Oxygen Demand

Calculation of Aeration Oxygen Demand from OUR Test

Once you have generated an OUR value you can use that value to calculate the oxygen demand in the bioreactor. You can also use the OUR value generated from a given sample volume to calculate the OUR in the wastewater entering the bioreactor. The OUR test result generates a value with the units of mg O2/L/min. This value, in itself, does not tell us all that much, other than how it compares to previous OUR values. The piece of information that is more useful is to know how many pounds (or kilograms) or cubic feet (or cubic meters) of oxygen are being consumed which can then be compared to the oxygen generation or oxygen supply capacity of the aeration system. So our goal is to move from the original OUR units of mg O2/L/min to units of lb O2/day as illustrated by Equation 1.

Equation 1: Using the OUR to Calculate Oxygen Demand

Oxidation Ditch
mg/L-to-pounds

An oxidation ditch at a sugar plant.

If a sample is collected from the effluent end of the bioreactor, the OUR of that sample measures the oxygen demand remaining in the MLSS. If treatment is complete, this will be a relatively low value or the endogenous value. If treatment is incomplete, the OUR value will be higher. When the OUR drops below  0.15 (mg O2/L)/min [9.0 (mg O2/L)/hr] the bioreactor is close to endogenous and when it falls below 0.10 (mg O2/L)/min [6.0 (mg O2/L)/hr] you can be certain that endogenous respiration has been achieved. So what if the OUR is 0.267 (mg O2/L)/min [16.0 (mg O2/L)/hr] in a bioreactor with a volume of 3,000,000 gallons.

We will begin the calculation of the oxygen demand by converting from OUR in units of minutes to units in hours as shown in Equation 2.

Equation 2: Conversion of OUR from Units of Minutes to Units of Hours

We can now calculate the oxygen demand in units of pounds per day remaining in the MLSS as it exits the bioreactor as shown in Equation 3. The key value to use in this equation is the volume of the aeration basin.

Equation 3: Oxygen Demand from OUR

Oxygen demand

How does the result obtained in Equation 3, where endogenous respiration had not been achieved, compare to having achieved endogenous respiration? Let’s recalculate the oxygen demand using an OUR of 0.10 (mg O2/L)/min [6.0 (mg O2/L)/hr] as shown in Equation 4.

Equation 4: Oxygen Demand from Endogenous OUR

Endogenous oxygen demand

When treatment in the bioreactor is complete and endogenous respiration is achieved the oxygen demand is only 37.5% of what it was when using the OUR value in Equation 3. We could also have simply multiplied the result from Equation 3 by 0.375 but the point here is understanding the use of the equation to calculate the oxygen demand in an aeration system of any size.

Using the same approach taken above we can also calculate the oxygen demand of any given sample by substituting the expected or estimated flow rate of that sample into the bioreactor. If we are using the total wastewater flow rate, as in the flow leaving an equalization tank, we replace the volume of the aeration basin in Equations 3 and 4 with the total flow in units of millions of gallons per day. If the total wastewater flow rate is 2,500 gallons per minute or 3,600,000 gallons per day, we replace the aeration volume with this new wastewater flow volume. If, for example, the OUR of the sample was 0.393 (mg O2/L)/min [23.6 (mg O2/L)/hr] the total oxygen demand exerted by the wastewater would be calculated as shown in Equation 5.

Equation 5: Oxygen Demand from Endogenous OUR

Wastewater oxygen demand

The calculation of the oxygen demand from the wastewater itself is a valuable piece of information for heavily loaded activated sludge systems. As shown by Equation 5, the oxygen demand is 16,977 pounds per day. If the oxygen supply system to the bioreactor cannot provide this much oxygen, plus the 3,597 pounds needed for endogenous respiration, oxygen will be a critical constraining factor for the bacteria which will reduce their ability to oxidize the organic load entering the bioreactor. This, in turn, can create a range of problems from effluent violations, filamentous bacterial growth, and odor problems.