Factors Affecting Oxygen Transfer Rate (OTR) in Aeration Tanks | Wastewater Treatment Optimization

Factors Affecting Oxygen Transfer Rate in Aeration Tanks

The Importance of OTR in Aeration Tanks

Oxygen transfer rate (OTR) is a critical parameter in biological Wastewater Treatment, directly impacting the efficiency of organic pollutant degradation and the energy consumption of aeration systems. In activated sludge processes, microorganisms rely on dissolved oxygen (DO) to break down organic matter, and an optimal OTR ensures stable treatment performance while minimizing operational costs.

This article explores the key factors influencing OTR.Understanding these factors helps engineers optimize aeration systems, reduce energy usage, and maintain compliance with effluent standards.

Factors Affecting OTR

The oxygen transfer rate (OTR) in aeration systems is influenced by multiple physical, chemical, and operational factors. Below are the key determinants of OTR efficiency:

1. Oxygen Saturation Concentration (C_s)

• OTR is directly proportionalto the saturation concentration of dissolved oxygen (DO).

• Temperature dependence:
  • C_s decreasesas temperature rises (e.g., 9.1 mg/L at 20°C vs. 7.6 mg/L at 30°C).

2. Water Temperature

• Impact on K_Laa (Mass Transfer Coefficient):
  • K_Laincreases with temperature (faster diffusion).
  • C_s decreaseswith temperature (lower solubility).

• Net effect: In the 10–30°C range, these opposing trends nearly cancel out.

3. Wastewater Characteristics

1）Impurities (α Factor)

• Surfactants/organics reduce OTR.

2）Correction: Multiply K_La(clean water) by α (0.3–0.8 for wastewater).

• Salinity (β Factor)
• Dissolved salts lower oxygen solubility.

• Correction: Multiply Cₛ (clean water) by β (0.9–0.97).

3）Oxygen Partial Pressure (p)

• Higher atmospheric pressure raises C_s.

• Correction:

p = Local atmospheric pressure(Pa) / 1.013×10⁵ Pa

4）Water Depth (For Diffused Aeration)

• C_sis highest at the diffuser depth (due to hydrostatic pressure).

• Effective C_s= Average of values at diffuser depth and water surface.

4. Bubble Dynamics & Hydrodynamics

• Bubble size: Smaller bubbles ↑ surface area (↑Kₗa) but ↓ turbulence.

• Contact time: Longer exposure ↑ OTR (but stagnant liquid films limit transfer).

• Turbulence: Bubble rupture and liquid mixing renew oxygen-depleted films.

5. Diffuser Performance

• Fine-pore diffusers (e.g., membrane discs) generate small bubbles (↑OTR).

• Coarse-Bubble Diffusers rely more on turbulence.