Oxygen Utilization Rate (OUR) Estimation

Oxygen Utilization Rate

OUR = oxygen utilized / oxygen supplied (1)

In most cases, it is difficult to obtain the total oxygen utilized data, so it is common practice to approximate the oxygen utilization rate using the following formula (2):

OUR = calculated oxygen demand / oxygen supplied (2)

The result calculated by formula (2) is accurate only when the dissolved oxygen is 2 mg/L; otherwise, the oxygen transfer efficiency is either underestimated (when DO > 2 mg/L) or overestimated (when DO < 2 mg/L). This can be seen from the following formula:

AOTR = (α·SOTR·θ/C_S20) × (τ·Ω·β·C_S20 − COP) (3)

C_ST = τ·Ω·β·C_S20

Assuming that other factors are the same as in the design, the actual oxygen transfer rate (AOTR) depends only on the value of (C_S20 − COP). The higher the operating oxygen concentration (COP), the lower the actual oxygen transfer rate (AOTR). Aerator manufacturers usually set the OUR design point at COP = 2 mg/L. Therefore, it is not surprising that the value calculated by formula (2) deviates from the manufacturer’s data when COP exceeds or falls below 2 mg/L.

Three Steps to Evaluate OUR

To evaluate the OUR more accurately, it is recommended to follow the three steps below:

1. Measure and record the DO value, while recording the corresponding time period;
2. Calculate the weighted average of the DO as the COP value;
3. Use the weighted-average COP and formula (3) to correct the data calculated from formula (2).

Correction Formula and Calculation Example

OUR^* = (calculated oxygen demand / oxygen supply) × (τ·Ω·β·C_S20 − 2) / (τ·Ω·β·C_S20 − COP)

Example:

Assume τ, Ω, β are all 1, C_S20 = 9.08, the weighted average of DO = COP = 3.8, and (calculated oxygen demand / oxygen supply) = 70%

Then OUR^* = 70% × (9.08 − 2) / (9.08 − 3.8) = 93.86%

OUR^* — the true oxygen utilization rate