Equivalent Flow

Calculation of an Equivalent Flow Rate

When using a given sample volume (e.g., 1 mL or 3 mL or 5 mL) in a 300 mL BOD bottle an equivalent flow rate can be calculated. The concept is a little confusing so we’ll begin by using one of the graphics shown in the Level 3 OUR Testing. In Figure 1 we are adding different sample volumes to the BOD bottle which is then filled with MLSS so the OUR test can be run. Using the concept of ratios we can proceed to calculate an equivalent flow rate.

Figure 1: Level 3 OUR Sample Volumes

Sample volumes used in OUR testing

Let us assume that we have an aeration basin with a volume of 1,411,200 gallons (5,342 m3) and the wastewater flow rate is 2,500 gallons per minute (9.46 m3/min). The equivalent flow rate given these two conditions for several sample volumes is shown in Table 1.

Table 1: Equivalent Flow Rates for a Given Sample Volume

OUR equivalent flow table

These equivalent flow rates were calculated using the ratio logic portrayed in Figure 2. You always need to start with two values to calculate the equivalent flow rate: 1) the influent flow and 2) the aeration basin volume.

Figure 1: Level 3 OUR Sample Volumes