Wastewater Calculations - Mathematics and Statistics for the Environment - page 687

24.7.1.4 Food-to-Microorganism Ratio
The food-to-microorganism ratio (F/M ratio) is a process control approach based on maintaining
a specified balance between available food materials (BOD or COD) in the aeration tank influent
and the aeration tank mixed liquor volatile suspended solids (MLVSS) concentration. The chemical
oxygen demand (COD) test is sometimes used, because the results are available in a relatively short
period of time. To calculate the F/M ratio, the following information is required:

• Aeration tank inluent low rate (MGD)
• Aeration tank inluent BOD or COD (mg/L)
• Aeration tank MLVSS (mg/L)
• Aeration tank volume (MG)

Primaryeffluent COD/BOD(mg/L)f × low(MGD) × 8.34 lb/mg/L/MG

F/Mratio =

(24.33)

MLVSS(mg/L) × aera tor volume(MG) × 8.34 lb/mg/L/MG

Typical F/M ratios for activated biosolids process are shown in the following:

BOD (lb)/MLVSS (lb) COD (lb)/MLVSS (lb)

Conventional 0.2-0.4 0.5-1.0

Contact stabilization 0.2-0.6 0.5-1.0

Extended aeration 0.05-0.15 0.2-0.5

Pure oxygen 0.25-1.0 0.5-2.0

■ EXAMPLE 24.43
Problem: The aeration tank influent BOD is 145 mg/L, and the aeration tank influent flow rate is 1.6
MGD. What is the F/M ratio if the MLVSS is 2300 mg/L and the aeration tank volume is 1.8 MG?

Solution:

145 mg/L × 1.6 MGD × 8.34 lb/mg/L/MG

F/Mratio = = 0.0.6 lb B OD perlbMLVSS

2 300 mg/L × 1.8 MG × 8.34 lb/mg/L/MG

Key Point: If the MLVSS concentration is not available, it can be calculated if the percent volatile
matter (% VM) of the mixed liquor suspended solids is known.

MLVSS = MLSS × % Volatile Matter (%VM) (24.34)

Note: The F value in the F/M ratio for computing loading to an activated biosolids process can be
either BOD or COD. Remember, the reason for biosolids production in the activated bio-
solids process is to convert BOD to bacteria. One advantage of using COD over BOD for
analysis of organic load is that COD is more accurate.

■ EXAMPLE 24.44
Problem: The aeration tank contains 2885 mg/L of MLSS. Lab tests indicate that the MLSS is 66%
volatile matter. What is the MLVSS concentration in the aeration tank?

Solution:

MLVSS (mg/L) = 2885 mg/L × 0.66 = 1904 mg/L