ozone solubility is complex due to its inherent instability in water and sensitivity to various environmental factors.

This guide explores the fundamental principles behind ozone solubility and mass transfer, key influencing factors, and common injection techniques essential for optimizing ozone-based water treatment processes.

Understanding Ozone Solubility

The solubility of a gas in water is generally described by Henry's Law. However, for ozone, which undergoes chemical changes in water, specific coefficients like the Bunsen and Solubility Ratio factors are often employed to better characterize its dissolution.

Henry's Coefficient

Henry's Law posits that the solubility of a gas in a liquid is directly proportional to the partial pressure of that gas above the liquid. While fundamental, its direct application to ozone is limited because ozone reacts and decomposes in water. It is primarily applicable to gases that do not chemically interact with the solvent during transfer.

Two forms of Henry's Coefficient are commonly referenced:

1. Molar Fraction Form:

   • Formula: Y = H * X
   • Where:
     • Y = Partial pressure of the substance over the fluid [atm]
     • X = Molar fraction of ozone (gaseous) [-]
     • H = Henry's constant [atm/(mol/L)] or [atm·L/mol]

   In practice, ozone generators produce ozone as part of a gas mixture (e.g., in oxygen or air), necessitating the use of partial pressure. This form helps describe the equilibrium between ozone in the gas phase and its dissolved molar fraction.

2. Dimensionless Henry's Constant (Hc): This form is often more practical for water treatment engineers as concentrations are expressed in mass per volume.

   • Formula: Hc = (Concentration of gas above the liquid in equilibrium) / (Concentration of gas in the liquid in equilibrium)
   • Where:
     • Hc = Dimensionless Henry's constant [-]
     • Concentration in gas phase (e.g., mg/L of gas)
     • Concentration in liquid phase (e.g., mg/L of liquid)

Bunsen Coefficient (β)

The Bunsen adsorption coefficient (β) is another method used to calculate ozone solubility. It describes the volume of gas at Normal Temperature and Pressure (NTP: 0°C/32°F and 1 atmosphere), which is dissolved at equilibrium by a unit volume of liquid at a given temperature, when the partial pressure of the gas is one atmosphere.

• Formula for Dissolved Gas Concentration: Cs = β * M * P

• Where:

  • Cs = Concentration of dissolved gas [kg/m³ (lb/ft³)]
  • β = Bunsen adsorption coefficient [-]
  • M = Density of the gas [kg/m³ (lb/ft³)]
  • P = Partial pressure of ozone [Pa (psi)]

• Definition of Beta (dimensionless): β = Vg / Vl

  • Vg = Volume of gas dissolved
  • Vl = Unit volume of liquid

Solubility Ratio Coefficient (S)

The solubility ratio coefficient (S) is a widely used and practical method for estimating ozone solubility, relating to the Henry coefficient. It is defined as milligrams of ozone per liter of water per milligrams of ozone per liter of gas, making it a dimensionless factor.

• Empirical Formula for Solubility Ratio (S) at varying temperatures: log₁₀S = -0.25 – 0.013T [°C]
  • Where:
    • S = Solubility ratio coefficient [-]
    • T = Temperature in degrees Celsius [°C]

This formula serves as a guideline, as actual ozone solubility is influenced by multiple factors within the water matrix. Temperature, however, is consistently one of the most critical influencing factors.

• Example Calculation at 20°C (68°F): log₁₀S = -0.25 – (0.013 * 20) log₁₀S = -0.25 – 0.26 log₁₀S = -0.51 S = 10^(-0.51) S ≈ 0.31

  At 20°C (68°F), the solubility ratio (S) is approximately 0.31 mg/L water per mg/L of carrying gas. This means if the ozone concentration in the gas phase is 20 mg/L, approximately 20 * 0.31 = 6.2 mg/L of ozone will dissolve in water at 20°C (68°F).

Factors Influencing Ozone Solubility

Optimizing ozone solubility is critical for treatment efficiency. The degree of solubility is affected by:

• Ozone Concentration in Gas: Higher ozone concentration in the feed gas (or higher partial pressure) directly increases solubility. Modern ozone generators can produce high concentrations (e.g., up to 240 g/Nm³).
• Gas Pressure: Increasing the overall gas pressure within the contactor enhances ozone dissolution.
• Water Temperature: Decreasing water temperature significantly improves ozone solubility. Colder water can hold more dissolved gas.
• Amount of Solutes: High concentrations of dissolved solids or other solutes can reduce ozone solubility.
• pH: Generally, decreasing pH can improve ozone stability and, consequently, solubility in some contexts, as ozone decomposes more rapidly at higher pH.
• UV Light Exposure: Excess UV light can cause ozone decomposition, reducing effective dissolved ozone concentration.

Ozone Mass Transfer

Mass transfer is the process by which ozone moves from the gas phase into the liquid phase. This occurs across a gas-liquid interface, driven by a concentration gradient. A simplified double-layer model illustrates this, showing resistance in both phases.

The stages of ozone mass transfer include:

1. Diffusion of ozone across the gas/liquid interface: Ozone molecules move from the bulk gas to the interface.
2. Dissolving into the liquid film: Ozone dissolves into the thin liquid layer directly at the interface.
3. Diffusion into the bulk liquid: Dissolved ozone then disperses throughout the main body of water.

The rate of mass transfer is dependent on several factors:

• Physical properties of gas and liquid: Density, viscosity, and diffusivity influence how easily ozone moves.
• Concentration difference across the surface: A larger difference (driving force) leads to a faster transfer rate.
• Turbulence: Increased turbulence or mixing in both the gas and liquid phases enhances the contact area and reduces boundary layer thickness, thereby improving transfer efficiency.

Ozone Injection Techniques

Efficient ozone dissolution requires appropriate injection techniques. The most common methods are diffusers and Venturi injectors.

Diffuser Systems

Diffusers typically consist of porous discs, plates, or bars installed at the bottom of contact tanks. They work by injecting ozone gas under pressure, creating a column of fine bubbles that rise through the water column, maximizing gas-liquid contact time and surface area.

• Advantages:
  • High transfer yield (especially with sufficient contact depth).
  • Relatively simple construction.
  • Economical for large water volumes, such as in drinking water systems.
• Disadvantages:
  • Not very compact; requires deep contactors.
  • Efficiency heavily depends on contact column depth and bubble size.
  • Prone to clogging (stuffing-up), which can reduce efficiency over time, requiring maintenance.

Venturi Systems

Venturi injectors operate by creating a pressure differential within a constricted flow path. As water flows through the Venturi throat, its velocity increases, and pressure drops, creating a vacuum that draws in ozone gas. The gas is then mixed intimately with the water as the flow expands downstream.

• Advantages:
  • Very compact installation, requiring less footprint than contact tanks.
  • No moving parts within the injector itself, leading to low maintenance.
  • High transfer yield due to intense mixing and shear forces.
  • Excellent for sidestream injection and smaller systems.
• Disadvantages:
  • Requires a motive water flow, meaning energy consumption for pumping.
  • Can create backpressure in the system.

AquaChain Engineering Tip

When troubleshooting low dissolved ozone concentrations in a contactor, beyond checking generator output and feed gas pressure, ensure your ozone diffuser system is regularly inspected for fouling. Clogged pores significantly reduce bubble size consistency and gas-liquid contact, leading to poor transfer efficiency and wasted ozone. Consider chemical cleaning or replacement schedules based on water quality and operational history.

Frequently Asked Questions

Q1: Why is ozone solubility more complex to predict than other gases? A1: Ozone is an unstable gas that reacts and decomposes in water, unlike inert gases, making its effective solubility dependent on reaction kinetics in addition to physical dissolution principles.

Q2: How does water temperature specifically impact ozone solubility? A2: Ozone solubility is inversely proportional to water temperature; colder water can dissolve and hold significantly more ozone than warmer water, which is a critical consideration for system design.

Q3: What's the primary benefit of a Venturi injector over a diffuser for ozone transfer? A3: Venturi injectors are generally more compact, require less contact volume, and offer very high mass transfer efficiency due to intense mixing, making them suitable for installations with space constraints or when high dissolution rates are needed in a smaller footprint.

Relevant AquaChain Guide: For details on pre-treatment or post-treatment steps that might influence ozone effectiveness, refer to our guide on Filtration.