Oxygen Utilization Rate (OUR) Estimation

Oxygen utilization rate OUR = Amount of oxygen utilized / Amount of oxygen supplied. (1)

In most cases, it is difficult to obtain the total oxygen utilization data, so people tend to use the following formula (2) to approximate the oxygen utilization rate:

OUR = calculated oxygen demand / oxygen supplied. (2)

The results calculated using equation (2) are only accurate when the dissolved oxygen is 2 mg/L; otherwise the oxygen transfer efficiency will be either underestimated when DO > 2 mg/L or overestimated when DO < 2 mg/L. This result can be obtained from observing the following formula:

AOTR = (α · SOTR · θ / Cs20) × (τ · Ω · β · Cs20 - Cop). (3) 
                    Cst = τ · Ω · β · Cs20.

Assuming that other factors are the same as in the design condition, the actual oxygen transfer efficiency (AOTR) only depends on the value of (C_s20 - C_op). The larger the operating oxygen concentration Cop, the smaller the actual oxygen transfer efficiency AOTR. Aerator manufacturers usually set the OUR design point at C_op = 2 mg/L. Therefore, when C_op is above or below 2 mg/L, it is not surprising that the value calculated using equation (2) deviates from the manufacturer's data.

To better evaluate OUR, it is recommended to follow these three steps:

1. Measure and record the DO value, and record the corresponding time period;
2. Calculate the weighted average of DO as the C_op value;
3. Correct the data calculated from equation (2) using the weighted average Cop and equation (3).

OUR* = (calculated oxygen demand / supplied oxygen demand) × (τ · Ω · β · Cs20 - 2) / (τ · Ω · β · Cs20 - Cop).

example

Assume that τ, Ω, and β are all 1, C_s20 = 9.08, the weighted average DO value C_op = 3.8, and (calculated oxygen demand / oxygen supply) = 70%.

OUR* = 70% × (9.08 - 2) / (9.08 - 3.8) = 93.86%.
OUR* is the true oxygen utilization rate.