Factors Affecting Oxygen Input

The required air flow rate is determined by the process oxygen demand and the actual oxygen transfer efficiency achievable by the aeration system. In practice, the process oxygen demand is first determined as the Actual Oxygen Requirement (AOR) and then converted into the required Standard Oxygen Transfer Rate (SOTR) under standard conditions using correction factors for wastewater characteristics, temperature, diffuser submergence depth, and operating dissolved oxygen concentration.

A commonly used relationship is:

SOTR = AOR / [α · β · ((C*T,h − CL) / C*20) · θ^(T−20)]

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• α = wastewater correction factor
• β = salinity/surface tension correction factor
• C*T,h = oxygen saturation concentration at operating temperature and submergence depth
• CL = operating dissolved oxygen concentration (DO)
• C*20 = oxygen saturation concentration in clean water at 20°C under standard conditions
• θ = temperature correction factor
• T = wastewater temperature (°C)

Once the required SOTR has been determined, the air flow rate can be calculated based on the oxygen transfer performance of the aeration system. A practical relationship is:

QN = SOTR / (SSOTR · hE)

Where:

• QN = standard air flow rate (Nm³/h or Nm³/min)
• SSOTR = specific standard oxygen transfer rate (oxygen transfer performance), expressed in g O₂/(Nm³·m)
• hE = diffuser submergence depth (m)

The higher the oxygen transfer efficiency and the greater the submergence depth, the lower the air flow rate required to meet a given oxygen demand. Therefore, aeration system design should always be based on measured performance data or manufacturer test results obtained in accordance with EN 12255-15 or DWA-M 209.